Traditional celadon

Recent firing with traditional porcelain stone glaze. In the past I’ve tried but failed to use modern materials like feldspar and kaolin to capture the beautiful, unctuous surface and depth of porcelain stone celadons. In this glaze the coloration is completely due to iron occurring naturally in the material.

Colors of Celadon: Iron and Titania

Following images from Bonham’s 2014 auction, The Feng Wen Tang Collection of Early Chinese Ceramics

The best resource I’ve found about color in Chinese glazes is Nigel Wood’s Chinese Glazes.  Chapter 8, Iron in Chinese Glazes, covers iron in detail, while celadons are covered throughout the book.

There’s a great range of colour in Chinese celadons.  In traditional celadons, color is mostly the result of materials containing naturally-occurring iron being fired in a reduction atmosphere, with the color modified by the balance of other oxides in the glaze as well as the underlying clay body.  Relatively small amounts of titania, manganese, copper and even cobalt can affect the color in significant ways.  Ancient kilns like Yue, Hutian, Longquan, and Yaozhou became associated with certain colors and qualities of celadon, as the materials used at those kiln sites naturally contained particular blends of oxides.  In Jingdezhen, celadon glazes seem to have evolved from fairly simple geographically specific material-based recipes (e.g. 10 parts glaze stone from Yaoli, 1 part glaze ash from Leping) to extremely refined and intentional recipes with similar base glazes that incorporate additional materials for their specific ability to modify color.  This lead to glazes named not just for kiln sites but also specific colors and qualities:   天青 (sky blue with a small < .2% addition of cobalt),豆青(”bean” celadon pure green iron celadon),影青(”shadow” lake-green),粉青,玉青 (jade celadon),冬青 (winter green),鸭蛋青 (bright duck-egg green), just to name a few.

But celadons in the Song dynasty were mostly restricted to local materials.  Thus if your local clay and glaze stone contained only trace amounts of titania and low amounts of iron (such as in towns near Jingdezhen), then you could produce the famously pure blueish-green qingbai glazes.  While if you were in the Northern Yaozhou kilns where materials naturally contained more titania, your celadons would tend towards olive-green.  (These are greatly over-simplified and generalized statements.  Regarding Yaozhou celadons, in Chinese Glazes, Chapter 6, The Stonewares of North China, Woods also mentions the blueish-grey glazes of Yaozhou, as well as the possible influence of coal firing on the color of Yaozhou celadons.)

For me, part of the beauty of Chinese ceramics is the ability of those ancient potters to reveal the beauty of their local materials, and how the resulting aesthetic of each kiln’s wares was in large part driven by the nature of those materials.

Of course, these days most of us make glazes by blending various “standardized” materials, with color variations resulting from additional coloring oxides.

The following test is a simple biaxial blend showing the influence of iron and titania on the color of a base glaze.  I’m now using Pinnell Clear for all of my additive tests as I find it a much better glaze than the traditional Leach 4321.  Further tests could be done using small amounts of manganese, copper and cobalt, as well as varying the fluxes and silica:alumina ratio.  For this test I substituted Grolleg for New Zealand Halloysite, but I did not adjust the recipe to account for the slightly different chemistry.  For this test it is important that the base glaze has as little titania as possible, so it’s best not to use “dirtier” clays.  The test tiles are made from Jingdezhen “super-white” porcelain, which serves as a good blank canvas for the glaze colors.  These glazes look quite different on dirtier porcelains and stoneware.

Glazy: One Year Old

One year old!

Exactly one year ago, Glazy registration was opened to the public. Since then, we’ve made a ton of improvements and added many more recipes.

Thank you!

94% of website server fees have been paid with your generous donations. Thanks to all of you who have added recipes, photos, and contributed valuable ideas to Glazy. Special thanks to Pieter Mostert and Matt Katz for all their help.

Notable new additions to Glazy:

Stull SiO2:Al2O3 Charts

Si:Al Charts now include a Stull overlay as well as color-coded R2O:RO Ratios. To learn more about Stull, R2O:RO ratios, and other illuminating aspects of glazes, see Matt Katz’s Introduction to Glazes Online.

Extended Search

Simply click the eyedropper icon or one of the photo swatches to search by color. Keyword search is now greatly improved with natural language text search and the ability to search for numbers.

Material Safety Information

Newly added this month are hazard warnings for each material in the recipe. There is still a lot of work to be done in Glazy to provide accurate, easily understandable safety information for potters.

Todo

There are more improvements planned. The most important change in the next few months will be the addition of material lists, including regional and supplier lists. Material lists can be shared between users and rated.

WE NEED YOUR PHOTOS!

Ceramics recipes “do not travel well” and are very sensitive to differences in materials, preparation, application, firing, and cooling. The best way to compare, critique, and refine our recipes is to share photos of our results.

If you have photos you would like to share but find the Glazy interface too complicated, contact us and we will help. If you represent a school or studio with a lot of tests, we can help add the photos and recipes for you.

Glaze Transparency Test

Recently I’ve been wondering if there’s a reliable way to test glazes for transparency.  A method that would allow one to compare results from different firings and glaze types.

Paint manufacturers have a system for testing paint opacity that uses a black and white card from which a contrast ratio can be calculated. The primary manufacturer is Leneta.

I couldn’t find any parallels in the ceramics industry.

I wanted to try a similar method using porcelain (white) and stain/colored porcelain (black), adjusting the results to account for the fact that our whites and blacks are not pure.

Using my whitest porcelain, I created a colored slip adding 8% of a local black stain.  (Ideally one would use a standard mason stain.)  Adding Darvan, I made a thin slipcast slab that I then cut into small squares.

Cutting square slabs of stained porcelain

Using the same casting porcelain I made a thicker slipcast slab which was then cut into square test tiles.  The black stained squares were then applied to each test tile and rubbed flat.  Finally, the tiles were bisque fired in the hopes of minimizing contamination of the glaze when dipping.

Test tiles after adding black-stained squares.

Two 100 gram batches of glaze were prepared:  Pinnell Clear and Pinnell Clear with added 10% Zircopax.  Using volumetric blending I created tests in 2% increments.  The tiles were dipped in the test glazes.  Ideally, steps would be taken to ensure even thicknesses of glaze.

Test tiles after firing in reduction to Orton cone 10.

The fired tests display a nice opacity gradation as zircopax is added to the glaze.

Unsure of the best way to measure transparency (or opacity) using these test tiles, I tried the simplest approach I could think of.  Adjusting the image to greyscale, I averaged the colors of the white test tiles as well as each black-stained square.  Below are the Brightness levels measured in Photoshop using the HSB scale.  If these tests were going to be made consistent across firings, I suppose one could normalize the photos based on the color of the unfired white porcelain body.

For the opacifying power of Zircopax relative to this specific test, I created an opacity scale in Photoshop using the 0% glaze as a baseline and then matched the tests to this scale.  According to the scale, a 4% addition of Zircopax opacifies the glaze by 30%, while a 10% addition of Zircopax opacifies the glaze by 70%.  I’m probably vastly over-simplifying things.  For instance, I didn’t take into account the fact that the entire test whitens as Zircopax is added.  Also, there will probably be few times in ceramics where there is a neat linear relationship, for instance adding 14% Zircopax to the glaze won’t necessarily get me to 100% opacity.

Below is a closeup of the black squares.  If I had made these tiles more consistently, with a crisp, straight border between the black and white porcelain, it might also be possible to compare diffusion.

Close-up of colored squares.

Spraying Glaze

Spraying glaze is a fairly complicated process.  There are craftspeople in Jingdezhen whose only job is going from workshop to workshop spraying glaze.  There are so many factors involved with spraying (the type of work, thickness of work, type of glaze, glaze consistency, air pressure, spray head type, even weather) that it requires years of experience to be able to master the art.

I hope to slowly add to this article in the future.  For now I will just lay out the basics of how I spray glaze.

The Spraying Booth

My spray booth is made locally in Jingdezhen.  It’s a simple stainless steel frame with glass.  A large fan is attached to the back, sucking out particles.  Water is pumped from a bucket through a hose that leads to the top of the booth interior.  The water is channelled along the top of the glass and then exits through small holes, forcing the water to run down the glass, washing away glaze.  The water finally exits through a hole in the bottom of the spray booth, pouring back into the water bucket.

The Air Compressor

I have an old, noisy air-tank compressor that I rarely use.  I much prefer the Jingdezhen method- a cheap magnetic air compressor used in fish tanks.  I’ve used my current compressor for six years and it still runs great, with no need to worry about adding oil or filtering the outgoing air.

I’ve found that a 520W compressor is ideal.  In the past I had a smaller compressor that didn’t spray as well.

The sprayer does a great job of mimicking traditional Jingdezhen glaze spraying using just the breath.  A normal air compressor using a paint sprayer head will give you a finely atomized cloud of glaze resulting in a powdery glaze application.  But a traditional mouth sprayer connected to the fish tank compressor will give you relatively large glaze droplets that soak into the clay, leaving a more compact glaze application.

The fish tank compressor method also sprays less glaze into the air.  I often just run the water pump and leave the booth fan off (but of course I wear a good respirator).

Note that this type of spraying results in more water being absorbed into the ware.  Especially for thin pieces, care needs to be taken not to overload the ware with water.  I usually spray the outsides one day and the insides the next, giving the ware sufficient drying time in-between sprays.

If while spraying you notice the glaze stays wet and shiny on the surface it means you are either spraying too close or have already reached saturation.  This is bad.  There’s a good chance that the entire glaze layer will separate from the ware.

Mouth sprayers

The glaze sprayers widely used in Jingdezhen were originally meant to be sprayed using only one’s mouth.  Since then, the mouth stem has been modified from conical (larger end towards mouth) to tapered at both ends for a tight fit into an air compressor hose.

Making these sprayers is a specialized craft.  The sprayers come in dozens of different configurations.  The sizes of the container, nozzle, and mouth stem as well as the distances between these parts, all determine the characteristics of the spray pattern.  In general, larger containers are used for larger work (e.g. sculpture), while the smallest containers are used for spraying underglazes and details.

Comparing spray patterns. On the left, Paasche L Sprayer #4 attached to air-tank compressor, approximately 30-40 psi. On the right, Jingdezhen glaze canister with fish-tank magnetic air compressor.

The Paasche L Sprayer #4

The Paasche L Sprayer #4

Like Jingdezhen glaze canisters, the Paasche allows you to make fine adjustments in distance between the nozzle and container tube.  Along with adjusting air pressure and glaze thickness, a number of different spray patterns can be achieved.

Spraying

It’s difficult to write about actually spraying glaze, because each session is different.  The basic process is:

  • Spray outsides.  Do not rest ware directly on turntable or plaster disc, but rather elevate it with a stable item such as a smaller plaster column.  If the inside is already glazed, on top of the support you can add a sponge disk.
  • After spraying the bottom, you can scrape glaze off of the feet.
  • Ideally, wait one day while the bottoms dry completely.  If in a rush, blow air over the ware with a fan.
  • Spray insides.  Take care that feet are not resting on a surface that will become wet during glazing.  The dry plaster turntable disk helps with this issue.
  • Clean glaze off the feet by trimming or with a sponge.

To spray:

  • Using a notch in the turntable disk as a guide, keep a mental note of how many revolutions you make and the resulting thickness of the glaze (checked by scraping).  The number of revolutions will vary each glaze session, and is influenced by the glaze canister, air pressure, glaze consistency, size of ware, etc.
  • Keep the glaze canister in constant, steady motion- up & down, side to side, or circular.  You may need to vary the motion to get consistent application.

Mixing test glazes

It’s important to wear a NIOSH certified mask whenever using dry glaze materials.

I guess mixing up glazes isn’t that big of a deal, but I’m sharing my technique just in case there are some absolute beginners out there.

I find it easier to use a digital scale, see my article here.

Glazes “don’t travel well”, in other words materials, application, and firings vary from studio to studio.  Even for well-known glazes, it’s important to first make a small tests.  For these tests, I use 50g or 100g of material and apply the test glaze to a number of different clay bodies.

I use the Glazy Batch Calculator on my phone which will show you the subtotals for arbitrary amounts of total glaze materials.

Once I’m happy with a test, I mix up a larger batch of 1-2Kg.  1Kg is enough material to glaze small cups, 2Kg is a good amount for small bowls.  These larger tests should reveal any problems with glaze suspension (is bentonite required?), application (cracking, peeling, etc.), and fired glaze defects.  Once you have some nice results with 1Kg, you can finally move on to a big bucket of 5-10Kg.

Mixing up a test

Flat test tiles require the least amount of glaze for application.  Here’s my article about how I make test tiles.

Sieve Mesh Size

For “natural” glazes containing large-grained materials or ashes, or in cases where homogeneity is not a concern, it’s fine to use a larger screen of 60-80 mesh.  But in all other cases I use 120 mesh or smaller.  Small mesh size is very important for glazes that contain small amounts of very important materials such as coloring oxides (e.g. cobalt and iron).  But it’s also important to ensure that materials are adequately broken up and mixed (such as clays).

Below you can see two tests of the same batch of glaze fired in the same kiln.  The glaze on the left was applied after passing the materials three times through an 80 mesh screen.  The glaze on the right is the result of passing that same glaze once more through a 120 mesh screen.

Poorly dispersed colorants like iron are easy to see in fired glazes.  But keep in mind that other “invisible” glaze ingredients like clays, feldspar, etc. also need to be well-dispersed and mixed in order to ensure the glaze melts properly.  If you use a 60-mesh screen for tests and then a 120-mesh screen for large glaze batches, there will be differences between the fired results.

Seeing the cones

I’ve seen a few techniques for seeing into the kiln at high temperature.  An old friend of mine still prefers blowing into the peephole, unfortunately on more than one occasion it has resulted in the particles resting in the peephole to be blown in as well, settling on the ware.  The Jingdezhen firing masters I’ve met just put on an old pair of sunglasses and squint (on the rare occasions they actually need to look at a cone).

I’m currently using #5 welding goggles, the only pair I could find for sale here but they work really well.  If you have a choice, go for IR rated lenses which protect from harmful infrared light.  Here’s a really good article about eyeware for potters.

Combined with the goggles, a strong flashlight will give you a really good view inside the kiln.  This year, my old LED flashlight finally gave out, and at around 400 lumens it was still a little difficult to see in the kiln.  The LED flashlight I purchased as a replacement was on sale for about $40USD, a little expensive but to be honest I just wanted to know what 2000 lumens would look like.  It’s blinding!  But using this flashlight I can see all the way to the back of the kiln even in reduction at 1300° C (my kiln is only 1 meter long).  You can even see glazes start to glisten in the light of the flashlight as they begin to melt..

So if you’re getting a new flashlight for the kiln, I think you should go for at least 1000 lumens.

Welding goggles and the flashlight

Glazy open source ceramics recipe library

I invite all of you to join Glazy, a ceramics recipe library that allows anyone to browse and add pottery recipes for free.

Glazy was built using the latest open source tools, including Laravel and Bootstrap.  The database of ceramic recipes was originally seeded with data from Linda Arbuckle’s GlazeChem database and John Sankey’s glaze database. John Britt, Alisa Liskin Clausen, Terry Rorison and Tara Hagen have also included their glaze tests with images.

Since it’s release, many new features have been added to Glazy, including improved charts and color search.  More information can be found on the Glazy help page.

If you have ceramics recipes that you would like to add, or if you would like to help organize the recipes already in the database, please contact us.

Glazy is constantly improving and evolving.  I hope you will join us!

Glazy: Glaze recipes most used materials

With the data stored in Glazy it is possible to visualize recipes using graphs and charts.

In the future, these visualizations and more will be added to Glazy at http://glazy.org/graphs

Below are simple pie charts showing the most commonly used glaze materials for both Mid-Fire and High-Fire glazes.

For those who are just starting out with glazes, these charts could be a useful guide when stocking glaze materials.

The charts on the left are for “base” materials, the materials that form the actual glaze.  On the right are charted the “additional” materials- typically colorants and opacifiers- that are added to the base glaze materials.

There are still some strange and untrustworthy recipes in the Glazy database that may have skewed the results somewhat.

Mid-Fire Glazes

High-Fire Glazes

Below are the charts for High-Fire glazes.

Digital Scales for Weighing Glazes

After years of using simple balance scales to measure out glazes, I finally decided to invest in a better setup. I couldn’t find any triple-beam scales for sale in Jingdezhen, so instead I purchased a cheap 200-gram digital scale from a local shop.  I was delighted at how much simpler and faster it was to mix up tests with the digital scale.  It was only a few months later when I compared the digital scale to my old balance scales and discovered that the digital scale was consistently inaccurate, even just after calibration.

After having wasted 600RMB, I decided to just buy the best reasonably priced scales I could find.  The only imported brand in my price range and available in China was the Ohaus Scout Pro line.  I purchased two- one for tests and measuring colorants (model SP202, up to 200 grams with 0.01 gram readability) and one for mixing up bigger batches of glaze (model SP4001, up to 4000 grams with 0.1 gram readability).

The SP202 is very accurate, great for when you are making very small test batches.  The scale can also be used to measure colorants for big batches of glaze.

I use the SP4001 to directly measure out 1-3kg batches of glaze, or for measuring out each ingredient in larger glaze batches.

After a couple years, the Ohaus scales are still performing very well, especially considering that they are stored on the glazing patio and subjected to the weather.  The scales cost me much more than I wanted to spend, but they are well worth the money.

In conclusion:

  • If you’re looking to purchase scales for small glaze batches but don’t have a lot of money to spend, go for a triple-beam scale.  A good triple-beam will be much more trustworthy than a cheap digital scale.
  • If you only have enough money to buy one digital scale, get a 200-gram scale, preferably with .01 readability.  This will allow you to make accurate test glazes, as well as accurate colorant additions to larger batches of glazes.
  • If you do buy a digital scale, don’t forget you will need to calibrate it from time to time.  (I do so each glaze-making session.)  You will need accurate calibration weights in order to so, adding to the final cost.
Calibrating the Ohaus SP202 and SP4001 digital scales

Simple Microscopy for Ceramics

Supereyes digital USB microscope connected to laptop and using Quicktime to view a small Qing wine cup.

The first day of Ecology class we went out to a local pond, gathered water, and returned to the lab.  I’ll never forget the amazement of viewing the water under a microscope, exploring that hidden world.

I finally got my first microscope for viewing ceramics.  There are multiple hand-held digital microscopes available now, I went with a Chinese company, Supereyes.  The A005+ is their most expensive version and has a 5MP sensor.  It was about $150USD.

The camera comes with Windows software.  On a Mac, you just plug in the USB cable, open Quicktime and select File->New Movie Recording.  You can either record a movie or take a screenshot.  Results are best if Quicktime is in Fullscreen mode.

The quality of the A005+ sensor is not great.  There is a lot of noise in the image and resolution seems poor for 5MP.  Focusing works fairly well, although the focusing mechanism (rotating the top of the microscope) can be confusing.  The A005+ has built-in lights, however they produce a lot of glare on reflective surfaces.  Using ambient light or shining a flashlight at various angles works better.  It is also very interesting to view translucent ceramics with a light shining through them.

Shining a flashlight through a late Ming porcelain export dish to view birds painted in blue & white.

In retrospect, I probably should have gone with a regular microscope, using a handheld digital camera to record images.  (John Skaley has more information about microphotos at the bottom of his iron glazes article.)  But the A005+ is still a pretty fun toy.

High-Fire Cone 10 clear reduction glazes compared under the microscope

As a first test of the microscope, I thought it would be interesting to compare various clear glazes.  Personally, I prefer clear glazes that do not contain too many bubbles visible by the naked eye.

These clear glazes were all fired in a reduction atmosphere to Orton Cone 11.  You can find the recipes to these glazes on my new recipe website, Glazy.org.

Photomerge

Teadust glaze test tile

In microphotography it’s common to take multiple images of a subject at various focus depths, finally merging those images into a single image with greater depth of field.

We can also physically move the subject around on the microscope platform without varying focal length, finally merging those images into a single, larger image.  (Just like taking a “panorama” shot on your mobile phone.)  Various software exists to merge these photos together, including Photoshop and Lightroom.

For Photoshop the process is very simple.  Just gather your panoramic images into a single folder.  Open Photoshop and select File->Automate->Photomerge.  Select all your images and modify settings if desired.  I find that the default settings work just fine in most cases.

Here is a recent teadust glaze I have been working on and the resulting Photoshop photomerge of the same test tile.

Photomerged image of a teadust glaze

An old Porcelain Stone Mine

Unprocessed porcelain stone

It’s surprising to me how often archaeological discoveries seem to be made in Jingdezhen, but then I remember that wherever I walk in this place there are deep layers of shards beneath my feet.

A friend of mine was given samples from a recently found porcelain stone mine dating from the Five Dynasties Period.  Apparently the find has not gone unnoticed- professional antique makers have been secretly mining the site.  Luckily we have the chance to acquire some of this porcelain stone.

I’m often dealing with unfamiliar, traditional materials of which chemical analyses are lacking or unreliable.  In these cases, I usually create a series of line blends to get a basic idea of what I’m working with.  From those first tests, one can further refine glazes using more line blends and triaxials.

For this porcelain stone I created the following initial tests:

  • Pure porcelain stone, crushed, milled and sieved.
  • Porcelain body using porcelain stone and kaolin at 15-45%.
  • Lime-fluxed celadon glazes:
    • Porcelain stone and 10-20% Er Hui (Glaze Ash)
    • Porcelain stone and 10-20% Wollastonite
    • Porcelain stone and 10-20% Whiting

Idealized “traditional” recipes are also based on two-component mixtures.  For glazes, porcelain stone was mixed with a flux like glaze ash.  For porcelain bodies, porcelain stone was simply mixed with a proportion of kaolin.

Usually a single line blend of either Whiting or Wollastonite could tell you a lot about a porcelain stone.  However, porcelain stone mixed with Glaze Ash or Whiting often results in fuming/carbon trapping, so I wanted to test each flux separately.  I usually also create Dolomite or Talc tests.

I also prepared two sets of test tiles for cone 10 and 12 firings.

Stones of all types can be used in glazes.  Joseph Grebanier’s Chinese Stoneware Glazes lists many recipes that use locally sourced granite.  And Brian Sutherland’s Glazes from Natural Sources contains a wealth of information on the subject.

Triaxial testing

A lot of potters in China still seem to mix glazes the old-school way- one cup of this, two cups of that.  And strangely enough this technique seems to work pretty well for complex traditional materials.  Being a foreigner I tend to make things overly complicated.  I am also terrified of mixing up a great glaze but not remembering the exact composition or, even worse, not knowing how to adjust it if it doesn’t come out right.  So I rely on a lot of line blend and triaxial glaze testing.

Line blends are a useful tool for comparing two different recipes.  Commonly, the two recipes are different glazes.  For instance, you can blend two different celadon glazes in different proportions to create a new celadon recipe.  Line blends are also commonly used for testing additions of coloring oxides, for instance the effect of incrementally adding iron oxide to a clear glaze.

The range of the line blend is arbitrary- you can start each variable from 0% and go up to 100% or you could choose any range in-between.

For example, when adding red iron oxide to a reduction-fired clear glaze like Leach 4321, a line blend from 0% to 10% in 1% increments is sufficient to see the gradual transition from clear to blue celadon (1%), light green celadon (2%), dark green celadon (3-4%), brown, and tenmoku (7-10%).

To illustrate a line blend, here is a sample using two RGB colors blending by opacity.  If you already know the result of the outlying 100% blends, you could remove them from the test.  However, each firing is different and I usually leave in the 100% blends regardless.

Line blend of red and blue RGB colors by opacity

You could mix each test in a line blend individually, but a much less time-consuming method is to only make the left and right-most 100% solutions.  Using a syringe, you can easily create each mixed blend.

Ian Currie popularized volumetric blending.  (See his article here.)  First, mix the same weight of each glaze you will be blending.  Second, add some water to the glazes and sieve thoroughly.  Third, add water to each glaze so that their volumes are equal.  Now the two glazes are ready for blending.

Preparing glazes for volumetric blending

Below is an illustration of the blends in a 20ml syringe.  For my test tiles, 20ml is the minimum volume I need in order to completely cover a test tile (including a second dip).  See here for more information on how I make test tiles.

Using a syringe to perform volumetric blending

As you can see, for 10% increments using a 20ml syringe we need at least 20+18+16+14+12+10+8+6+4+2 = 110ml of each 100% recipe.  Preparing 100 grams of glaze material for each glaze should be sufficient.

Each time before you take glaze into the syringe, be sure to re-mix the glaze and confirm it has not settled.

Although I use 20ml test batches, it’s easier to use a 30ml syringe so there is extra room for drawing in air, making it easier to mix the glazes.

Volumetric line blending of 20ml glaze in a 30ml syringe
A triaxial volumetric line blend. Test tiles and glazes are arranged in order.
On the back of each test tile is written the full glaze information.
Test tiles resulting from a volumetric line blend. Increasing amounts of red iron oxide added to a clear glaze.
Fired result of the same test tiles. Reduction Orton cone 10.

Here is a line blend of Dolomite added in 2% increments to a Chinese porcelain stone.  Normally, I would test from 4-14% Dolomite, as I know from experience that this is the most useful range.  Below are the tests from 4-12% Dolomite in which the porcelain stone turns from a satin-matte to light celadon glaze.

Mixtures of porcelain stone and 4-12% Dolomite

However, it’s often good to test beyond the limits.  We might expect that even more Dolomite added to this glaze stone will result in a runnier, more transparent celadon/clear glaze.  But ceramics is much more complicated than simply mixing two colors together.

Here is the same test carried further, from 16% to 20% Dolomite.  We’ve gone from a creamy satin-matte to celadon and now to Dolomite matte with crystals.

This test also shows the importance of making small increments when doing line blends.

Mixtures of porcelain stone and 16-20% Dolomite

Triaxial Blends

Ceramics recipes are a complex interaction of multiple variables.  Triaxial blends involve three variables, making them more useful for exploring ceramic glazes and bodies.

Below is a four-row opacity triaxial blend of three colors (red, green, and blue) in RGB with a range of 0%-100% for each color. The outer edges of the blend are simply line blends of two colors, while the middle area of the blend contains mixes of all three variables.

Four-row triaxial blending RGB colors by opacity

The size, variables, and ranges of a triaxial are completely up to you.  A four-row triaxial with ranges starting at 0% is of limited use in testing mixes of all three variables.  You could adjust the four-row triaxial for ranges of 20%-80% so that each test includes at least some percentage of each variable.  Or, you could move up to a five-row triaxial for even more results.

Below is a five-row opacity triaxial blend of three colors (red, green, and blue) in RGB with a range of 20%-100% for each color. Because the range starts at 20%, each test includes all three variables.  If you instead wanted the outer blends to represent line blends, the ranges would start at 0%.

Five-row opacity triaxial RGB blend, 100-20%

Moving up to six and eleven-row triaxials you will notice that the number of tests we are creating is growing very quickly.  Because the range is from 0%-100%, the outer edges of the triaxial represent simple two-variable line blends.

Six and eleven-row triaxial RGB opacity blend, 100-0%

Based on these simple RGB color-blend triaxials, it might seem as if larger numbers of rows are unnecessary since we can easily infer the range of colors from a smaller four or five-row triaxial.  However, as mentioned before, ceramics recipes are a complicated mix of ingredients and sometimes results do not transition gradually from one test to the next.  Using too small of a triaxial with too large of an increment might miss important changes in glazes.  For instance, if Whiting were one of our variables in an eleven-row triaxial, the difference between each Whiting increment would be drastic.

Triaxial Size

The number of tests required for a given triaxial size can be determined by the triangular number sequence.  For a triaxial with only one row we of course need only one test.  But the number goes up very quickly as we add rows:  2 rows has 3 tests, 5 rows has 15 tests, 11 rows has 66 tests, and so on.

If we want to create a triaxial where each variable is in the range of 0%-100% and tests are in 10% increments, we will need an 11-row triaxial with 66 tests.  Unfortunately, it takes a lot of time to create so many test tiles!  It would be nice if we could create meaningful tests while at the same time reducing the size of the triaxial.

Reducing Triaxial Size

Often we already have idea of the ranges we want for each variable in a triaxial.

For example, say we’d like to find a nice Chinese blue & white qinghua underglaze.  Nigel Wood’s Chinese Glazes shows that qinghua is not simply cobalt blue but rather a complicated mix of oxides.  We decide upon three basic variables for our triaxial:  cobalt (top), iron (left), and manganese (right).  In order to do a comprehensive test, we decide to use steps of 10% for each variable.  Usually we would need a full 11-row triaxial for this type of test, necessitating 66 tests.  However, we decide that the left side (a line blend of cobalt and manganese) and the bottom side (a line blend of iron and manganese) are not interesting to us, so we can leave those out.  Now we are left with only 45 tests, in other words a 9-row triaxial.

Triaxial with eliminated tests. The sides are simply line blends. For this test we leave out the left and bottom sides.

Volumetric Blending for Triaxials

In our blue & white (qinghua) underglaze triaxial we still have 45 tests to make.  But instead of mixing each test individually, we can use Ian Currie’s volumetric blending to create intermediate tests.

Starting at the top of the triaxial pyramid, remove each adjacent glaze.  Now we are left with only 15 glazes to mix.  To produce the intermediate glazes, simply mix the two glazes in a 1-to-1 relationship by volume.

Note that the more times you need to mix a glaze, the more total glaze material you will require.  For the type of triaxial below I usually mix 200 gram batches in order to ensure that I have enough glaze.  My test tiles only require 20ml of glaze each.

Volumetric blending of triaxial, 50% mixes

We can further reduce the number of glazes to prepare, with the trade-off of more numerous and more complicated volumetric blends.  In the example below, we are only left with 6 glazes to make.

Further volumetric blending of triaxial

The actual fired result of this triaxial is much less balanced than the computer-generated RGB diagrams.  It’s apparent that cobalt oxide is a much stronger colorant than both iron oxide and manganese.  It seems the most interesting results are in the bottom two rows of the triaxial.  It might be interesting to “zoom in” on the bottom portion of the triaxial to refine the color even further.

In designing the tests, we also take into consideration that the glaze that covers an underglaze will affect the resulting color.  So for each test tile, the top half is covered with a basic transparent glaze (Limestone), while the bottom half is covered in a traditional chinese glaze (灰釉).  The narrow unglazed band in the middle gives us further information.

Fired result of cobalt, manganese, iron triaxial.

The changes in coloring oxide triaxials are usually straightforward- colors gradually shift.  Results are not so certain, though, when performing triaxial tests on other glaze components such as glass formers, melters, stabilizers and opacifiers.  Sometimes huge changes in a glaze can occur within only one or two-percent changes of the recipe.  So ideally, for each glaze test we would create a 51-row (range of 0%-100% in 2% increments) or 101-row (range of 0%-100% in 1% increments) triaxial.  But a 51-row triaxial needs 1,326 tests, while a 101-row triaxial needs 5,151 tests!

So in designing triaxials with small percentage increments it is often necessary to eliminate vast swaths of the triaxial.

For example, in the partial triaxial below I am searching for a nice teadust glaze.  The triaxial is based on 2% increments of Chinese glaze stone, whiting, and silica.  From experience and prior testing, I have already determined fixed percentages for ingredients not included in the test (red iron oxide and talc), and I have set fixed ranges for the three variables.  For instance, from past tests I know that I do not want too much or too little whiting- a range of 10%-16% is enough.  In this manner I have pared down a 1,326 test triaxial to only 13 tests.

As you can see in the results below, there is indeed a great range of glazes even within 2% increments.  If I had created a smaller triaxial with larger 10% increments I might have entirely missed the teadust crystal effect.

Further reducing size

By plotting out all of the tests (or even just the extremities) in the large triaxial above on a Silica/Alumina chart, we can further reduce the number of glaze tests.  From experience, it’s obvious that a cone 10 glaze with 60% Whiting and no Feldspar or Silica won’t work out very well.  But by looking at the chart, we can see entire areas of the test triaxial that are “out of bounds” for a good glaze and thus probably don’t need to be included.

Having said that, there are some interesting glazes (like Shinos) out of the ranges of commonly accepted glaze limits.  And in my example above, the tests that look most to me like teadust are quite high in silica and fall just inside the blue “underfired” zone.

For more information, see R.T. Stull’s original article in Transactions of the American Ceramic Society, Volume 14, pages 62-70.  Also see Matt Katz’s Introduction to Glaze Formulation Online

Large 2% triaxial plotted on SiO2/Al2O3 chart with Stull overlay

Conclusion

Tests build upon each other.  Coarse triaxials can be later refined, and specific glazes targeted within smaller ranges.  The more experience you have, the more you know where to look.  If you’re just starting out, I recommend a large, 11-row triaxial of Potash Felsdpar, Silica, and Whiting which will reveal a range of glaze types, from celadons to mattes.  Once you have a good feldspar/silica/whiting glaze you could try adding coloring oxides, stabilizers like ball clay or kaolin, opacifiers, etc.

Triaxial Worksheets Download (PDF)

Triaxial Worksheet

http://www.derekau.net/wp-content/uploads/2014/12/TriaxialWorksheet.pdf

http://www.derekau.net/wp-content/uploads/2014/12/PRINT_BLENDS_printA4.pdf

Tea Dust Glaze

Tea Dust Recipes on Glazy

The following glazes and more can be found on my new website, Glazy: https://glazy.org/search?base_type=460&type=550&cone=high

The Complete Guide to High Fire Glazes Tea Dust Recipes

Some tea dust glazes from The Complete Guide to High Fire Glazes.  I fired these glazes according to my usual firing schedule, probably too hot and not ideal for the development of crystals necessary for good tea dust glazes.  I crash cool to 1000C and then completely shut up the kiln.

Currie 10 Tea Dust

http://glazy.org/recipes/3311

Currie 11 Tea Dust

http://glazy.org/recipes/3310

Coleman Tea Dust

http://glazy.org/recipes/3312

Tea Dust Black

http://glazy.org/recipes/3308

Tessha Tea Dust

http://glazy.org/recipes/3309

Chinese Traditional Tea Dust Glaze

Although the glazes above call all be considered tea dust, in China teadust glaze usually refers to Qing dynasty wares like the ones below.  Although tea dust can result in a range of colors and with varying crystal sizes, usually the crystals are very evenly dispersed.  (The example below is a Qianlong vase sold through Sotheby’s.)

Qianlong Teadust Vase
Various Teadust Colors

Chinese Traditional Tea Dust Glaze Recipes

The recipe above comes from 颜色釉 (Colored Glazes), published in 1978 by the Jingdezhen Ceramics Company.  The components of the recipe can be difficult to find today, even in Jingdezhen.  Those materials that are still produced probably have different compositions than in 1978.  For instance, I’ve noticed differences in Glaze Stone (釉果) and Zijin Stone (紫金石) from year to year.  What this recipe has in common with the Western tea dust glazes above is the addition of magnesium oxide in the form of talc, which helps form the tea dust crystals.

The glaze preparation is quite interesting.  The ingredients (excluding the white clay (白土)) is ball milled for 30 hours, then white clay is added and the full glaze is milled for another four hours.  After adding water, the glaze is sprayed approximately 1mm thick.  Firing is to 1250°C in a weak reduction atmosphere.

Tea Dust Triaxial – First Attempt

In part because I’ve had a difficult time finding a reliable chemical analysis for the glaze stone I use, but mostly because I often don’t have much clue what I’m doing, I often use triaxial blends to find interesting glazes.  I’m also interested in making glazes for my normal firing schedule, rather than changing my firing for a single type of glaze (like the recipe above that fires to 1240°C).

Because triaxials are limited to three variables, the following tests make some assumptions about tea dust glazes.  The top axis of the triaxial will be glaze stone (the “unknown” ingredient), the left axis whiting (the flux), and the right axis silica.  I’m certain I need magnesium in the form of talc, dolomite, or magnesium carbonate- I choose 8% talc.  I also need some iron, based on past experience and other glaze recipes I choose red iron oxide at 8%.

From the resulting triaxial it seems that crystal growth improves as silica is increased while glaze stone and whiting are decreased.

Traditional teadust glaze. Partial Triaxial, 2% Increments

Tea Dust Triaxial – Second Attempt

For each row of the first triaxial, I like what’s happening in the third column.  For the next test, I extend this third column down a few rows to see what happens.

Extended Triaxial. Left- Porcelain, Right- Stoneware

It’s nice to see my educated guesses are working out.  Beginning from the fourth row of this second test, it seems like I’m coming close to a glaze that I like.

You’ll notice that the glazes look completely different on the two types of test tiles (porcelain on the left and stoneware on the right).

Just to check that my initial amounts for Iron and Talc are in the right ballpark, I choose the fourth row of this test and make some adjustments.  It seems that Red Iron Oxide at 8% additional and Talc at 10% are ideal.

After a few more tests further adjusting the silica, iron, and talc I ended up with the below glaze.  While it’s not an ideal tea dust glaze by Qianlong standards, what’s important is that I like it and I can fire it using my usual temperature and schedule.

Photomerged microphoto of the same test tile.
The teadust glaze on a small bowl

August 2015 Update

Just to see what would happen if I continued adding silica to the recipe, I extended the bottom-right corner of the original triaxials.  As you can see below, incrementally replacing glaze stone with silica leads to an increasingly uniform crystal distribution more closely approximating Chinese teadust glazes.  (A feldspar/whiting/silica triaxial for some of the High-Fire Glazes recipes above may have similar results.)  Note that in the following tests I increased Red Iron Oxide to 9% (additional).

Second triaxial extension for teadust test glaze
Complete tested triaxial for teadust glaze.
Microphoto of the extended triaxial’s bottom-right test

Adjusting Coleman Tea Dust Black

As I did with the traditional tea dust glaze above, I wanted to adjust one of the tea dust recipes in John Britt’s High Fire Glazes to increase coverage of the crystals.  The most promising candidate I had found was Coleman’s Tea Dust Black.  Starting with that recipe, I designed a triaxial that incrementally increased silica while reducing feldspar (also lowering the alumina).

The original recipe for Coleman Tea Dust Black:

http://glazy.org/recipes/3308

Custer Feldspar 39.81
Silica 25
Whiting 15.74
Kentucky OM #4  12.04
Talc, Magnesium Silicate 7.41
Red iron oxide, RIO 9.3
Total: 109.3

Since I can only change three variables in the triaxial, I choose to modify Custer Feldspar, Silica, and Whiting.  I keep the Ball Clay at 12%, raise Talc slightly to 8% (which was a good amount in my traditional tea dust glaze) and set Red Iron Oxide at 9.5%.

Triaxial adjusting Coleman Tea Dust Black. Porcelain, Reduction Cone 10

The most instructive column is the last, with Whiting set at 14%.  As silica is increased and feldspar decreased, crystal coverage becomes more even.

Cooling

The above triaxial was slow-cooled.  Below is a comparison of the same glaze with a crash-cool to 1000°C.

John Sankey has an excellent article about iron glazes and cooling: http://johnsankey.ca/glazeiron.html

I added the recipe for this glaze on Glazy at http://glazy.org/recipes/3557

Potash Feldspar: 28%
Silica: 40%
Whiting: 12%
Ball Clay: 12%
Talc: 8%
Red Iron Oxide: 9.5%

Actually, this glaze is probably too high in silica, and the resulting stony surface not at all like the satiny Chinese teadust glazes.  I will try again, increasing the talc and adjusting iron levels rather than simply adding silica.  Ideally I would also attempt different firing/cooling schedules, but I won’t change my firings just for a single glaze.

I hope this article shows how to refine a glaze using triaxial testing.  With some general knowledge about the type of glaze, educated guesses, experience and a bit of luck, one can design triaxials that reveal interesting new glazes.

Note:  I did not cover glaze limits in this article.  There are various glaze limits that describe “good glazes”.  For instance, on the Glazy page for the triaxial-derived Tea Dust glaze above (http://glazy.org/recipes/3557), you will notice that the glaze sits just outside the Hesselberth & Roy Δ9-10 Glaze Limits.  It is up to you to ensure your glazes are functional, especially if they contain oxides that are toxic when leached.  However, I find that a lot of interesting glazes are outside of established glaze limits.

Ash Glazes

All of the following ash glaze recipes and more can now be found on my new open-source ceramics recipes website, Glazy: https://glazy.org/search?base_type=460&type=1060

I’m not particularly an ash glaze aficionado, and I’m far from an expert.  But it’s surprisingly easy to make an interesting ash glaze, and it’s nice to have some “natural” glazes which give interesting surfaces on functional ware.

The digitalfire website has safety tips for mixing and using ash glazes.

At the bottom of this article is a list of resources for learning more.

Wood stove ash with local stoneware

A great way to make an ash glaze is to mix any type of ash with your stoneware body.  A line blend of ash from 40-60% is a good place to start.

Below are tests of a local Jiangxi stoneware body, Tianbao, mixed with unwashed ash from my wood stove.  Glazes dipped onto bisqued porcelain and dark stoneware tiles, then fired in reduction to Orton Cone 10.

Rice Straw Ash with Local Stoneware

Below are tests of a local Jiangxi stoneware body, Tianbao, with Rice Straw Ash.

Glaze Ash with Local Stoneware

Jingdezhen glaze ash before sieving

Below are tests of a local Jiangxi stoneware body, Tianbao, with a traditional Jingdezhen glaze ash called Er Hui.

High Fire Glazes Ash Recipes

Some ash glazes from The Complete Guide to High Fire Glazes  All tests fired to Orton Cone 10 in reduction.

Basic Ash

Titus-Zella Wood Ash

This recipe, listed in High-Fire Glazes, is simply 50% wood ash and 50% custer feldspar.

Basic Aerni Ash

Zellar Ash

Other Ash Glaze Recipes

Leach Ash

As posted by Tom Turner (link below).  He says: “I do not wash ash as I believe much of the character is in what is washed away. Dry sieve through a 30 mesh sieve.”  I used a 60 mesh sieve.

Leach’s Basic Ash Glaze
Wood Ash 40
Feldspar 40
Ball Clay 20

Libby Pickard Ash Glaze

From Phil Rogers Ash Glazes, p. 85:
https://books.google.com/books?id=th3JZzIFFYQC

Also listed on Glazy:  http://glazy.org/recipes/4407

Ash glaze tile. Natural light, 18MP EOS Rebel T2i photo

Tenmoku with Rice Straw Ash

This glaze is from a University of Texas online glaze database (approx 2004) at: http://general.utpb.edu/fac/stanley_c/
http://general.utpb.edu/fac/stanley_c/formulae/glaze/tenmokus.htm

More information on Glazy:  http://glazy.org/recipes/3500

Synthetic (Fake) Ash

You must burn a lot of wood or plant matter to make a small amount of useable ash.  The sieved, unwashed ash collected after a Winter burning my small wood furnace only gives me enough material to make a couple buckets of 50% clay 50% ash glaze.  Furthermore, I burn whatever type of wood happens my way, and it often contains impurities like dirt and nails.  So besides the limited quantity I have available, there is also a question of consistency.

Substituting part or all of real ash with synthetic ash in a glaze recipe is one method to address the problems of impurities, variability, and supply.

For most of these tests I just made a simple 50/50 mix of the synthetic ash recipe with a local stoneware body in order to compare to my previous real wood ash glazes.  The following tests were all fired to Orton Cone 11 in reduction.

Robert Tichane’s Recipe

Robert Tichane’s excellent book Ash Glazes has a chapter devoted to synthetic ash glazes.  Based on Dr. Emil Wolff’s analysis of Beech wood ash, Tichane creates the following synthetic ash:

  • Limestone: 75 g (43.2%)
  • Dolomite: 50 g (30.8%)
  • Potassium Carbonate: 25 g (15.4%)
  • Bone Ash: 6 g (3.7%)
  • Sodium Carbonate: 5 g (3.1%)
  • Calcium Sulfate: 3 g (1.9%)
  • Silica: 2 g (1.2%)
  • Sodium Chloride: 0.2 g (0.1%)
  • Ferric Oxide: 1 g (0.6%)

Robert Tichane’s Synthetic Ash 100%

As you can see in the third picture, the soluble salts tend to permeate bisque ware, possibly decreasing body melting temperature and increasing warping.  (It would be interesting to compare the effects of solubles on a raw-glazed tile.)  Solubles are troublesome for other reasons, of course.  Water should not be removed from an already mixed glaze batch, and safety is a concern.

Robert Tichane’s Synthetic Ash 50%, Stoneware 50%

Tichane’s recipe is interesting because of the soluble components that emulate unwashed wood ash.  And of all the synthetic ash recipes I tested, Tichane’s comes closest to the feel of real unwashed wood ash glazes.

Maritaro Onishi’s Recipe

Ash Glazes also contains a recipe for synthetic ash by Maritaro Onishi:

  • Limestone: 62%
  • Feldspar: 12%
  • Bone Ash: 7%
  • Magnesite (Magnesium Carbonate): 5%
  • Kaolin: 10%
  • Silica: 3%

Maritaro Onishi’s Synthetic Ash 50%, Stoneware 50%

Tichane notes that impurities in the feldspar and kaolin may add iron and manganese.  However, in my test I added an additional 2% red iron oxide and 1.2% manganese dioxide.

Etsuzo Katou’s Recipe

John Neely on the Clayart mailing list mentions Etsuzo Katou who published another synthetic ash recipe, Ueda synthetic ash #3:

  • Whiting: 59%
  • Potash Feldspar: 22%
  • Magnesium Carbonate: 11%
  • Bone Ash: 5%
  • Red Iron Oxide: 2%
  • Manganese Dioxide: 1%

Etsuzo Katou’s Synthetic Ash 50%, Stoneware 50%

I accidentally used 2% Manganese in my test glaze.

Joseph Grebanier’s Recipe

In chapter 13 of Chinese Stoneware Glazes, Synthetic Wood Ash, Joseph Grebanier compares various wood ash analyses and questions the accuracy of Onishi’s synthetic ash formula.  Using Herbert Sanders’ ash analyses in The World of Japanese Ceramics,  Grebanier creates the following recipes:

Grebanier’s Batch Recipe for Synthetic “Common Ash”:

  • Whiting: 35.73%
  • Buckingham Feldspar: 24.35%
  • Kaolin: 10.39%
  • Flint: 10.11%
  • Magnesium Carbonate: 6.86%
  • Bone Ash: 4.09%
  • Soda Ash: 3.90%
  • Red Iron Oxide: 3%
  • Manganese Dioxide: 1.32%

Grebanier’s “Common Ash” recipe was later simplified by Phil Rogers in Ash Glazes:

  • Whiting: 36%
  • Buckingham Feldspar: 25%
  • Kaolin: 10%
  • Flint: 10%
  • Magnesium Carbonate: 7%
  • Bone Ash: 5%
  • Soda Ash: 4%
  • Red Iron Oxide: 3%
  • Manganese Dioxide: 1%

Grebanier’s Batch Recipe for Synthetic Pine Ash:

  • Whiting: 44.37%
  • Orthoclase: 36.17%
  • Magnesium Carbonate: 6.36%
  • Soda Ash: 4.38%
  • Bone Ash: 4.1%
  • Red Iron Oxide: 2.31%
  • Manganese Dioxide: 2.28%

Grebanier’s Pine Ash recipe was later simplified by Robert Tichane in his Ash Glazes:

  • Whiting: 44.4%
  • Feldspar: 36.2%
  • Magnesium Carbonate: 6.4%
  • Soda Ash: 4.4%
  • Bone Ash: 4.1%
  • Red Iron Oxide: 2.3%
  • Manganese Dioxide: 2.3%

Grebanier’s Synthetic Common Ash Recipe (simplified)

In the following tests I substituted Buckingham Feldspar for my local Potash Feldspar.

Grebanier’s Synthetic Pine Ash Recipe (simplified)

Synthetic Ashes in Low-iron Glazes

Many white, clear, and celadon glazes in antiquity were at least in part comprised of plant or tree ash.  By varying the amount of synthetic ash and coloring oxides (mostly iron and manganese) you can quite easily produce some nice glazes.

One of the most important lessons to learn from using synthetic ash is not necessarily to reproduce the look/effect of natural ashes, but rather to understand the result of introducing a wider variety of oxides into the glaze mix.  For instance, take a common celadon recipe, Hamada 5-3-2 (50% custer feldspar, 30% silica, 20% whiting) which is primarily fluxed with lime.  What happens when we introduce extra sodium, potassium, magnesium, phosphorus, iron and manganese to the mix?

Grebanier Synthetic Common Ash and Porcelain Body

One of my favorite ways of making new glazes is to simply mix a flux or ash with a clay body.  This method is usually used with iron-rich stonewares but works just as well for cleaner stoneware and porcelain bodies.

Grebanier Synthetic Common Ash, reduced Iron and Manganese

For the following three tests, the amount of Iron and Manganese in the synthetic ash was reduced to 1/3 of the original amount.

The first test results in a lovely semi-matte glaze, while the last two tests are passable celadons.

Grebanier Synthetic Common Ash and Feldspar

This is basically the Titus-Zella Wood Ash above.

Adjusting the Feldspar/Synthetic Wood Ash glazes

For the potash feldspar/synthetic wood ash mixes above, I reduced the amount of iron and manganese (1/3 of original recipe) as well as added silica to reduce crazing.  With 10% Silica we’ve eliminated the crazing.

Making Your Own Synthetic Ash

Chemical analyses of ash vary widely, even analyses by different people of the same type of ash.  But chemical analyses are a good starting point for experimentation.  Synthetic ash recipes can be calculated by hand (see Chapter 13 of Nigel Wood’s Chinese Glazes) or by using glaze software.

I was interested in Issu wood ash, commonly used in Japan for making celadon glazes.  The chemical analyses for Issu wood  (see list by Linda Arbuckle below) are:

SiO2 Al2O3 TiO2 Fe2O3 CaO MgO K2O Na2O MnO P2O5
Herbert Sanders, The World of Japanese Ceramics 34.6 4.38 0.49 47.71 5.99 2.51 0.06 0.33 3.93
Cardew 16.19 4.16 0.92 36.68 6.6 1 0.2 0.48 0.92
J.B.E. Patterson (via Leach, A Potter’s Book) 71.96 0.63 0.28 15.95 1.57 0.84 0.42

As you can see, there’s a huge difference between these three analyses, it’s difficult to know if any of them are to be trusted.  So I checked Grebanier’s Chinese Stoneware Glazes and was surprised to find that he had come across the exact same problem (Chapter 13, Synthetic Wood Ash).  Grebanier seems to have abandoned hope of finding an Issu ash substitute, but I went ahead with Sanders’ analysis as it seemed more trustworthy.

(Actually, perhaps it’s not that Sanders’ analysis is more trustworthy as much as the fact that it’s more interesting to me.)

I calculated two recipes, one using only Whiting and one minimizing the amount of Whiting by using Wollastonite (without adding too much Silica) and replacing Magnesium Carbonate with Dolomite (in order to provide MgO as well as CaO).

Recipe 1 (Whiting):

  • Whiting 53.36
  • Custer Feldspar 13.97
  • Silica 13.76
  • Magnesium Carb 9.72
  • Bone Ash 6.63
  • New Zealand Kaolin 2.03
  • Iron 0.31
  • Manganese Dioxide 0.23
  • Total: 100.01

Recipe 2 (Minimize Whiting):

  • Whiting 21.35
  • Wollastonite 29.86
  • Dolomite 23.14
  • Custer Feldspar 15.46
  • Bone Ash 7.34
  • New Zealand Kaolin 2.25
  • Iron 0.35
  • Manganese Dioxide 0.26
  • Total: 100.01

Ash Glazes Resources

Ash Glazes by Robert Tichane.

The Complete Guide to High Fire Glazes by John Britt.  Besides being my favorite introduction book on glazes, there is a section on ash and synthetic ash glazes.

Chinese Stoneware Glazes by Joseph Grebanier.  Joseph uses ash in many of his glazes in order to re-create ancient Chinese glazes.

Ash Glazes by Phil Rogers.  I have not yet read this book, but the reviews are good and the book preview on Google Books looks promising.

Ash Glaze on the Digitalfire.com website.  Also information on wood ashhardwood ashsoftwood ashrice straw ash,  and rice husk ash.

Nigel Wood’s Chinese Ceramics and Science and Civilisation in China: Volume 5, Chemistry and Chemical Technology, Part 12, Ceramic Technology both contain a wealth of information of the role of wood and plant ash in the development of Chinese glazes, including chemical analyses of ashes and comparisons with glaze compositions.

Some ash glaze recipes can be found on Rick’s Bricks and Tom Turner’s website.

Chemical Analyses of Various Ashes

The following information is from a Linda Arbuckle handout, GlazeChem Materials.

Bamboo ash

% 4.8 K2O 0.3 CaO 86.4 SiO2 0.4 Fe2O3 8.1 LOI

Bamboo ash supplies several oxides, especially SiO2.

Analyses:

Cardew:

% 4.8 K2O 0.3 CaO 86.4 SiO2 0.4 Fe2O3 8.1 LOI

(‘Bamboo sugar’, Java) (Bourry, 1st English edition, 1901)

Barley straw ash

% 5 Na2O 22 K2O 3 MgO 8 CaO 57 SiO2 5 P2O5

Barley straw ash supplies several oxides, especially SiO2 and K2O.

Analyses:

Hamer & Hamer:

% 5 Na2O 22 K2O 3 MgO 8 CaO 57 SiO2 5 P2O5

Beech ash

% 8.34 Na2O 24.29 K2O 8.20 MgO 42.00 CaO 3.01 SiO2 6.2 P2O5 0.62 Fe2O3 4.52 MnO 2.10 SO3 0.72 Cl

Beech ash supplies several oxides, especially CaO and K2O.

Analyses:

Rogers:

% 8.34 Na2O 24.29 K2O 8.20 MgO 42.00 CaO 3.01 SiO2 6.2 P2O5 0.62 Fe2O3 4.52 MnO 2.10 SO3 0.72 Cl

Rogers (appendix):

% 4.0 Na2O 16.5 K2O 10.9 MgO 55.5 CaO 5.45 SiO2 5.45 P2O5 1.0 Fe2O3

Birch ash

% 9 Na2O 22.6 K2O 14.3 MgO 29.6 CaO 11.5 SiO2 7.9 P2O5 1.3 Fe2O3 0.4 MnO 3.4 LOI

Birch ash supplies several oxides, especially CaO and K2O.

Analyses:

Conrad:

% 9 Na2O 22.6 K2O 14.3 MgO 29.6 CaO 11.5 SiO2 7.9 P2O5 1.3 Fe2O3 0.4 MnO

Rogers (appendix):

% 7.69 Na2O 12.53 K2O 7.69 MgO 57.5 CaO 3.84 SiO2 7.69 P2O5 1.0 Fe2O3

Hamer & Hamer:

% 9 Na2O 18 K2O 11 MgO 45 CaO 4 Al2O3 8 SiO2 4 P2O5 1 Fe2O3

Cedar ash

% 3.7 Na2O 4.3 K2O 6 MgO 44.2 CaO 0.52 Al2O3 24.28 SiO2 10.6 P2O5 1.01 Fe2O3 0.3 MnO 5.09 LOI

Cedar ash supplies several oxides, especially CaO and SiO2.

Analyses:

Conrad:

% 3.7 Na2O 4.3 K2O 6 MgO 44.2 CaO 0.52 Al2O3 24.28 SiO2 10.6 P2O5 1.01 Fe2O3 0.3 MnO

Common ash

% 2.33 Na2O 3.91 K2O 3.3 MgO 22.42 CaO 8.91 Al2O3 30.99 SiO2 1.91 P2O5 3.04 Fe2O3 1.26 MnO 21.44 LOI

Common ash supplies several oxides, especially SiO2 and CaO.

Analyses:

Sanders:

% 2.33 Na2O 3.91 K2O 3.3 MgO 22.42 CaO 8.91 Al2O3 30.99 SiO2 1.91 P2O5 3.04 Fe2O3 1.26 MnO 21.44 LOI

Desert plant ash

% 28.0 Na2O 5.5 K2O 0.5 MgO 21.1 CaO 1.8 P2O5 34.0 CO2

Desert plant ash supplies several oxides, especially Na2O and CaO.

Analyses:

Tichane (TCB): (adds to 91%)

% 28.0 Na2O 5.5 K2O 0.5 MgO 21.1 CaO 1.8 P2O5 34.0 CO2

Elder ash

% 2 Na2O 17 K2O 16 MgO 38 CaO 14 SiO2 13 P2O5

Elder ash supplies several oxides, especially CaO.

Analyses:

Hamer & Hamer:

% 2 Na2O 17 K2O 16 MgO 38 CaO 14 SiO2 13 P2O5

Fern ash

% 0.56 Na2O 4.81 K2O 7.44 MgO 8.59 CaO 19.32 Al2O3 55.02 SiO2 0.3 TiO2 0.92 P2O5 1.67 Fe2O3 1.36 MnO

Fern ash supplies several oxides, especially SiO2 and Al2O3.

Analyses:

Rogers (appendix):

% 0.56 Na2O 4.81 K2O 7.44 MgO 8.59 CaO 19.32 Al2O3 55.02 SiO2 0.3 TiO2 0.92 P2O5 1.67 Fe2O3 1.36 MnO

Hamer & Hamer: (Bracken and fern)

% 3 Na2O 26 K2O 8 MgO 12 CaO 10 Al2O3 33 SiO2 6 P2O5 1 Fe2O3 1 MnO

Grass ash

0.2 KNaO 0.3 MgO 0.5 CaO 0.2 Al2O3 2.0 SiO2

Grass ash supplies several oxides, especially SiO2 and CaO.

Generic grass ash.

Green: (weed and grass ash)

0.2 KNaO 0.3 MgO 0.5 CaO 0.2 Al2O3 2.0 SiO2

Green: (unwashed lawn clippings, from Leach “A Potter’s Book”)

0.41 KNaO 0.29 MgO 0.30 CaO 0.27 Al2O3 1.09 SiO2 0.12 P2O5 0.03 Fe2O3

Green: (washed lawn clippings, from Leach “A Potter’s Book”)

0.15 KNaO 0.32 MgO 0.53 CaO 0.38 Al2O3 1.52 SiO2 0.15 P2O5 0.05 Fe2O3

Rogers: (Lawn grass)

% 6.20 Na2O 6.19 K2O 5.65 MgO 12.88 CaO 16.60 Al2O3 39.64 SiO2 9.00 P2O5 3.44 Fe2O3

Hamer & Hamer: (Lawn grass)

% 3 Na2O 5 K2O 5 MgO 10 CaO 11 Al2O3 59 SiO2 5 P2O5 2 Fe2O3

Mixed hardwood ash

% 9.12 Na2O 14 K2O 12 MgO 30 CaO 0.1 Al2O3 15.3 SiO2 13.1 P2O5 2.4 Fe2O3 0.1 MnO 3.88 LOI

Hearth ash

% 0.55 Na2O 1.49 K2O 5.44 MgO 35.9 CaO 3.69 Al2O3 14.08 SiO2 2.14 P2O5 0.94 Fe2O3 34.32 h3O 0.14 MnO

Hearth ash supplies several oxides, especially CaO and SiO2.

Analyses:

Cardew:

% 0.55 Na2O 1.49 K2O 5.44 MgO 35.9 CaO 3.69 Al2O3 14.08 SiO2 2.14 P2O5 0.94 Fe2O3 34.32 h3O 0.14 MnO

(Hearth ash (‘Dobai’), Japan (E. Kato, Interceram, 2 (1962), 110).)

Heather ash

% 9 Na2O 12 K2O 10 MgO 21 CaO 41 SiO2 5 P2O5 2 Fe2O3

Heather ash supplies several oxides, especially SiO2 and CaO.

Analyses:

Hamer & Hamer:

% 9 Na2O 12 K2O 10 MgO 21 CaO 41 SiO2 5 P2O5 2 Fe2O3

Issu-wood ash

% 0.06 Na2O 2.51 K2O 5.99 MgO 47.71 CaO 4.38 Al2O3 34.6 SiO2 3.93 P2O5 0.49 Fe2O3 0.33 MnO

Isu-wood ash supplies several oxides, especially CaO and SiO2.  Issu (Distylium racemosum) is the source of a very popular wood ash in Japan. Note the incredible variations in the analyses.

Analyses:

Sanders:

% 0.06 Na2O 2.51 K2O 5.99 MgO 47.71 CaO 4.38 Al2O3 34.6 SiO2 3.93 P2O5 0.49 Fe2O3 0.33 MnO

Cardew:

% 0.2 Na2O 1 K2O 6.6 MgO 36.68 CaO 4.16 Al2O3 16.19 SiO2 3.67 P2O5 0.92 Fe2O3 30.08 h3O 0.8 SO3 0.48 MnO

(Isu Ash from Hagi, Japan (E. Kato, Interceram, 2 (1962), 110).)

Leach (“A Potter’s Book”):

% 0.84 K2O 1.57 MgO 15.95 CaO 0.63 Al2O3 71.96 SiO2 0.42 P2O5 0.28 Fe2O3 8.29 LOI

(from work done by J.B.E. Patterson)

Ivy ash

% 20 Na2O 26 K2O 8 MgO 25 CaO 12 SiO2 6 P2O5 3 Fe2O3

Ivy ash supplies several oxides, especially CaO, K2O, and Na2O.

Analyses:

Hamer & Hamer:

% 20 Na2O 26 K2O 8 MgO 25 CaO 12 SiO2 6 P2O5 3 Fe2O3

Larch wood ash

% 9 Na2O 21 K2O 8 MgO 27 CaO 1 Al2O3 11 SiO2 8 P2O5 4 Fe2O3 11 MnO

Larch wood ash supplies several oxides, especially CaO, K2O, SiO2, and MnO.

Analyses:

Hamer & Hamer:

% 9 Na2O 21 K2O 8 MgO 27 CaO 1 Al2O3 11 SiO2 8 P2O5 4 Fe2O3 11 MnO

Mahogany ash

% 10.98 Na2O 9.49 K2O 4.39 MgO 9.49 CaO 3.81 Al2O3 51.51 SiO2 2.08 P2O5 4.53 Fe2O3 3.72 LOI

Mahogany ash supplies several oxides, especially SiO2.

Analyses:

Cardew:

% 10.98 Na2O 9.49 K2O 4.39 MgO 9.49 CaO 3.81 Al2O3 51.51 SiO2 2.08 P2O5 4.53 Fe2O3

(Australian white mahogany) (Forestry Commission of New South Wales, 1956)

Maple ash

% 6.04 Na2O 6.2 K2O 12.05 MgO 27.5 CaO 0.9 Al2O3 13.8 SiO2 8.1 P2O5 2.5 Fe2O3 0.5 MnO 22.41 LOI

Maple ash supplies several oxides, especially CaO.

Analyses:

Conrad:

% 6.04 Na2O 6.2 K2O 12.05 MgO 27.5 CaO 0.9 Al2O3 13.8 SiO2 8.1 P2O5 2.5 Fe2O3 0.5 MnO

Meadow hay ash

% 7.0 Na2O 25.67 K2O 5.0 MgO 11.56 CaO 29.57 SiO2 6.2 P2O5 1.0 Fe2O3

Meadow hay ash supplies several oxides, especially SiO2 and K2O.

Analyses:

Rogers (appendix):

% 7.0 Na2O 25.67 K2O 5.0 MgO 11.56 CaO 29.57 SiO2 6.2 P2O5 1.0 Fe2O3

Hamer & Hamer: (Meadow grass)

% 4 Na2O 15 K2O 5 MgO 10 CaO 3 Al2O3 58 SiO2 4 P2O5 1 Fe2O3

Mixed wood ash

0.125 Na2O 0.266 K2O 0.187 MgO 0.422 CaO 0.016 Al2O3 0.375 SiO2 0.109 P2O5 0.062 Fe2O3

Oak ash

% 9.12 Na2O 14 K2O 12 MgO 30 CaO 0.1 Al2O3 15.3 SiO2 13.1 P2O5 2.4 Fe2O3 0.1 MnO 3.88 LOI

Oak ash supplies several oxides, especially CaO.

Analyses:

Conrad:

% 9.12 Na2O 14 K2O 12 MgO 30 CaO 0.1 Al2O3 15.3 SiO2 13.1 P2O5 2.4 Fe2O3 0.1 MnO

Tichane (TCB):

% 3.9 Na2O 9.5 K2O 3.9 MgO 72.5 CaO 2.0 SiO2 5.8 P2O5

Rogers: (English oak)

% 9.12 Na2O 14.00 K2O 12.01 MgO 30.02 CaO 0.13 Al2O3 15.30 SiO2 13.8 P2O5 2.40 Fe2O3 0.10 MnO 0.05 CuO 2.61 SO3 1.18 Cl

Rogers (appendix): (China)

% 1.47 Na2O 5.77 K2O 4.09 MgO 23.54 CaO 15.11 Al2O3 39.81 SiO2 2.3 P2O5 3.58 Fe2O3 4.32 MnO

Rogers (appendix): (Japan)

% 1.52 Na2O 5.68 K2O 4.14 MgO 23.69 CaO 16.34 Al2O3 39.62 SiO2 2.62 P2O5 3.83 Fe2O3 1.01 MnO

Hamer & Hamer:

% 6 Na2O 11 K2O 9 MgO 51 CaO 1 Al2O3 10 SiO2 10 P2O5 1 Fe2O3 1 MnO

Straw ash

0.4 KNaO 0.2 MgO 0.4 CaO 2.7 SiO2 0.1 P2O5

Straw ash supplies several oxides, especially SiO2.  Generic straw ash.

Analyses:

Green: (straw ash from cereal crops)

0.4 KNaO 0.2 MgO 0.4 CaO 2.7 SiO2 0.1 P2O5

Tallowwood ash

% 6.84 Na2O 2.41 K2O 25.58 MgO 52.15 CaO 1.09 Al2O3 8.96 SiO2 0.4 P2O5 1.93 SO3 0.38 Cl2 0.18 MnO

Tallowwood ash supplies several oxides, especially CaO and MgO.

Analyses:

Cardew:

% 6.84 Na2O 2.41 K2O 25.58 MgO 52.15 CaO 1.09 Al2O3 8.96 SiO2 0.4 P2O5 1.93 SO3 0.38 Cl2 0.18 MnO

(Eucalyptus microscorys III (Tallowwood) Forestry Commission of New South Wales, 1956)

Thatching grass ash

% 0.22 Na2O 2.55 K2O 3.67 MgO 6.14 CaO 5.42 Al2O3 76.96 SiO2 1.58 P2O5 0.22 TiO2 1.06 Fe2O3 0.15 SO3 0.67 MnO 1.4 LOI

Thatching grass ash supplies several oxides, especially SiO2.

Analyses:

Cardew:

% 0.22 Na2O 2.55 K2O 3.67 MgO 6.14 CaO 5.42 Al2O3 76.96 SiO2 1.58 P2O5 0.22 TiO2 1.06 Fe2O3 0.15 SO3 0.67 MnO 1.4 LOI

(Abuja, Nigeria. Wahsed and calcined at 900C) (Overseas Geological Surveys, London, 1966)

Turpentine ash

% 4.08 Na2O 1.2 K2O 1.03 MgO 1.88 CaO 1.26 Al2O3 89.74 SiO2 0.39 P2O5 0.42 LOI

Turpentine ash supplies several oxides, especially SiO2.

Analyses:

Cardew:

% 4.08 Na2O 1.2 K2O 1.03 MgO 1.88 CaO 1.26 Al2O3 89.74 SiO2 0.39 P2O5

(Australian turpentine) (Forestry Commission of New South Wales, 1956)

Wheat straw ash

% 2.8 Na2O 11.5 K2O 2.5 MgO 6.1 CaO 66.2 SiO2 5.4 P2O5 2.8 SO3 3.8 S

Wheat straw ash supplies several oxides, especially SiO2 and K2O.

Analyses:

Cardew:

% 2.8 Na2O 11.5 K2O 2.5 MgO 6.1 CaO 66.2 SiO2 5.4 P2O5 2.8 SO3 3.8 S

(Air dried) (B.C.S., (1959), 59P)

Green:

0.67 K2O 0.14 MgO 0.19 CaO 0.71 Al2O3 4.95 SiO2 0.10 P2O5

Tichane (TCB):

% 2.8 Na2O 11.5 K2O 2.5 MgO 6.1 CaO 66.0 SiO2 5.4 P2O5 2.8 LOI

Rogers:

% 1.4 Na2O 13.6 K2O 2.5 MgO 5.8 CaO 67.5 SiO2 4.8 P2O5 0.6 Fe2O3

Hamer & Hamer:

% 2 Na2O 13 K2O 4 MgO 6 CaO 70 SiO2 5 P2O5

Willow ash

% 2.50 Na2O 49.80 K2O 8.26 MgO 20.21 CaO 0.05 Al2O3 4.44 SiO2 10.00 P2O5 1.25 Fe2O3 0.18 MnO 1.22 SO3 0.08 Cl

Willow ash supplies several oxides, especially K2O, CaO, and P2O5.

Analyses:

Rogers:

% 2.50 Na2O 49.80 K2O 8.26 MgO 20.21 CaO 0.05 Al2O3 4.44 SiO2 10.00 P2O5 1.25 Fe2O3 0.18 MnO

1.22 SO3 0.08 Cl

Hamer & Hamer:

% 3 Na2O 51 K2O 9 MgO 21 CaO 5 SiO2 10 P2O5 1 Fe2O3

Ash wood ash

0.125 Na2O 0.266 K2O 0.187 MgO 0.422 CaO 0.016 Al2O3 0.375 SiO2 0.109 P2O5 0.062 Fe2O3

Ash wood ash supplies several oxides, especially CaO and SiO2.

Analyses:

Hamer & Hamer:

% 8 Na2O 17 K2O 12 MgO 27 CaO 1 Al2O3 24 SiO2 7 P2O5 4 Fe2O3

How I Make Glaze Test Tiles

I’ve tried all manner of methods for making test tiles- thrown, extruded, and slab.  Each type has advantages, but I’ve finally come to the safest, most economical and useful method for my needs.

Assuming you have a fast way to make slabs, using this technique you can easily make a couple hundred test tiles in an afternoon.

Hand-rolled slabs work fine.  The irregular edges can be saved and used to make the supporting triangles.  To attach the supports I use dry clay trimmings and pure vinegar.  The triangles are dipped into the vinegar slip for about two seconds, then wiggled into place on the test tile until the two slabs are firmly joined.  (I have tried this with all of my clays, from porcelains to stonewares, without problems.  However, your clay may differ.)

Just after joining, the test tiles should be covered and dried slowly for the first day or so.  You may notice some cracking along the join line, but the join should be strong enough to last through bisque and glaze firings.

If you have casting slip, the fastest way that I have found to make test tiles is to make a slipcast slab using a large, flat plaster table.  Supports are joined to tiles using the same casting slip.

These test tiles will not fall over (especially important as I often put tests in public kilns).  They can be stacked very closely in the kiln, and the height can be varied by choosing either the long or short end of the triangular support.  Because they are flat, they require less glaze for coverage (handy when using very small batches of material) but also show more area of glaze than an extruded column.  The hole in the base allows the tile to be hung on a glaze bucket or on a display.  I stamp the back of the tile with a code for the type of clay, and there is ample space to write information about the test using an underglaze pencil.

The tiles should be quite thick in order to better absorb and support thicker layers of glaze.

After dipping the face of the tile into a glaze, I dip one side (not the top) again in order to see different thicknesses.  (Double-dipping the top of the tile often results in the thicker glaze running down into the thinner area, sometimes resulting in a completely even coat after firing.)

Using flat, square tiles I can dip twice while only using 20ml of glaze.  This is especially useful when doing triaxials or biaxials using the syringe method.

The back of the tile has a lot of space for writing glaze information.  Before firing use an underglaze pencil (I use Amaco Black Pencils), after firing a permanent marker can be used to record the date and temperature.

Another advantage of these square tiles is that they photograph very well.

For my purposes, the best size for a test tile is 6cm or 7cm square.  This size is large enough to get a feel for what the glaze looks like, while small enough to be covered by only 20ml of mixed glaze (including a second dip).

Conveniently, the supporting triangles are half the width of the tile, which allows for first cutting the slab into 6cm or 7cm columns.

I know it seems really anal, but this cutting pattern saves clay and ensures the tiles all rest at the same angle, allowing for more accurate comparisons and tighter packing in the kiln.

Test tile dimensions

Qinghua Stone

Pictured above is a type of stone (叫珠子) mined in Zhejiang province and sold in Jingdezhen.  It is primarily used for making Chinese underglaze blue & white (qinghua).  These hard nuggets are quite expensive, while the softer stone surrounding these nuggets is about six times cheaper.

In the magnified view you can see that this stone is a composite of many different minerals.  I am not an expert but there are definitely chunks of silica and mica.  From tests, it seems that the darker bits are primarily iron and manganese.  In ancient times I assume that this stone also contained a small amount of cobalt, however from my tests that no longer seems to be the case.  The proprietor of the materials shop confirmed this, although he mentioned that ores containing cobalt are still sometimes unearthed and fetch a premium price.

In Science and Civilisation in China: Volume 5, Chemistry and Chemical Technology, Part 12, Ceramic Technology, Part 6, Pigments, Enamels, and Gilding (pages 680-692), Nigel Wood writes extensively about the composition of Chinese blue & white underglaze pigments, with analyses of cobalt bearing absolites and other ores.  Nigel Wood also covers the subject in Chinese Glazes (pages 61-66).

For me, the basic lessons to be learned from Wood’s research are: a) that underglaze blue pigment composition varied substantially throughout history and between kilns (especially between the official and folk kilns), b) the importance of the ratio between the primary coloring oxides (cobalt, iron, and manganese), and c) the influence of other oxides (especially silica and alumina) present in the underglaze pigment and covering glaze upon the final color.

Above is pictured a quick first test of the qinghua stone.   An underglaze was made by crushing the rock and sieving it through an 80 mesh screen.  From left to right:  100% ore, 90% ore/10% CoO, 80% ore/20%CoO.  As you can see from the 100% test it appears that the ore contains little or no cobalt.  This is obviously a poor first test, as underglaze blue & white is usually ground very finely.

Below is a more refined test.  The qinghua stone was first bisqued to 900 degrees, crushed, and sieved through a 100 mesh screen.  Finally, the stone and cobalt were ground with a mortar and pestle.  In my opinion, the combination of Cobalt, Iron, and Manganese result in a much more interesting range of colors than Cobalt used alone.