Plaster Calculator

Calculate the amount of water and plaster you will need for various solid forms.

https://plaster.glazy.org/

Bookmark this link on your phone’s browser for easy access!

The code is open source and available on Github:

https://github.com/derekphilipau/vue-plaster-calculator

Inspired by Keith Simpson’s (@earlyamericanrobotpottery) plaster handout and the USG plaster calculator.

The calculations are from USG (https://www.usg.com/), Keith Simpson (Alfred University), and Andrew Martin (“The Essential Guide to Mold Making & Slip Casting”). 

Continue reading “Plaster Calculator”

Mixing plaster in a plastic bag

I’m not a plaster master, so I don’t know if this is a good or even original idea.  By mixing plaster in a plastic bag, it seems easier to remove bubbles from the mixture, while pouring is much more controlled.  It’s the same idea as using a garden watering can for pouring glaze:  You pour from the bottom where pressure is highest and bubbles are fewest.

Blades

I use X-acto blades all the time, some modified for specific tasks like carving porcelain or scraping glaze off of feet.

I’m not sure if it’s all part of a vast X-acto conspiracy, but it seems that a lot of people don’t know that these blades can be easily & quickly sharpened?  While there are a number of sharpening methods (even just using bare fired porcelain) that will work, it can be tedious to get the sharpening angle right.  The most convenient method I have found is an angled sharpener (pictured).  Just a few quick passes through the ceramic sharpener gets the blades useable again.  It’s faster for me to sharpen the blade than switch out a dull blade for a new one.  (Unfortunately my sharpener is approximately 45% degree sharpening angle (> 20 degrees per side), it might be better to have a narrower-angled sharpener.)

Also note that not all X-acto blades are stainless steel.  You don’t want rusty blades all over your studio or in your reclaim.  A 10 or 100-pack of stainless steel #11 blades might last you a lifetime.

An alternative to X-acto blades are stainless steel surgical blades. They come in a wide variety of shapes and sixes perfect for a number of jobs. I usually use the blades without a handle. They come in ten-packs and last a really long time if you sharpen them.

Pouring Glaze with a Watering Can

I’m sure that using a garden watering can for pouring glazes is a common technique, but when I came up with the idea I thought I was a genius 🙂 The design of a watering can ensures a constant, strong stream of liquid during pouring that is perfect for glazing. Bubbles are reduced since the watering can pours liquid from the bottom of the can.

Pouring the Outside

Once the inside is glazed, I will wait until the next day to glaze the outsides.  It’s important not to overload the bisque ware with water.

I use an old electric wheel for pouring the outsides.  It’s important to rotate the wheel at sufficient speed so that glaze does not gather on the inside rim of the pot.

Here’s a good video by John Britt about pouring the outsides on a turntable.

Low-fire Electric Kiln

I’ve finally gotten a new low-fire electric kiln. This kiln is designed to fire up to 1000°C, so it’s useful only for on-glaze enamels and bisque. Total cost was 2900RMB, which is about $420USD.

Slow drying

I have a couple “wet boxes”.  These are plastic bins with lids into which a layer of plaster has been poured.  The plaster is kept wet in order to maintain humidty, slowing (if not stopping) the drying process.

However, I haven’t used the wet boxes in a long time.  I’ve found it much easier and more convenient to simply wrap each piece in it’s own plastic trash bag.  Pieces stored in this manner can be trimmed weeks or even months later.

Ware placed on MDF bats and wrapped in plastic trash bags.
Ware is placed mouth-down on an MDF board to prevent warping. The boards mold easily, alternatively only wrap the ware.
After two months the ware is still wet, although the moisture is now distributed more uniformly.
Bags of clay are double-bagged in large heavy-duty plastic bags. A piece of soaked plaster is placed in the bottom of the bag.

Kiln & Firing

This page is in progress and will cover my kiln and firing.  For now it is just a place to store my notes.

The older bottles can be very dangerous. Sometimes the top pressure-release valves leak, even after closing.
Electric water heater and gas filter.

Razor trimming

I’m not sure if double-edged safety razor are still available in the West, but here in Jingdezhen they are an essential trimming tool.  These razors are thin, sharp, and most importantly flexible.  Great for wheel-trimming details on small forms, or for scraping hand-built objects.  The most used brand is Flying Eagle.  I get the more expensive stainless steel ones.  At 5RMB for a 5-pack, each blade is about 15¢ USD.

Two brands of safety razors available in China

Using a Dremel or similar tool, edges of both thin and thick razors can be ground for specific uses, like scraping glaze off these plate feet.

A Dremel tool was used to create a notch in this blade for use in scraping glaze off this unique foot.

Scanning Test Tiles

Having purchased a scanner for digitizing my family’s old photos, I had the brilliant idea to also scan glaze test tiles.  I thought I was a genius until Matthew Katz mentioned that he had been scanning tiles for the past ten years.

Matthew noted that CCD scanners have a greater depth of field, which is great for three-dimensional objects like test tiles.  Because of his recommendation I purchased the Canon 9000F Mark II.

I’m not a scanner expert and have never calibrated a scanner before.  I already have an X-Rite ColorChecker Classic for photography, and this color card can be used with X-Rite’s i1Profiler (i1Publish) software to create a scanner profile.  Unfortunately, the software license seems to be very expensive.

I tried Argyll CMS (http://www.argyllcms.com/) but results using the generated ICC profile were worse than the default output.

Here’s a scan of some test tiles.  I had to adjust the Exposure in Photoshop by about +1 stop.  Notice the reflections on some test tiles that were not flat.

Enable large image scans on Canon 9000F Mark II

The Canon software is really frustrating- by default it wouldn’t let me scan a file greater than a set limit (10208 x 14032 pixels, or larger than 100MB).  I finally found a solution hidden away in the software settings.

Comparison with DSLR

I have a relatively old and cheap Canon EOS Rebel T2i with a 18MP sensor.  In comparison with the Canon 9000F scans, the photos from my camera are smaller.  However, they seem to contain just as much if not more detail and better colors.  If needed I can adjust lighting conditions and camera settings to reduce reflections and adjust exposure.  On the other hand, the scan had some reflections that I could not eliminate.

It also takes less time for me to take photos than scan at 2400dpi.

Below are comparisons of the scan and the photos.  In particular, the dark glazes came out very poorly on the scanner.

Conclusion

In conclusion, while the Canon 9000F is great for scanning old photos and documents, I still haven’t found a way to scan glaze tiles that beats results from my old DSLR.

Smartphone Microscopy

Last year I purchased a USB microscope (see article).  It’s pretty fun, but ultimately I was really disappointed by the quality of the images.  The 5MP sensor seems pretty cheap and images have a lot of artifacts.  Furthermore I was never satisfied with the color.

The best choice would probably be a “real” microscope with a camera adapter.  However, this little hobby of mine doesn’t justify spending a lot of cash.

There are a few tutorials online for creating your own phone microscope using the lens of a laser pointer.  (I tried this and it worked pretty well, but I never found a way to conveniently attach the lens to the camera.)  Wired’s article Turn Your Cellphone Into a High-Powered Scientific Microscope has a good tutorial as well as background on the scientists who are using cellphones as biomedical devices.

Fortunately there are now multiple products for sale that make it easier to attach a lens to the phone.

There’s a former Kickstarter project that looks promising and is shipping, the 15x Micro Phone Lens and 150x Micro Phone Lens. (See my November 2016 update, below.)  In China there are a number of cheap alternatives.

Supereyes Smartphone Microscope (Not recommended)

I purchased the Supereyes Smartphone Microscope for about $7USD.  The images below were taken with this lens.  Unfortunately the top of the plastic lens is not protected and I scratched it after playing with it for less than an hour.

The images from the iPhone with attached lens look much better than my USB microscope.  The photos below were taken in natural light.

The lens attached to the iPhone. On top is a plastic cover which is easily scratched. I removed it with a Dremel.

Eyeskey 12x Micro Lens

It was only after using the Supereyes lens that I realized it’s horrible design (does not fit on camera lens, slides off the camera lens, plastic cover easily scratched, etc.) that I bought another cheap microscope lens, the Eyeskey 12x Micro Lens (also about $7USD).

The Eyeskey model is designed much better.  An adapter slides perfectly over the phone and the lens is screwed into the adapter.  The plastic tube that is visible using the Supereyes lens is not a problem here.  The magnification is also greater than the Supereyes lens.

(Unfortunately I could not find this model for sale in the West.  However, I think the 15x Micro Phone Lens might be even better.)

The Eyeskey lens with phone adapter. Much better design than the Supereyes.

Comparison of Supereyes and Eyeskey Lenses

Microphonelens 8x Macro & 15x Micro Lens

On a recent trip to the US I ordered the Microphonelens 8x Macro lens and 15x Micro lens.  These lenses are different than others- they simply stick to the phone’s camera lens and have no outer support column.

In use, I found it more difficult than the other lenses because I could not directly rest the lens at the correct distance against the viewed object, resulting in more blurry photos due to camera shake.  Also, while the micro lens is designed to stick on the phone lens it falls off if touched and gets dirty in the process of handling.

These drawbacks are forgivable, though, as the quality of the images seems superior to either of the other lenses I tested.  Also, the lens cleans up easily with just a bit of pure or soapy water.  The soft material also scratches less easily than hard plastic lenses.

For viewing glazes, both the 8x and 15x lenses are useful.  I would recommend the 15x (although of course it has a narrower depth of field).

Stitching photos

I tried the iPhone’s panorama feature but it did not work.  The iPhone panorama gets confused.  However, you can take multiple photos (moving the camera slightly each shot) and then stitch the photos together using software.  Adobe Lightroom (Photo Merge->Panorama) and Photoshop (Automate->Photomerge) have this feature , however it takes a lot of time and seems limited to 100 or so photos.  I tried a free program called AutoStitch that worked pretty well.

AutoStitch image stitch of Iron Red (Kaki) glaze. Autostitch options: Multiband->Blending Bands = 2 (default). You can see a lot of ghosting where images weren’t aligned properly. Photos from Supereyes lens.
AutoStitch image stitch of Iron Red (Kaki) glaze. Autostitch options: Multiband->Blending Bands = 10 (needed over an hour to process 78 photos). Photos from Eyeskey lens.
AutoStitch image stitch of crazy Tenmoku iron glaze by Kimura Moriyasu. Photos from Eyeskey lens.
Photoshop photo merge of Song dynasty saggar with natural wood ashes. Photos from Eyeskey lens. Photoshop seems to do a better job than AutoStitch (in default mode).

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

Yaoli Village and Raonan Outdoor Ceramics Museum

Yaoli Ancient Village (瑶里古镇) is a fairly well-known tourist destination located about 1 1/2 hours by car from Jingdezhen.

During the past few years I have visited the village a handful of times, and each time I’m even more disappointed by the continuous development, poor management, and flocks of tourists.

But the countryside around Yaoli is beautiful.  If you continue driving past the ancient village you will find numerous small villages with restaurants offering local cuisine.  Drive up the mountain and you should come across wonderful views of the valleys as well as waterfalls.

One of the waterfalls to be found in the mountains

Yaoli is also the home of a type of porcelain stone known as “glaze stone”.  This stone is a major component of traditional Jingdezhen glazes.

There is a very nice outdoor museum in Yaoli called Raonan (绕南陶瓷主题园区) which runs along a small river.  The river powers hammer mills that continuously crush Yaoli porcelain stone.  There are also ancient dragon kilns and even pottery wheels where you can try throwing Jingdezhen porcelain.

The river running through the Raonan ceramics museum
A water wheel powers large hammer mills used for crushing porcelain stone
A water wheel powers large hammer mills used for crushing porcelain stone
The crushed stone powder is washed, mixed, and dried in large pits.
The porcelain paste is formed into bricks and air-dried.”
Large piles of waste saggars and shards surround the kiln sites.

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