During the next few days I’ll be releasing a series showing how to create a glaze using Glazy and volumetric blending.Continue reading “Iron Saturate Glazes”
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.
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.
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
Currie 11 Tea Dust
Coleman Tea Dust
Tea Dust Black
Tessha Tea Dust
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.)
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.
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.
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.
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).
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:
Custer Feldspar 39.81
Kentucky OM #4 12.04
Talc, Magnesium Silicate 7.41
Red iron oxide, RIO 9.3
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%.
The most instructive column is the last, with Whiting set at 14%. As silica is increased and feldspar decreased, crystal coverage becomes more even.
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%
Ball Clay: 12%
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.
Jingdezhen Porcelain Stone
There are a number of types of porcelain stone mined throughout Jingdezhen and the surrounding countryside. Some are more suitable for making porcelain clay, while others are traditionally used for glazes. It is difficult to know how similar modern-day porcelain stone is to traditional materials. During the few years I have lived in Jingdezhen, some mines have been closed off to private mining, while others have simply run out of material. Those still operating often mix poor-quality material with good material in order to increase production. And plaster is added in ever increasing amounts in order to make the porcelain stone bricks less likely to break in transit.
Below are three types of porcelain stone fired to 1310 Celsius in a reduction atmosphere. From left to right: San Bao porcelain stone, Yaoli glaze stone #1, Yaoli glaze stone #2.
Most porcelain stone is made from a combination of rocks. The stones below are used to make Yaoli glaze stone. On the left is the more common, dirtier stone, in the middle is the higher quality stone, while the right is the mixed, washed and purified stone before adding plaster. All examples were fired to 1310 Celsius in reduction atmosphere.
San Bao Porcelain Stone (三宝瓷石)
Below are some simple tests of porcelain stone from San Bao Village (三宝瓷石). This stone is often used for making traditional porcelain clay, but it can also be used for glazes. (However, usually “glaze stone”, or 釉果， is used for glazes.)
The only chemical analysis I have found for San Bao Porcelain Stone comes from 陶瓷艺术釉工艺学 published by 江西高校出版社:
SiO2: 7.013, Al2O3: 17.64, Fe2O3: 0.69, CaO: 0.54, MgO: 0.09, K20: 4.02, Na2O: 4.68. LOI 2.01
(The SiO2 amount is most likely a typo, they probably intended to write 70.13.)
However, this analysis can only be used as a basic guideline. There are noticeable differences between the porcelain stone produced by various families in San Bao, and quality seems to change every year.
Traditional glaze recipes usually call for whiting, but unless fired very carefully, porcelain stone has a tendency to carbon-trap when whiting is the flux. Replacing whiting with wollastonite eliminates this problem.
Below are a few glazes containing San Bao porcelain stone, silica, wollastonite and kaolin. It is quite easy to make a very nice celadon with porcelain stone.
Yaoli Glaze Stone (瑶里釉果)
Yaoli Glaze Stone (瑶里釉果) is traditionally used for creating glazes. As with San Bao porcelain stone, there are a number of families mining, cleaning, and creating the glaze stone bricks sold in Jingdezhen.
陶瓷艺术釉工艺学 has the following analysis:
SiO2: 73.99, Al2O3: 15.55, Fe2O3: 0.37, CaO: 1.76, MgO: 0.33, K20: 2.88, Na2O: 2.63. LOI 2.88
But again, the material varies from seller to seller and from year to year.
At the left is a simple melt test of 75% glaze stone with 25% Wollastonite and 80% glaze stone with 20% Wollastonite. The 20% Wollastonite version is already a perfectly useable celadon.
Below are a few simple celadons made with glaze stone, silica, wollastonite, and kaolin.
A test of 80% porcelain stone and 20% kaolin (New Zealand halloysite). This porcelain is particularly white and fairly translucent. It is quite nice to throw but fragile when bone dry. Mixtures of various types of porcelain stone with between 10-40% kaolin produce porcelain bodies suitable for a range of temperatures.