During the next few days I’ll be releasing a series showing how to create a glaze using Glazy and volumetric blending.
The first step is familiarizing oneself with the glaze type. For this demonstration I’m interested in creating a Cone 10 Iron-Saturate Red microcrystalline glaze also known as “Kaki”, “Tomato Red”, and “Persimmon”. Some historically examples are Chinese Song Dynasty Ding Persimmon-glazed wares as well as many of Shoji Hamada’s works.
For each search, Glazy shows Recipe Cards with photos as well as a Stull Chart. The Stull Chart has five major regions: Unfused, Matte, Semi-Matte, Bright/Gloss, and Under-fired. There is another area, Crazed, that overlaps the other regions. In the next step, I will create a Biaxial Test using the Stull chart as a guide.
From the analyses of Iron-Saturate glazes in Glazy, it is not clear what the ideal amount of Silica and Alumina (and the Si:Al ratio) should be. So our first step will be to re-create the Si:Al Stull Chart with a prototype Iron-Saturate glaze. In an Si:Al grid we can only adjust the amounts of Silica and Alumina, so we must set in stone the other characteristics of the glaze. Looking at the analyses of recipes in Glazy, it is apparent that we will need a good deal of Iron as well as Phosphorus. For our fluxes, apparently some MgO is required. As an educated guess, or initial prototype Iron Saturate glaze will have variable Silica and Alumina, while the following are set for all glazes: KNaO 0.2, CaO 0.6, MgO 0.2, Fe2O3 0.22, P2O5 0.12.
We will use volumetric blending to magically create 25 glaze tests from only 4 batches of glaze. The four corner glazes composed using the Glazy Recipe Calculator. The columns originate from the Origin so that each column represents a specific Si:Al ratio. It is decided to put the “Left” column in the Stull Matte Region, while the “Right” column is pushed close to the Under-Fired Region.
Batches of 500 grams are created for each corner glaze, and the test glazes are mixed using volumetric blending with samples of 20mg. The tests are ready to be fired!
Note: The “educated guess” for our initial prototype glaze is informed in large part by the amazing work of Carol Marians. Carol has posted 8 years of glaze research on her website at:
I made a mistake on the Si:Al ratios for each biaxial column. The ratios should be approximately 4.3, 5.4, 7, 9.3, and 12.8
Here are the results of the Iron-Saturate Biaxial. While the reduction firing with uncontrolled cooling results in brown, metallic surfaces, the oxidation firing with a controlled cool and hold at 1700°F (925°C) gives us more interesting results. The biaxial reveals that an Si:Al ratio of around 9-9.5 (the fourth column) promotes redder glazes. In particular, tile D4 (4th row, 4th column) seems promising.
The oxidation firing schedule is adopted from Carol Marians, but simplified to:
150°F/hr to 250°F (65°C/hr to 120°C)
400°F/hr to 2050°F (200°C/hr to 1120°C)
120°F/hr to 2250°F (50°C/hr to 1230°C)
60°F/hr to 2290°F (16°C/hr to 1250°C)
40°F/hr to 2310°F (4°C/hr to 1265°C)
Hold of 10 minutes at 2310°F (1265°C)
400°F to 1700°F (925°C) – Down-fire
Hold 2 hours at 1700°F (925°C)
See http://carol.knighten.org for many more examples of firing schedules for iron red glazes.
From the Iron-Saturate Biaxial we choose tile D4 to work with. We can now “zoom in” and refine the Si:Al ratio for this tile. At Si:Al 8-8.5 the glaze seems more evenly covered in crystals. As the Si:Al ratio is increased the coverage becomes more splotchy.
It seems the Si:Al ratio for biaxial tile D4 was already pretty good. Now we can move on to testing factors other than Si:Al in the prototype glaze. In this test, we increase the level of R2O (KNaO, or K2O & Na2O) while decreasing the amount of Calcium. I was surprised by the result for 0.3 KNaO, perhaps there would have been a better result if both Calcium and Magnesium were decreased? Or decrease Si & Al? Anyway, based on this test I’ll just stay at R2O:RO 0.2:0.8
Test of Iron-Saturate Biaxial tile D4 replacing Mahavir Potash Feldspar with Minspar 200 Soda Feldspar. Not a 1-to-1 percentage replacement, but maintaining the same UMF (except K2O and Na2O).
Now that we’ve established an R2O:RO ratio of 0.2 using Potash Feldspar, we can test the best proportion of Calcia to Magnesia. With our R2O set at 0.2, 0.8 remains for the RO (including Calcia and Magnesia) portion of our UMF fluxes. The educated guess of 0.6 CaO and 0.2 MgO in our original Iron Saturate biaxial turns out to be a good choice. More than 0.2 MgO also gives some interesting glazes with a more metallic surface.
Same UMF, different sources of Magnesia.
Glazes are often split into two parts: A base glaze and additives like colorants and opacifiers. It’s like ordering a pizza (base) with toppings (additives). We’ve already tested many aspects of the Iron-Saturate base glaze including Silica:Alumina, R2O:RO, and Calcia:Magnesia. Now we can move on to the additives: Iron and Bone Ash. (The decision between what is part of the base recipe vs. an additive is somewhat arbitrary. Just as with a pizza that’s made it to your stomach, it all eventually ends up mixed together.) Additives are added in addition to the base glaze recipe. So in this test, I did not alter the base glaze at all, the only difference is increasing Red Iron Oxide and Bone Ash. I was surprised to see that additional bone ash didn’t alter the glaze a lot more.
A line blend of biaxial test D4 blended with the same recipe without Bone Ash (but maintaining the same fluxes to account for the missing CaO from the Bone Ash). Without P2O5, our glaze is a nice tenmoku. At 0.03 P2O5 very faint traces of crystallization appear. At 0.06 P2O5 crystallization is much more evident, and somewhere between 0.06 and 0.09 P2O5 there is a dramatic transformation.
Testing different sources of iron using tile D4 from the Iron-Saturate Biaxial. I’m not sure what’s going on with Yellow Iron Oxide. It would be interesting to see other sources of iron, especially iron phosphate.
Adding Titanium Dioxide in 1% increments to tile D4 from the Iron-Saturate Biaxial. Titanium Dioxide is often present in analyses of Song Dynasty Russet/Persimmon glazes as well as Japanese Kaki glazes, but usually in amounts of less than 1%.
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.
In Chinese Glazes, we learn from Nigel Woods that the cobalt used for underglaze blue & white underglazes and blue glazes came in a range of chemical compositions and grades of purity. Thus, there are many shades of blue due to the quality of cobalt-containing stone as well the overlying glaze.
In the same book, Nigel presents a lovely Chinese blue stoneware glaze which, in addition to cobalt, contains iron and manganese “impurities”.
In fact I’m personally not fond at all of glazes and underglazes containing only cobalt as a coloring oxide. Pure cobalt often comes out as a garishly blue color. In the triaxial blend below, I take a nice clear glaze (Sue’s Clear) with added 1% Cobalt Carbonate. Then I blend with 1.5% Red Iron Oxide (bottom left) and 1.5% Manganese Dioxide (bottom right). The resulting colors on the bottom row are much more pleasing to my eye.
The full image can be viewed here: http://www.derekau.net/wp-content/uploads/2014/12/BLUE_TRIAX_ALL.jpg
Having not fired cone 6 since college, I started by first testing a number of clear cone 6 glazes on https://glazy.org
Out of these tests, there were two glazes that I preferred. The first, Sue McLeod’s Clear, is a soft clear with minimal clouding and has B2O3 at 0.18 which, according to Matt’s information, is ideal for cone 6 glazes.
The second glaze is a shop glaze available at the Wellsville Creative Arts Center called WCAC Celadon Clear. With B2O3 at 0.45, it is really high in boron and possibly less durable than the lower-boron clears I tested. However, WCAC Celadon Clear is by far the clearest glaze I’ve tested, almost like a layer of pure glass or honey. Even on dark stoneware it’s really clear with almost no clouding.
Being new to cone 6, I was curious as to the effect of boron levels on clear glazes. So, I created two biaxials, both with R2O fixed at 0.2. In the first Sue’s Clear inspired biaxial, B2O3 is set at 0.18. In the second biaxial inspired by Celadon Clear, B2O3 is doubled to 0.36.
Each biaxial resulted in a nice clear, with the higher Boron clear being almost completely transparent and glossy, while the Boron 0.18 clear is translucent and soft.
Standard Cone 6 Porcelain Body #551
Same chart but with words describing each test glaze:
Best Clear at B2O3 0.18
The best clear resulting from the B2O3 0.18 biaxial is here: C6 R2O 0.2 B2O3 0.18 Best Clear
B2O3 0.36 Biaxial
In order to test the effect of higher B2O3 levels, I doubled the amount of Boron in the initial biaxial from 0.18 to 0.36 while maintaining the same R2O:RO ratio. I also made the boundaries of the tests a little higher (see map comparison). I was surprised to see that the only clear glazes in the 0.36 Boron test appear much farther down (lower in Si & Al) in the chart. But the “clear” region is still in the same Si:Al Stull region.
After testing WCAC Celadon Clear and seeing the results of my B2O3 0.36 biaxial, it seems there is definitely a region of very glossy, very clear glazes at higher boron levels.
Coincidentally, I tested an old glaze recipe posted to the Clayart mailing list by Laura Speirs in 1996: https://glazy.org/recipes/21102 As with the WCAC Celadon Clear, the Speirs recipe is also very high in Boron (0.51), and it also fires very clear and glossy:
VC Easy Glossy
One afternoon I began discussing the WCAC Celadon Clear with a WCAC member, Nancy Alt. I was very surprised to discover the interesting history of this glaze.
In 2009 Nancy Alt had visited Val Cushing’s home and purchased a vase with a lovely blue-green celadon glaze. Nancy asked Val if he could share the glaze recipe, and he not only shared it but converted it from cone 9 to cone 6 (the temperature Nancy was firing). Val’s email is copied below. It shows the extremely generous nature of this amazing potter and teacher:
From: Val Cushing
Subject: Re: celedon glaze
Date: May 12, 2009 at 12:46:57 PM EDT
To: Nancy Alt
This glaze is one I made for C/9 oxidation electric firing, so that it would appear to be a blue green celadon. I have revised it for you to be the same color and texture only for C/6 ox. electric . I will give you two to try , first VC Pale Emerald, C/6 , glossy , blue/green , celadon looking. as follows………. Kona F/4 feldspar 24, Ferro Frit 3134 24, Dolomite 4, whiting 14, barium carbonate 2, zinc oxide 2, flint 24, and EPK 6. ADD TO THAT , 1/2 % COPPER CARBONATE for blue green. VC/easy glossy, C/6 ox. , electric , celadon looking , green. Cornwall Stone 46, Gerstley Borate 20, Ferro Frit 3124 26, Ball Clay 8. — add 2% copper carb. and 1/2 % red iron oxide for celadon looking green color. Test these two Nancy and if the color is not exactly what you expected let me know and we can make a revision. We may have different “tastes” about color , but we can get what you want…My pale emerald should be quite a bit like the glaze on the jar of mine you now have. and THANK YOU . Val
So it turns out that the glaze I liked so much, WCAC Celadon Clear, was actually a Val Cushing recipe called “Easy Glossy”. I checked Cushing’s Handbook for the recipe and didn’t find it. Nor could I find similar recipes in the Glazy database. So it’s quite possible this is a newly discovered Val Cushing glaze recipe.
However, the WCAC Celadon Clear had been modified from the original “Easy Glossy”, most notably subbing Gerstley Borate for Gillespie Borate. I wanted to see not only the original recipe but also the color variations that Cushing was working with. So I created a triaxial blend.
Below is the triaxial blend using Copper Carbonate and Red Iron Oxide.
Some of Robert Tichane’s glaze tests and reproductions of Chinese Glazes donated to the Freer and Sackler Galleries: https://
archive.asia.si.edu/ collections/edan/ default.cfm?searchTerm=tich ane&btnG.x=0&btnG.y=0&btnG =Search
For the total work required to make a single cup, it must pass through 72 hands, and only then can it become a vessel.
72 Hands is an effort to document all types of ceramics techniques. The video style is very simple- a single take of each technique focusing on the artisan’s hands. Each video is accompanied by an article with a description of the technique and photos. Videos are shot in high-resolution 4K Ultra-HD resolution which gives a clear view of the technique.
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To be updated.
Throwing large pieces on plaster bats reduces cracking issues.
Using chamois leather to attach a plaster bat to the wheel
Large sheets of chamois leather for drying cars can be purchased online very cheaply. Synthetic versions that I have tried do not work.
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.
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.
A step up from the 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.
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.
Well, that didn’t work out. The kiln master ended up over-firing, past Chinese cone 10. Orton cone 12 probably dropped around Chinese cone 8/9. Looking forward to doing a better test in my own 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.
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.
This page is in progress and will cover my kiln and firing. For now it is just a place to store my notes.