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Iron Saturate Glazes
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:
http://carol.knighten.org/
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%.
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Cone 6 Oxidation Blue Triaxial Blend
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
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Orton Cone 6 Clear Glazes
Having not fired cone 6 since college, I started by first testing a number of clear cone 6 glazes on https://glazy.org
I also studied up on cone 6 glaze chemistry via Matthew Katz’s Advancing Glazes course and his papers: Boron in Glazes, Mid-Temperature Glaze Science, Glaze Safety/Durable Glazes Presentation.
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.
Best Clear at B2O3 0.36
The best clear resulting from the B2O3 0.36 biaxial is here: C6 R2O 0.2 B2O3 0.36 Best Clear
It is similar to WCAC Celadon Clear in it’s glossy, transparent quality.
High-Boron Clears
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
Dear Nancy,
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.
From Safety & Durable Glazes Presentation
From Boron in Glazes
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Tichane’s Tests
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 
Glaze sample: Jun with copper red; Small bowl with two copper-red spots on outside, two on inside
Modern reproduction of Yaozhou ware bowl from original mould in Metropolitan Museum of Art
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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.
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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.
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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.
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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.
Paint Opacity Chart from Leneta
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.
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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.
A typical Jingdezhen spray booth
A gap between glass and stainless steel reservoir evenly distributes water down the glass.
The fan at the back of the spray booth blows out particles.
Detail of the fan label
The pump sucks water from a bucket and up through the spray booth.
Detail of the water pump
Inside the booth I place a large plastic basin for collecting glaze. Inside the basin is a turntable.
On top of the turntable I place a plaster disk. The added weight results in more even turning, while the plaster absorbs glaze. A notch in the plaster helps with counting revolutions. After spraying, glaze can be scraped off and collected.
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.
My air compressor is actually just a cheap fish tank pump. It's much quieter than normal air-tank compressors.
Detail of the 520W fish tank magnetic air pump, rated at 0.04MPa (approx 6PSI)
The spray canister is attached via rubber hose. A shut-off valve to controls air flow.
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.
The parts that make up a glaze canister.
Some of my locally made glaze spraying canisters
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.
Using a toilet brush for mixing up the glaze each time I fill the canister
It's difficult to see in this photo, but the center of this dish was trimmed thin, and the sprayed glaze has saturated the ware. The surface of the glaze is no longer powdery. Stop spraying.
I've found no consistent way to check glaze depth other than scraping with a knife.
After spraying the bottom, glaze can be scraped off with a box cutter blade or metal rib. Take care not to scratch the ware.
After spraying the bottom, a board is placed on the foot, then flipped over and placed on a ware board.
A large circular piece of foam is used to flip over glazed ware, protecting the insides.
Ware is placed on a damp, firm foam pad and rotated using even pressure, resulting in a clean glaze line.
Foam after cleaning a bottom. Foam firmness and hand pressure determines glaze line height.
Another method for creating a clean glaze line- using a notched rib.
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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
I use cheap, reusable restaurant soup containers with lids. The size fits my small sieves perfectly, and they are easier to use than plastic cups. Glaze name & recipe is written on container with permanent marker.
Carefully measure out each ingredient into the bowl, placing into separate piles so that any extra material can be easily removed.
Dry mix the ingredients with a spoon until well dispersed.
For 100 grams of material, add about 50ml of water (less if your glaze has little or no clay). I am paranoid and use water from my reverse osmosis filter, as my tap water is hard and sometimes of questionable quality.
Wait a few minutes until the water has thoroughly soaked the materials, then stir. Glaze should be fairly thick, do not add too much water as you will be adding more as you go along.
I use a stiff rib to scrape the glaze through the sieve. Do yourself a favor and get containers that perfectly fit your sieves.
The first pass takes the most work as the clays are broken apart.
Try not to lose any material, especially if preparing for volumetric blending. Use a water sprayer to clean the container, spoon, and sieve after each pass. But don't add too much water.
I do two or three passes through the sieve. After the final pass, the glaze should be creamy without any large grains or lumps.
Now you can slowly mix in a bit of water. I keep the glaze thicker than normal. Flat test tiles require the least amount of glaze.
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.
A test celadon glaze. 100g of material passed through an 80 mesh screen 3 times. Note the iron spots.
Passing the exact same batch of glaze one more time through a 120 mesh screen adequately disperses the glaze materials.
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.
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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
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Qing Dynasty Test Tiles
Spotted in a friend's book. Unfortunately I forgot to record the title.
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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!
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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.
Below are the charts for High-Fire glazes.
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Orton Cone 10 Reduction Glaze Line Blends
Leach 4321 is a simple, reliable glaze that we can use to compare coloring oxides.
All of the following glaze variations can be found on Glazy: http://glazy.org/search?search_words=leach&category=36&cone=high
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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.
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Simple Microscopy for Ceramics
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.
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
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.
Teadust glaze test tile
Photomerged image of a teadust glaze
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An old Porcelain Stone Mine
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.
A sledge hammer is used to break off pieces of the hard porcelain stone
A very hard mortar and pestle is used to further break down the porcelain stone.
The crushed porcelain stone is ball milled for four hours.
Even after milling, the mixture needs to be sieved.
After sitting and decanting excess water, the mixture is dried on a plaster slab.
Because I’m in a hurry, the mixture is further dried on the stove.
Porcelain body tests with increasing proportions of kaolin.
Fired porcelain body tests.
Fired porcelain glaze tests with increasing proportion of flux.
