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%.
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
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.
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.
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 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.
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.
- 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.
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.
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.
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!
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.
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.
- 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.
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.