The body is Plainsman H570 (0.5-1% porosity). Notice the EBCT test bars in front (engobe compatibility). These sandwich the body and the engobe together in a thin strip, differences in fired shrinkage curl the bar during firing (toward the higher shrinker). The straighter the bars fire the better the fit. My regular engobe for use on our buff stoneware, L3954N (Plainsman H550, 2-3% porosity), has lower fired shrinkage than this (since that body is less vitreous). This one increases the shrinkage (with 5% more nepheline syenite, 5% less silica and 3% less ball clay). This employs 10% Mason 6600 stain to produce the jet-black fired product.

Many cone 6 black clay bodies are available commercially. Some manufacturers are using manganese dioxide but the fume hazards seem far too serious to risk. Is it practical to slurry up your own from a recipe? Yes. Probably the most common method is the addition of raw umber or burnt umber. Umber behaves as a flux so when added to middle temperature bodies the resulting clay becomes too vitreous (so warping, bloating and glaze blisters are common). The secret is too add the umber to a cone 10 stoneware and the umber fluxes it to a cone 6 stoneware. Another method is the use of a black stain. Stains carry no danger of bloating and they work really well. However, the elephant in the room here is cost (that’s also why cobalt is not even on the table). But there is a way to make black stain affordable: Using a jet-black engobe.

All these bars were fired to cone 6 (although they are numbered 5).
Top bar: L4768D, a white cone 10R porcelain with 10% umber (the result is too vitreous and colour has gone brown).
L4768E is a cone 10 red burning stoneware stained with 5% umber. The red fireclay in its recipe is the secret of it being sufficiently refractory to host the umber.
L4484D is MNP plus 6% Mason 6666 black stain. Normally more stain would be needed if it were a white burning porcelain.
P6729 and P7159 are both Plainsman Coffee clay, it is made using 10% burnt umber.

"It Starts With a Lump of Clay", a step-by-step Insight-Live.com tutorial (from its help system and the link below) on how to document every step (in an account at insight-live.com) of testing a raw clay. You will learn about drying shrinkage, drying performance, particle size distribution, plasticity, firing shrinkage, fired porosity, fired color, soluble salt content, fired strength, etc. We will not just observe these properties, but measure them. In doing so we will characterize the material. We will answer simple questions about how the material forms, dries and fires across a range of temperatures. In doing the testing I will be generating a lot of data. No single factor is more intimidating to new technicians than what to do with this data, how and where to store it, how it can be searched, learned from, compared.

Giant thin meter-square tiles are fired flat and crack-free by tile companies. How? Kilns that heat evenly from above and below (the tiles are on rollers). But these round tiles are being fired in an electric kiln, a device incapable of heating a large slab evenly. They are so large they reach almost to the outer walls. That means the outer edges receive direct radiant heat from the elements, this sets up a temperature gradient running from the edges toward the center. Passing such a piece up and down through quartz inversion thus creates a wave of sudden expansion and contraction moving through the piece. The artist was losing every one of these to dunting. It is not really advisable to even try this - but he was determined to do it anyway. One change in the process brought this one through: Slowing down to 50F/hr up and down through the quartz inversion (950-1150F).

Suddenly ware is coming out of your production kiln warped or cracked or off color. Unless the answer is obvious the first action should be to compare its drying and firing test data with past runs. If you are doing that as a routine then SHAB test bars (and the test result data they bring) will already be available for any finished ware that comes out of the kiln. That data is a characterization of your clay body. It is not possible to put a value on this kind of data gathering until a disaster happens (or better yet is prevented). Clay bodies have plasticity, dry performance, dry strength, fired density, fired shrinkage, fired strength, etc. If you have historical data (accompanied by firing schedules, recipes, etc) you have an invaluable tool. Where does one gather the data? In spreadsheets? No, in a database. And account at Insight-live.com is specifically intended for this.

These are made from L4005D red cone 6 stoneware. Both are cast and thin-walled (half of what a thrown piece would be). They were glazed only on the inside to encourage cracking/splitting if the glaze is under excessive compression (that is, the thermal expansion of the glaze is significantly less than that of the body). And that is what happened here. The piece on the left cracked after a couple of taps with a hammer. Notice how the crack has opened. The piece is "spring-loaded" (press it together and it reopens on release). The glaze is GA6-B. The piece on the right is glazed with G1214Z1. It spontaneously blew in half, with a loud crack, a few 5 hours after exit from the kiln. On further taps with a hammer these pieces shattered into dozens of smaller ones! The white glaze is certainly under too much compression. Obviously, neither is under any danger of crazing. Is the compression too great on the dark glaze? It did not shatter the way the white one did on further taps. And, another thicker-walled piece exiting the same kiln was glazed inside and out with that glaze. It was very strong. The lesson: Glaze compression, if not too much, is good for ware strength - but pieces must be glazed both outside and inside. And, thin ware like this must have better-fitted glazes.

White agglomerates of New Zealand kaolin (NZK) have ruined both glaze and body (Zero4 fritware). Both were slurried up by propeller mixing (the latter dewatered on a plaster table). But in both cases, the action of our lab mixer, a very capable device, was not enough to break up the NZK agglomerates! The glaze appears to be the easiest to fix: Sieve it at 100 mesh. But does that really work? No. The particles are 10-20 times smaller than the openings so agglomerates of hundreds could easily remain intact. The body is another matter. It is just about impossible to sieve because it contains significant VeeGum that gels the slurry. However, I am a potter and don't need to make thousands of gallons. Blender mixing is the answer, on high speed it smashes the agglomerates. Even if I need to do multiple gallons it is easy to process the slurry in batches in the 2 litre jar of my mixer.

Your studio or workshop can save a lot of money making and even selling your own glazes to students (at least for common colors and types). The supplies shown here enable printing quality labels on ordinary paper and applying them to jars that cost about $2. What about the glazes themselves? Just do what glaze companies do: Add stains to a base transparent glossy or matte glaze (for example G1916Q for cone 04 or G2926B for cone 6). Each 16oz jar needs 350g powder, 450g water, 5g CMC gum and 5g Veegum (leave out the Veegum and use less water for thicker coats). Stains are expensive (5-20 cents/gm here in 2024), at 7.5% (25g) that is $1-5 per jar. The materials for the G1916Q base cost $10/kg, and for G2926B $4/kg. Thus each jar costs $3.33 and $1.33 respectively. Making recipes of 5000g at a time would enable making 15 jars. A best-case is thus about $5/jar, worst case about $10/jar (compare that with commercials that are about $15-70). Of course, you can save money by looking for deals on materials and stains and recycling jars. Consider other advantages of making your own: You can tune the rheology, you know what is in it, you can adjust recipes to fit your clay bodies (to prevent crazing or shivering).

Are you worried about labelled, testing and SDSs? Take a look at ASTM D-4236 and see if you still need to worry.

The glazes are G1947U glossy, G2571A matte and GR10-C Ravenscrag Slip talc matte. The clay is H550. The firing schedule is C10RPL. The beauty of cone 10R is that no frits are needed, ordinary mineral mixes melt well. Cone 10 glazes can have high levels of SiO2 and Al2O3, those oxides are what produce durability and hardness. Because these mugs are thin-walled and have an even cross-sections there is no reason they could be not fast fired.

Do you really need to age clay when you make your own? No. In ancient Japan they did not have power blenders and propeller mixers. We do. To illustrate: I just sieved out the +80 mesh and +200 mesh particles from this raw clay (from one of our stockpiles) and then propeller-mixed it as a slurry. That wetted the particles very well and made it easy to sieve. Then I poured the slurry on to a plaster table and thirty minutes later it was ready-to-use. Slurry mixing is just as good as deairing in a pugmill. No wait! Particles wet even better. The plasticity of this clay is wonderful, and, it will not get any better with aging. Ancient Japanese potters used non-plastic, coarse particled clays so they needed to squeeze every last bit of plasticity out of them. Today, fine particled plastic clay materials are readily available. And we have something else the ancients did not: Micro-fine bentonite. A few percent of that and any clay can be made super-plastic (provided you have a good mixer to wet and separate all the particles to release their full power).