Absolutely Jet-Black Cone 6 Engobe on M340

This could also be super white

This is the L3954B engobe. 15% Mason 6600 black body stain has been added (instead of the normal 10% Zircopax used for white). Of course, a cover glaze is needed for a functional surface. A lot of development work went into producing a recipe that fits this body, M340. It works even when thickly applied because it has the same fired maturity as the body. Lots of information is available on using L3954B (including mixing and adjustment instructions). Engobes are tricky to use. Follow the links below to learn more. L3954B is designed to work on regular Plainsman M340 (this piece), M390 and Coffee Clay. Most importantly, adjusting its maturity, and thus reducing firing shrinkage, is documented. These bodies dry better than porcelains and are much less expensive, so coating them with an engobe to get a surface like this makes a lot of sense. Ed Phillipson discovered this 80 years ago, enabling selling pieces made from these clays as white hotelware.

Related Pictures

How to test if an engobe fits a clay body

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This is part of a project to fit an engobe (slip) onto a terra cotta at cone 02 using the EBCT test.
Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way!
Right: SHAB test bars of the white slip and red body enable comparing their drying and firing shrinkages.
Center back: A mug with the white engobe and a transparent overglaze. The slip is going translucent under the glaze because it is too vitreous. Its higher fired shrinkage curls the bi-clay bars toward itself. Reducing the frit will reduce the firing shrinkage and make it more opaque (because it will melt less).
Front: A different, more vitreous red body (Zero3 stoneware) fits the slip better (the strips dry and fire straight).

Here is why porcelain engobe does not fit stoneware

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This is how bad the fit can actually be. In the front is a bi-clay EBCT test strip of a grogged cone 10R sculpture clay sandwiched with a porcelain. After drying this bar was relatively straight. The back bar bent quite a bit even after bisque. But the bend on the front bar really shows the misfit. But this is not a thermal expansion issue where volume changes are measured in 100ths of a mm - these plastic bodies shrink 5-8% during firing, that is up to 8mm change in these 10cm long cars, that is the kind of volume change needed to make this happen. The porcelain has the higher fired shrinkage so it pulls the bar toward itself. The internal stress makes this bar a time bomb, waiting for a mechanical or thermal trigger to burst it into a hundred pieces. Admittedly, putting a thin layer of this porcelain onto a piece of heavy ware is not going to bend it - but the stresses of the porcelain being stretched-bonded will still be there, seeking relief (likely exhibited by cracking or flaking).

Make a black engobe to fit your stoneware perfectly

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This unglazed white stoneware mug has been fired to cone 10R (Plainsman H570, 0.5-1% porosity) to demonstrate the utility of a stained engobe, in this case L3954J. This recipe employs 10% Mason 6600. The firing schedule is C10RPL. Notice the EBCT test bars in front; I make these by sandwiching the body and the engobe together in a thin strip, they demonstrate engobe/body compatibility because differences in drying and fired shrinkage curl the bar (toward the one of higher shrinkage). Thus, the straighter the bars dry and fire, the better the engobe-body fit. This makes it practical to tune an engobe to fit any clay body (e.g. my regular L3954N engobe recipe for buff stoneware adjusts this to have a lower fired shrinkage). This is not something to ignore, engobes that don't "fit" can flake off or exhibit issues similar to glaze compression. An organized testing program is ideal to be able to use engobes. While stains are not common at this high of a temperature, my testing has shown good results on many colors. The Mason Colors data sheets show this stain (and their 6666 cobalt-free one) to be suitable as body stains up to 2300F (1260C) - cone 10 is technically a little above this limit but I have not encountered issues. Black engobe work very well at lower temperatures, e.g. my L3954B recipe.

Stained engobes can be applied thinly yet fire opaque

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This black engobe, L3954F, is on a cone 6 buff stoneware (at leather hard stage). It contains only 7.5% Mason 6600 black stain. How is that possible? Why do people add so much more to their underglazes? Because this recipe has been tuned to have the same degree of maturity as the body, it therefore fires totally opaque. This contrasts with other underglaze/engobe recipes containing significant frit, among other issues, their vitreous nature renders them translucent. Thus, up to 40% stain is needed to crowbar their opacity enough to intensify color. And a thicker application (that carries other issues).

Notice how thinly and evenly this is applied. This cannot be done with a brush; it has to be done by dipping. This was possible because of another key factor: The slurry was adjusted to be thixotropic. The thinner layer enables drying more quickly. This engobe was designed to be compatible with this body (meaning fewer issues with flaking during drying, better fire-fit).

How to make a black cone 6 oxidation clay body

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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.

Videos

Links

Recipes	L3954B - Cone 6 Engobe (for M340) Dry and firing shrinkage fitted to Plainsman M390, M340
URLs	https://plainsmanclays.com/l3954b The L3954B engobe page at PlainsmanClays.com explains how to mix and use it on Plainsman clays bodies at cone 6.
Materials	Mason 6600 Black Stain A cobalt-containing stain useful in bodies, engobes and glazes at a wide range of temperatures.
Glossary	Thixotropy Thixotropy is a property of ceramic slurries of high water content. Thixotropic suspensions flow when moving but gel after sitting (for a few moments more depending on application). This phenomenon is helpful in getting even, drip-free glaze coverage.
Glossary	Engobe Engobes are high-clay slurries that are applied to leather hard or dry ceramics. They fire opaque and are used for functional or decorative purposes. They are formulated to match the firing shrinkage and thermal expansion of the body.

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