The best way to make black clay: Don't.

To prove that using an engobe is the best way to make a black cone 6 stoneware clay surface consider the mug on the left, a version of L4768E. This is not the best way but it is the one most used (and what Plainsman Clays does with Coffee Clay). This is a DIY mix, the darkest that it is practical (I used the hyper-red burning C-Red clay as a base, thus needing much less umber to darken it). While it looks black beside red burning bodies, beside this true black it doesn't, First, it is not a body, it is just a thin veneer of stained porcelain engoben (if the surface is all we see why stain it all the way through?). That being said, some people need a totally black and safe clay, like this one, and can afford to make it themselves (if that is you try using the L4053B recipe as a starter). Get a plaster table and a propeller mixer and you will be unstoppable!

What about using metal oxides to make the engobe? Black stain is expensive but is far superior (and safer) for black. Consider using 10-15% colorant in this 500g mug, which would require 50-75 grams of stain (~$5-8). 10% stain in an engobe applied in this thin layer likely needs only a few grams of stain (for pennies of cost). What about using a commercial black underglaze? There are normally vitreous, would stick to a kiln shelf and not cover evenly like this.

Related Pictures

A special gummed engobe made this possible

The fired pieces appear to be made from black porcelain. But this is L3954F cone 6 black engobe over a white porcelain. Multiple factors make this work. First, a non-gummed dipping engobe will not work for this (it will not apply by brush evenly or thickly enough). This brushing version mixes 500 grams of L3954F powder (with black stain) into 280g water and 75g of Laguna Gum Solution. Second, the engobe recipe is tuned to have the same degree of vitrification as the body, in this case M370, so pieces don’t stick to the kiln shelf during firing. Of course, this procedure rewetted the bases of the leather hard mugs, extending drying time so extra precautions were needed to avoid cracking. For example, I painted wax emulsion on the handle outers and covered them all in cloth and plastic for a day to slow down drying.

L3954J black engobe on a cone 10R whiteware body

The body is Plainsman H570 (0.5-1% porosity). This piece is not glazed. The firing schedule is C10RPL. Notice the EBCT test bars in front (engobe compatibility). These sandwich the body and the L3954J engobe together in a thin strip, differences in fired shrinkage curl the bar during firing (toward the one of higher shrinkage). 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 that 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. 6600 is their cobalt black stain but the 6666 cobalt-free one should also work, both are suitable as body stains and recommended to 2300F (1260C) - cone 10 is technically above this limit but we have not encountered issues.

Stained engobes can be applied thinly yet fire opaque

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 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 was possible because of another key factor: The slurry was adjusted to be thixotropic. The thinner layer enables drying more quickly. The body-compatible engobe recipe also means fewer issues with flaking during drying, better fire-fit.

Brushing engobe fitted to this white porcelain turns it black

The L3954F engobe is tuned to have the same degree of vitrification as this porcelain (P300). I made a pint of a brushing version of it by mixing a 500-gram batch with 75g of Laguna Gum Solution and 280g water (it does not require VeeGum because we want a lower specific gravity than would be typical for a brushing glaze). Mixing in a kitchen blender gets all the lumps out and makes it paint beautifully onto leather-hard ware. One coat covers. This enables presenting this normally white burning body as a black porcelain to match the glaze. I get so excited about these that I make my own labels for single-jar brushing glaze and underglaze tests. They clearly show the code number, we assign these, for the project, in our insight-live account. Subsequently, the piece was bisque-fired, black-glazed, and fired at cone 6. The band painted on the base, which I did as a fix-up for a few tiny white bare spots, demonstrates something unexpected about this homemade engobe: It paints well on a vitrified surface, dries there without cracking and even fires there without flaking off.

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