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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Here’s how we make a base brushing engobe

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This brushing engobe is thick and gooey (because it contains CMC gum), so it is practically impossible to sieve. Our regular propeller mixer is not able to break up the tiny agglomerates of New Zealand kaolin. But 30 seconds of blender mixing makes it as smooth as silk. To make this liter of brushing engobe we use 800g of powder and 10g of CMC gum in 800g of water. Because of the very high clay content this does not require Veegum to gel it. The CMC gum greatly improves the brushing properties. It also thins the slurry enough to enable its lower-than-normal water content, making it more suitable for painting onto leather-hard ware, minimizing the number of coats needed.

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

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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.

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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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