This is how bad the fit can actually be. In the front is a bi-clay EBCT test strip of a grogged vitreous 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 an 8 mm change in these 10cm long bars, 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 sculpture clay is vitreous, so it can flex during later stages of firing. The internal stresses built in make this bar a time bomb, waiting for a mechanical or thermal trigger to burst it into a hundred pieces. Admittedly, putting a thin slip layer of this porcelain onto a piece of heavy ware is not going to bend it - but apply it thickly as an engobe and the stresses of the porcelain being stretched-bonded will still be there, seeking relief (likely to be relieved by cracking or flaking).

This picture has its own page with more detail, click here to see it.
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.

This picture has its own page with more detail, click here to see it.
First, the black engobe must be tuned to have the same degree of vitrification as the porcelain (using EBCT test). Second, to brush like a paint onto leather-hard ware, it needs CMC gum. Third, it needs a low water content so one coat covers. Here's how I made a pint:
-Put 280 g water into a blender.
-Add 500 g of L3954F powder.
-Add 75 g of Laguna CMC Gum Solution.
-Blender mix thoroughly to get all the lumps out.
I was so excited about being able to have a black porcelain surface on P300 that I made my own label (showing its code number from my 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: The gum enables it to dry on the vitrified surface without cracking, and this thin layer even fired on without peeling.
| Tests |
Engobe Body Compatibility Test
A bi-clay test strip curls to demonstrate the drying and fired compatibility or fit of an engobe (or slip) on a ceramic body or of two bodies for marbling. |
| 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. |
![]() PayPal | No tracking, No ads, No paywall, No transient content! Just organized, concise information constantly updated and improved. Was this helpful? Consider supporting me. |
Buy me a coffee and we can talk