|Monthly Tech-Tip |
The white engobe on the inside of this mug does not fit, that is what these cracks are about. The misfit can be demonstrated using EBCT test bars (shown in the foreground). This pair sandwich the engobe and body as a bi-clay strip, they are dried and fired with alternative sides up. A bend demonstrates differences in the fired shrinkage of body and engobe. On drying these stayed relatively flat but after firing at cone 10R, the engobe's higher shrinkage pulled them concave. On this mug, made from the same clay and engobe, the latter has done likewise, shrinking more than the body and creating a crack pattern that even the glaze could not hide. This was a development version of L3954N that contained too much nepheline syenite.
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.
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.