This is part of a project to fit a fritted vitreous engobe (slip) onto a terra cotta at cone 02 (it fires harder there).
Left: On drying the red body curls the bi-clay strip toward itself, but on firing it goes the other way!
Right: Test bars of the white slip and red body compare their drying and firing shrinkages.
Center back: A mug with the white slip and a transparent overglaze. Notice the slip is going translucent under the glaze. Why? It is too vitreous. That explains how it can curl the bi-clay bars toward itself (it has a higher fired shrinkage). So rather than add zircon to opacify the slip, it is better to reduce its frit content (thereby reducing its firing shrinkage). Reducing the frit in the slip will also make it more opaque (because it will melt less).
Front: A different, more vitreous red body (having a frit addition) fits the slip better (the strips dry and fire straight).
Pages that reference this post in the Digitalfire Reference Database:
EBCT Engobe Body Compatibility Test, Solving a difficult engobe flaking problem, Bi-Clay strips test compatibility between engobe and body, Creating a Body/slip Equilibrium in Terra Cotta Ware, Slip, Firing Shrinkage, Majolica, Engobe
This post is one of thousands found in the Digitalfire Reference Database. Most are part of a timeline maintained by Tony Hansen. You can search that timeline on the home page of digitalfire.com.