|Monthly Tech-Tip |
At low temperatures glazes and slips/engobes are not stuck on nearly as well as with stoneware and porcelain. So the glaze fit has to be better (poor fit will be evidenced by flaking at the lip). But that is not what is happening here. In this case a pigmented slip, or underglaze, was applied first, at leather-hard stage (thus it is being used as an engobe). The integrity of two bonds must now be considered: Slip-with-body and glaze-with-slip. Slip-to-body bonding is never as good as glaze-to-body or glaze-to-slip. When an engobe, or underglaze, is refractory then the bond-with-body is especially poor. Ceramic stains are highly refractory in comparison with low-fire bodies, simply adding them to an underglaze base recipe will make it refractory also. In addition, stains vary widely in their refractory character and the percentage of stain needed varies greatly with color. Some underglaze manufacturers compensate by incorporating a compensatory percentage of frit in each underglaze recipe. Other manufacturers simply have one base and add all the colours to that. Claims that underglazes work well across wide temperature ranges do not get tested when they are brushed on as decoration, but when they are applied like this, as an engobe, disaster strikes! In this case we can see that the failure is occurring at the underglaze-body interface and the glaze/underglaze "sandwich" is releasing in large flakes.
A ceramic compound meant to be applied to green or bisque ware and covered using a transparent overglaze. There are good reasons to make your own underglazes if you are in production.
Creating a Non-Glaze Ceramic Slip or Engobe
It can be difficult to find an engobe that is drying and firing compatible with your body. It is better to understand, formulate and tune your own slip to your own body, glaze and process.