Is porcelain engobe good on stoneware? Nope. This demonstrates what happens.

Saturday 8th July 2017

This is how bad the fit can actually be. In the front is a bi-clay EBCT test strip of a insight-live.com/glossary/48">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).

Pages that reference this post in the Digitalfire Reference Library:

EBCT Engobe Body Compatibility Test, Absolutely Jet-Black Cone 6 Engobe on M340 This could also be super white, Fitting a brushing engobe to a porcelain: Three things are very important., 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.