Glaze cracking during drying? Wash it off and then do this.

Friday 22nd January 2016

If your drying pottery glaze is doing what you see on the left, a spider web crack pattern, do not smooth it with your finger and hope for the best. It is going to insight-live.com/trouble/1">crawl during firing. Wash it off, dry the ware and change your glaze or process. The first thing to check is water content. If the glaze has worked fine in the past then it is likely going on too thick because the specific gravity is too high - just repeat cycles of adding a little water and dip testing. But that was not the issue here. Glazes need clay to suspend and harden them, but if there is too much it can mean trouble. This was Ravenscrag Slip, a clay, being used pure as a cone 10R glaze. The glaze appeared to go in perfectly and it dried to the touch in ~20 seconds. But shrinkage continues after that, revealing after a couple of minutes. Fixing the issue was a matter of adding some roasted Ravencrag Slip to the bucket. That reduced the shrinkage and therefore the cracking. Any glaze containing excessive kaolin can be fixed the same way (trade some of the raw kaolin for calcined kaolin). Some glazes that contain plenty of clay also have bentonite - a simple fix for these is to simply remove it.

Pages that reference this post in the Digitalfire Reference Library:

Calcined Kaolin, Calcination, Crawling

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.