Six layers, 85% Alberta Slip in the glaze, yet no cracking. How? CMC gum.

Friday 23rd November 2018

Six layers of any normal dipping glaze would be impossible, (flaking usually starts with the second). Yet this slurry is 85% plastic clay, it shrinks so much that it would be like a "dried-up lake bed" on the first layer. By the second layer it would all just fall off! How was it possible to dip six layers of this with no cracking? A 1% insight-live.com/material/228">CMC gum addition added enough adhesive power to not only stick it on better but actually counteract the drying shrinkage! Of course, there is a downside: A drying period is needed between each layer, the time depends on the porosity and wall thickness of the ware and the amount of gum. This also demonstrates the difference between the function of Veegum (and similar materials) with CMC. The former, if added to this recipe, would gel the slurry, require more water and drastically increase the shrinkage, making the cracking much worse. We typically use a mix of calcine:raw Alberta Slip to control drying shrinkage enough to enable crack-free application for one coat. But that is not enough to use this as a first coat dipping glaze, a CMC addition is required.

Pages that reference this post in the Digitalfire Reference Library:

CMC Gum, Paint another layer onto a fired glaze? Yes. With CMC gum., Base-Coat Dipping Glaze

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.