glaze craking

SP:

Manufacturers of glazes and clay bodies are not usually concerned with how many different types of clay/glaze there are. No one glaze can possibly work on all different kinds of clay, just like not all tires fit your car. That's just the way it is. The reason your glaze started cracking might be because it does not "fit" the clay body you were using. Cracking (crazing) usually means that the glaze does not have the expansion coefficient that the clay under it does (it doesn't expand as much as the clay, and cracks when the clay expands.) You might need to adjust the glaze, or the slip you are using for casting to bring them closer together in terms of expansion. You don't mention whether you put the mugs in the dishwasher or not, but sometimes the heat from a drying cycle is enough to start a glaze crazing. I've seen it happen as much as a year or two after. One day, everything is fine, the next...wham...cracks.

If the glaze is coming off the mugs in large "slivers", that is known as shivering, and is the result of the glaze not sticking to the clay, either because the bisque was dirty/greasy/dusty when you glazed, or some other reason.

"Chunks" of glaze and clay coming off at the edge of the mug means you dropped it :>)

Hope that helps, Wayne Seidl

To Wayne's excellent answer, let me throw in another possibility for completeness: These are low-fired pieces, so they are probably not very vitrified. I'm wondering if the cracks are caused by moisture absorption, either from an unglazed foot or invisible pinholes in the glaze. This is one big reason why people go to high-fired clays (besides not liking bright shiny colors). ;-)

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

A common problem with white earthenware clays, especially over here is that they need a very high bisque. In the UK that often means cone 1. Fired below that the clay does not complete its shrinkage, and so when fired again higher than the bisque shrinks again. Curiously this can upset the glaze fit causing eventual crazing. If your 1080 fired bisque is still very porous, this could have some bearing on the problem, in which case it might be worth firing the bisque higher than the glaze, and seeing how that affects it. I test for crazing by chilling the pieces in the freezer and then putting hot liquid in them, and doing it several times. Brutal, but it's a good test.

Steve Bath UK

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