|Monthly Tech-Tip |
10% light magnesium carbonate has been added to this low-temperature terra cotta white glaze - it induces enough crawling to form islands and the MgO it sources also mattes the glaze. The bare areas between the islands have a glassy surface, thus the crawling begins at some point after the melt has begun to form an interfacial layer with the body. "Snakeskin" recipes like this effect can call for much higher percentages of magnesium carbonate (we have seen 30%) but such levels can produce an excessively matte surface and reduce thermal expansion so much that the islands could shiver. Rather than bring a new into your production (these recipes can be really weird, calling for unusual materials) it may be better to add just enough magnesium carbonate to your standard white gloss glaze. Even better, start with a fluid melt high expansion glaze (e.g. a crackle), it will provide more margin for thermal expansion reduction. Magnesium carbonate is highly refractory and works this way all the way to cone 10 (where higher percentages are likely required).
Light Magnesium Carbonate
A refractory feather-light white powder used as a source of MgO and matting agent in ceramic glazes
A ceramic glaze fault that occurs during firing of the ware, the molten glaze pulls itself into islands leaving bare patches of body between.