Kiln wash is a refractory coating, usually a mix of alumina or silica with kaolin, painted onto the top of kiln shelves. It stops dripping glaze from fusing to the shelf, so drips can be chipped away instead of ruining it.
What Is Kiln Wash?
Kiln wash is applied as a thin, even coat, often two or three layers brushed in different directions, to the top surface of shelves only. It should never go on the underside, because flakes can fall onto glazed work on the shelf below and stick to it.
When glaze runs onto a washed shelf, the drip can usually be ground or chipped away with a scraper and the area patched with fresh wash. Over time the wash builds up, cracks and flakes, and needs scraping back and redoing. Digitalfire suggests zircon-based washes for very runny glazes.
Kiln wash isn’t a cure for runny glazes. Waste slabs, cookies and keeping glaze off the bottom of the pot prevent most problems. Wear a mask when scraping and mixing wash, since it creates fine dust, and work outside or with a damp cloth nearby to keep the studio clean. A well-kept shelf can last for many years of firings.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.