Kiln forming techniques require separators between the glass and the shelf or mould on which it rests during the heating process. These separators have different generic names – kiln wash and batt wash are two.
There are a number of brands of kiln wash. All of them contain two main ingredients – alumina hydrate (sometimes called slaked alumina) and kaolin (also called china clay). Different producers use these ingredients in various proportions. A number also include a colourant that changes when fired above certain temperatures to indicate the wash has been fired. It also distinguishes between the unfired and the already fired kiln washed shelves.
An important thing to remember is that the kaolin changes its composition once it is fired over 700C. This change causes it to stick to the glass on subsequent firings. Thus, it is essential to change the kiln wash after every firing that reaches that temperature or higher.
It is possible to apply a fresh coat of kiln wash over the old one to save time. However, as soon as the kiln wash flakes you must scrape off all the old kiln wash and apply a new coat to the bare shelf or mould.