Kiln wash is usually a mixture of alumina hydrate and kaolin. If kiln shelves with this typical mixture of kiln wash are not scraped and cleaned in between firings and the kiln shelves are flipped between firings to prevent warping, pieces of kiln wash can flake off, drop onto glazed ware and ruin the surface.
I have found that using alumina hydrate mixed only with water eliminates the problem of flaking kiln wash and works just as well. I just brush the mixture on my kiln shelves and, after firing, it becomes powder again. Cleaning the shelves is as easy as brushing the powder from the shelf. You can even save the powder, remix it with water and use it again next firing. An added bonus is that the powder enables the ceramic pieces to move a bit during the firing, which cuts down on warping that my occur during shrinkage.