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Bat wash is a slurry of refractory material used to prevent kiln furniture from fusing together or pots fusing to shelves or to their lids when fired in position. The ideal wash is one which is relatively cheap and should be easy to make up into a wash which stays put during setting, and lastly it should easily wash off the foot rims, lids etc of the finished ware.
A mixture of aluminium hydrate two parts, china clay one part will be found excellent for most purposes both oxidized and reduced.