Sunday 3 June 2018

Kiln Wash

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 of makes 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 600C/1113F. This change of composition is completed by 900C/1620F.  The change is progressive.  It is so slow that slumping and draping moulds coated with kiln wash will last indefinitely. However this change is great enough by 770C/1419F that the kiln wash sticks to the glass on the next firing. Thus, it is essential to change the kiln wash after every firing that reaches tack fusing temperatures 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.

Some makers use much less of the binder (china clay) than others which makes them better for the popular casting moulds than those for shelves and slumping moulds as they can be brushed away without abrasion.

In addition, boron nitride is a suitable release from moulds.  It is very stable at reltively high temperatures and so can provide a smooth, "slippery" separator between the glass and its supports, whether shelves, moulds or kiln furniture.  It does seal porous surfaces, meaning that air cannot move through the treated surfaces.  It has to be removed with abrasion and so thought must be given to which surfaces it is applied.

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