Wednesday, 23 November 2016

Kiln Washing Kiln Lids

It is frequently recommended that the bottom of the kiln should be kiln washed to prevent any spilled glass from sticking to the kiln brick.  You should remember that this is applicable to brick lined kilns.

This in itself is a little clue.  You do not need to kiln wash any insulation fibre in the kiln. If any glass were to stick to the fibre, it would come away easily.  In any case, most insulation fibre blanket will not stick to the glass.

The recommendation often goes on to advocate kiln washing the sides.  There is a caution that the side elements (if any) should not be kiln washed. The caution comes from the knowledge that water and electricity should not be mixed.  The kiln should not be on when applying kiln wash anyway.  If kiln wash is splashed onto the elements, it is simply a matter of letting the whole kiln dry naturally with the lid open before firing.

The extension to this series of recommendations is that the whole of the kiln should be kiln washed, including the lid.  This is not a good idea.  The wash on the lid will soon fail and drop dust and debris onto and into your work.  The glass should never touch the top of the kiln anyway.  If the elements are in contact with the glass, the glass will either stick to them or break.  You have to ensure you do not put glass nearer than about 20mm to the elements or lid. In any case, the glass will fall to the bottom of the kiln, not the top or sides – unless the kiln is not level.


The whole idea of kiln washing the interior of the kiln is suspect in some ways.  Anyone who has had glass drip off the shelf and onto the brick during an over-firing will know the glass eats into the brick through the kiln wash.  Kiln wash will only protect the brick at full fuse or less temperatures. But it is a good precaution to keep the pieces of frit that fall off the shelf from sticking to the brick. It does not do much more than that.

The application of kiln wash to the kiln creates another source of dust within the kiln.  Dust and general uncleanliness in the kiln is a main potential source of devitrification. Thus, the application of kiln wash should be the minimum necessary and does not need to go up the side beyond the elements or the lowest shelf height, whichever is less.

There is a strong argument to be made that laying a sheet of 0.5 mm fibre blanket on the floor of the kiln will provide better protection of the kiln than any amount of kiln wash.  It is less likely to fail, it is not a source of additional dust, it provides a better protection during any kiln runaway, and it is easily replaceable.