I do not know the
exact chemical changes of kiln wash at fusing temperatures. But I do suspect it
has to do with the kaolin. The aluminium hydrate is stable to much higher
temperatures (melting point of 2,072°C/3,762°F). So, I don't believe that part
of kiln wash is changing.
The Influence of Kaolin
Some reading has
led me to learn that by 600°C/1113°F the kaolin begins going through a non-reversable
chemical change. Prior to that, water can rehydrate the kaolin. In the hydrated
state kaolin forms hexagonal plates that can slip over one another. Once
600°C/1113°F has been exceeded the crystallisation formed cannot be reversed. It is a gradual process. It does not
become fully crystalline until 935°C - 950°C/1717°F - 1744°F. The
crystallisation stops the lubricating effect of the kaolin. I suspect that on the second full fuse firing these crystals (which contain silicon dioxide) interact with the glass (also
silicon dioxide) and stick to the glass. Although it does not fully combine with the glass. Why
this does not happen in the first firing, I do not know.
The fact that the
crystallisation cannot be reversed must be the key as to why kiln wash with
kaolin cannot be re-used once fusing temperatures have been reached. The crystallisation at 800°C/1473°F is nearly complete. It begins to exhibit the "stickiness" to the glass.
People who consistently avoid contour and full fuse firings find they can get more than one firing from kiln wash. This will be because the crystallisation is only partially completed. But it indicates that repeated tack fusing on kiln wash will ultimately fail as the
crystallisation will gradually increase with each firing. The number of firings possible on one coating of kiln wash will be dependent on temperature and times, among a few other things.
However, at
slumping temperatures, it appears the crystal formation is so slow as to have
no effect with multiple firings. Many people experience no difficulty with kiln wash sticking to the glass over many firings, when low temperatures are used. High temperature slumps will reduce the life of the kiln wash (where life is taken to mean the degree of crystallisation).
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| Picture Credit: Amazon |
Avoiding Kaolin
There are of course, ways to avoid kaolin. There is a kiln wash, called Primo Primer that does not
have kaolin in it. And you could make your own kiln wash from aluminium
hydrate. This is known as slaked alumina in ceramics. It can be used on its own with or without a medium to assist the smooth application of the kiln wash. One good medium is CMC.
When selecting the aluminium hydrate, be aware there are finer and coarser particles. The grades used in ceramics are usually coarser than glass
people want. But it can be made finer by putting it in a rock tumbler with
stainless steel ball bearings. You can run the result through a fine screen.
Mix with water to brush on, or sprinkle dry over the shelf. Both these can be
re-used. Neither provide as smooth results as when the kiln wash contains
kaolin.
Chalk, also known
as whiting, is calcium carbonate. This is often used as a separator in vitreous
paint firings and some forming operations. It has low solubility in water, so
cannot be painted onto shelves or moulds. It needs to be used as a loose or
compacted powder. It goes through chemical changes too, making renewal after
firing advisable. Above 800°C/1473°F calcium carbonate changes to calcium
oxide, or quicklime. This corrosive form is another reason it is disposed of
after any higher temperature firings.
Kiln wash and
calcium carbonate can be fired many times at low temperatures, because their
chemical composition remains relatively stable. Once higher temperatures are
used, chemical changes occur. This seems to enable them to stick to the glass
or form undesirable compositions. This phenomenon requires removal and
re-coating of shelves and moulds after full fuse firings. Their advantages are ease of use and low cost.
