Kiln Wash Sticking to Glass

Causes and avoidance

 

Photo credit: Immerman Glass

 In general, kiln wash for glass is made up of aluminium hydrate with kaolin (China clay) as a carrier. 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.

 Some reading has led me to learn that by 600°C/1113°F the kaolin begins to go 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 crystallisation cannot be reversed. It does not become fully crystalline until 935°C - 950°C/1717°F - 1744°F. The crystallisation stops the lubricating effect. I suspect that on the second firing these crystals (which contain silicon dioxide) interact with the glass and stick, although not fully combining 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 used in a previous firing. It also indicates that repeated tack fusing on kiln wash will ultimately fail as the crystallisation will gradually increase with each firing.

 However, at slumping temperatures, it appears the crystal formation is so slow as to have no effect on multiple firings.

 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, although the incorporation of  binders makes the application easier. The grades used in ceramics are usually coarser than kilnformers want. But it can be made finer by putting it in a rock tumbler with some stainless steel ball bearings. You can run the result through a fine screen to remove the ball bearings. Mix with water to brush on, or sprinkle dry over the shelf. The aluminium hydrate can be re-used, if they are kept free of contaminants. Aluminium on its own does not 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.

Kaolin provides significant advantages in the smooth application of kiln wash.  Caution needs to be exercised in using it after it has been fired to fusing temperatures, although it can be used at low temperatures for indefinite numbers of firings.

 Methods for removal of kiln wash are in this blog post.