Kiln shelf maintenance is a much hated but very necessary part of having a kiln. Neglected kiln shelves can result in flakes of kiln wash landing smack dab in the middle of a beautiful glaze surface, or pots inadvertently sticking to shelves where a glaze drip once landed. So it’s best to stay on top of cleaning your kiln shelves and to know how to apply kiln wash.
In this post, an excerpt from Clay: A Studio Handbook, Vince Pitelka gives some tips on scraping kiln shelves, mixing kiln wash, and shares a couple of kiln wash recipes. – Jennifer Poellot Harnetty, editor.
As you load any kiln, examine the shelves and furniture carefully. Loose, flaky shelf wash should be scraped off shelves with a sturdy metal scraper or a heavy wire brush. Always wear safety glasses or a face shield and a good dust mask when scraping, brushing or grinding shelves, and if possible do the work outdoors. If there is a serious accumulation of glaze residue it must be chipped or ground off before the shelf or furniture is used. Minor glaze drips can be chipped off with hammer and chisel. More serious glaze runs must be ground off. Never chip or grind shelves while they are resting on any hard, unyielding surface—always place them on a cushioning bed of cloth or foam rubber. In a pinch, a bed of sand makes a good support. When chipping glaze accumulations with a chisel, never hold the chisel vertically against the shelf. Always sharpen the chisel so that only one edge is beveled, and hold the flat edge against the kiln shelf, so that the force is parallel to the shelf, against the glaze accumulation as shown in figure 1.
If the shelf needs to be ground, a standard angle grinder is ideal for the task. For most studio needs, a 4½ inch angle grinder is very manageable and effective. For commercial use, a heavy-duty 7 inch angle grinder is recommended. Always select a grinder with a 4 1/2-inch spindle with 11 threads per inch that will accept all the standard accessories.
For light grinding, fiberglass reinforced grinding disks work fine. For heavier grinding and for leveling irregular shelves, use a masonry-duty diamond cup wheel or silicon carbide cup wheel. For grinding the edges or surfaces of kiln posts an ordinary motorized bench grinder works best. Make sure to keep the ends of your kiln posts very flat to minimize wobble within the set. When unloading a kiln, note which shelves and furniture need cleaning and/or recoating with shelf wash and take care of this routine maintenance right away. In any group or academic studio where a variety of people use the kilns, each user should do all kiln cleanup and shelf/furniture maintenance immediately after unloading the kiln. If there are no glaze runs and the shelf wash is not flaking, you don’t need to do anything to the shelves before the next firing.
If you do need to chip and/or grind glaze or scrape flaky shelf wash from the surface, recoat the shelves with an appropriate shelf wash. A common kiln wash for gas and electric oxidation or reduction kilns is 50% kaolin and 50% flint, but that mixture must never be used in salt, soda, and wood firing. For those processes, use 40% kaolin, 10% ball clay, and 50% alumina hydrate. Mix kiln wash to a thick creamy consistency and apply with a wide brush or paint roller. In some cases you may wish to build up successive coats, especially in any situation where significant glaze runs are fairly common.
In all cases, avoid getting kiln wash on the edges of the shelves where it can flake off in the firing and fall on glaze wares below. A shelf wash of 90% alumina and 10% kaolin works well if you plan to periodically flip your shelves.
Wadding is always used in salt, soda, and wood firings under the ware and on every contact surface between kiln floor and posts, between multiple posts, and between posts and shelves to keep them from glazing themselves together. Wadding is made from the same recipe as shelf wash (40% kaolin, 10% ball clay, and 50% alumina hydrate), mixed to the consistency of plastic clay. For wood firing add a liberal amount of fine sawdust to the mix.