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We get a lot of questions about kiln wash. So here is the scoop.
Kiln wash is a sacrificial layer of material between your pot and your kiln shelves. It's primary purpose is to prevent glaze from sticking to your shelves. You can make your own kiln wash, as there are many recipes out there, but most people buy already mixed kiln wash, such as our regular and high fire kiln wash. The ingredients are materials with high melting points, with the formula adjusted based on what temperature you are firing to. For example, higher temperature washes will have more alumina which has a high melting point, or zirconium which has an even higher melting point. Higher temp formulas always work fine at lower temperatures, but are more expensive.
The primary purpose of kiln wash is to prevent your glaze from sticking to the kiln shelf, should it come in contact with it during firing. Kiln shelves are normally made of hard ceramic, just like your pots, so if glaze melts directly onto a kiln shelf, it will stick!
Glaze usually only comes in direct contact with the shelf when something goes wrong. Normally you try to prevent shelf/glaze contact by either stilting your pieces to keep them off the shelf (for low fire), or by wiping the glaze off the pot bottoms (for mid-high fire). However you cannot predict when you’re going to have an unexpected problem, such as...
Your glaze may run or spit, your pot may tip over, or you may over-fire the clay (so it melts-down). This makes kiln wash an important preventative measure for all glaze firings. (You don't necessarily need kiln wash on bisque firings, however some porcelain clays can stick especially when fired in gas kilns, and there is also the possibility of a clay meltdown.)
If you don’t have kiln wash to catch any glaze before it reaches your shelf, you will have a number of problems:
Your pot will stick to the shelf and be very difficult to remove.
You will have to grind all of the glaze off the shelf, which is hard work and messes-up the smooth surface of your shelf.
Any glaze you miss and that remains on the shelf, including down in the pores of the shelf, will continue eating through the shelf and weakening it.
Note: Everything mentioned above is equally true when firing glass (which is essentially the same thing as glaze). When firing glass it is always in contact with the shelf or a mold, so having kiln wash (or glass separator, or some other type of release agent) on your working surfaces is critical for every firing.
We've had the best results when mixing a small amount of kiln wash in a bowl with water, until it has the consistency of skim milk. Apply several thin coats, letting it dry thoroughly in between each coat.
Some people even fire it between each thin coat (building it up in bisque firings, or adding additional layers of protection during glaze firings). You can apply kiln wash with a brush, roller, or even a sprayer. This assumes you've purchased kiln wash in powder form. For those people who don’t want to mix their own kiln wash, we will soon be stocking a pre-mixed liquid kiln wash product. Mixing and applying kiln wash in thicker coats is not a good idea as it causes more cracking and peeling.
Only apply kiln wash to the top surface of your shelves. If you are planning to flip your shelves over then you need to scrape all the kiln wash from the bottom side of the shelf first, otherwise the kiln wash can flake off on to the pots below. For the same reasons, its best not to apply kiln wash past the edge of the shelf. Many people apply kiln wash and then sponge off ½” around the perimeter, just to be sure a chip if kiln wash won't go over the edge.
If you have brand new kiln shelves, it is recommended that you fire them once in an empty kiln, without any kiln wash on them. This drives-out any organic material from manufacturing and transport. Then apply your kiln wash to the shelves.
When you do have a mis-hap and the glaze reaches your shelf, you should be able to lift your piece up, and the kiln wash in that area will come with it. If your pot otherwise looks good, you can grind that glaze run off the bottom of your pot. Then you can fill that spot on the shelf with more kiln wash. However, eventually the shelf will become very uneven with kiln wash. At that point you should scrape all of the kiln wash off and start fresh again. If you continue to fire on a rough shelf, it can potentially cause pots to “hang up” on the high spots as they expand/contract and shrink during firing. The manual method for removing kiln wash is a Kiln shelf scraper, and the automated method is to use a grinder (while wearing a mask, of course.)
Every rule has some exceptions, so here they are:
Some shelves for glass kilns are made from fiber. This allows them to be made larger and flatter than regular ceramic shelves. DO NOT apply kiln wash to fiber shelves, as the shelves will absorb the water and be destroyed. If you have fiber shelves, use shelf paper instead of kiln wash.
Nitride bonded silicon carbide shelves generally do not need kiln wash because glaze does not stick to them. However some porcelains under some firing circumstance can become glaze-like and want to stick, especially in gas kilns.
Even with bisque firings, it's generally a good idea to have kiln wash on the shelves. As pots expand and contract during firing, pot "feet" can sometimes stick and hang-up on the shelf surface and break off.
It is generally recommended that you apply kiln wash to the brick on the kiln floor as well. In the event that a glaze runs bad enough to reach the kiln floor, this will help prevent it from penetrating through the brick and destroying it. However, if your kiln has heating elements in the floor, DO NOT apply kiln wash to it. In general, kiln wash needs to stay away from heating elements, as the ingredients in kiln wash will aggressively attack them, causing them to fail. This is true whether the kiln wash is accidentally applied to the element, or whether some flakes or chips-off and ends up on the element. (Another good reason to vacuum your kiln every now and then.) You do not apply wash to the side walls of a kiln, again because you want keep kiln wash away from elements.
When salt or soda glazing, the salt/soda environment can create a layer of glaze out of the kiln wash, especially if it has silica in it. In that case, you should mix your own kiln wash with a salt specific formula, or use "wadding", which are small balls made of similar materials that you place under your pots.
Many glass artists use kiln shelf paper instead of kiln wash, even on ceramic shelves. This is because brush strokes or imperfections in the kiln wash will show through transparent glass and make your piece less attractive. Shelf paper can be used by ceramic artists as well, it’s just more expensive and not necessary.