What Is Kiln Wash? Everything You Need to Know + How to Apply It.
Kiln wash is a barrier to prevent unexpected glaze runs or drips from ruining your pottery kiln shelves, and your work. You brush it directly onto your shelves; it looks similar to a glaze when being applied.
Why do I need kiln wash?
Pottery Kiln shelves are made of a hard material that is similar to fired clay. If you don’t use a kiln wash, any glaze that comes into contact with your kiln shelf gets absorbed and becomes permanently fused to it. If a piece is fused to a bare shelf, something will have to break in order to remove it. Usually this means we have to sacrifice the pot to save the shelf. (An unfortunate necessity, but kiln shelves are expensive!) Kiln wash can also save our runny glazed pots from cracking when removing them from the shelf. 
A good kiln wash should prevent “plucking” as well. Plucking is when some porcelains, due to their high silica content, become so fluxed (melted) during the firing that they stick to an unprotected kiln shelf and small bits or splinters of clay fuse to the shelf, breaking free from the piece itself. It’s a disappointing way to ruin a beautiful piece of pottery.
A good kiln wash is not too flaky or powdery but comes free from the kiln shelf with little effort. It requires minimal maintenance when your glazes are behaving.
How to make kiln wash: a recipe
Here at Seattle Pottery Supply, we didn’t like the performance of commercially-made kilns washes (and didn’t know exactly what was in them), so we made our own kiln wash. All you need to do is add water.
But if you want to make your own, here's what to do:
Dry mix:
- 15 parts silica
- 15 parts kaolin (epk)
- 3 parts om4
Mix all of the ingredients above with water, using a high-speed mixer like a hand blender, until it's the consistency of paint (if you're of the scientific leaning, that's a density/specific gravity of about 1.8-1.9).
How to apply kiln wash
Kiln wash - whether you make it yourself, or purchase it ready-made - can be applied by paintbrush, roller or by spraying.
Make sure your kiln wash is well mixed before applying. You want it to be homogenized. Stir with a whisk before dipping the brush in the wet kiln wash. When brushing kiln wash, use a large wet sponge to dampen the kiln shelf first. Pre-wetting the kiln shelf prevents the water in the kiln wash from being absorbed into the shelf too quickly. It’s best if the kiln wash stays wet until you have an even coat.
- Dip the brush into the kiln wash and start painting the shelf. Use a wide, soft-bristled brush that will hold as much kiln wash at a time as possible. Don’t rub the bristles on the edge of the bucket to remove the excess; you want as much as possible on the brush. And make sure you don’t get any kiln wash on the bottoms of your shelves. Tops only!
- As soon as the brush is dry, go back for more. Apply the kiln wash very liberally with long strokes. The key is to get a nice even coat. Using many small brushstrokes can make for an uneven layer.
- Once the shelf is covered, allow it to dry to the touch and then add another layer.
- When finished, wipe any drips off the edges of your shelves. If you leave them, they could flake off and land in your glazed pots during the firing.
- Always make sure your kiln wash is completely dry before firing. It will change from a darker color to lighter when it’s dry. If the kiln wash is still damp and you put it in the kiln, you run the risk of it flaking and not adhering properly to the shelf.
Your shelves may take a couple of days to dry. To speed up drying, use a fan or put them near a hot kiln.
If possible, put your freshly kiln-washed shelves through a bisque firing before you do a glaze firing. Bisque firings are generally slower, so any leftover moisture has time to escape. This isn’t always possible, of course but it’s what we do when we have the time.
After the first firing, check your shelves. If you can see the shelf through the kiln wash, add another layer. You want the kiln wash to be thick enough to prevent glaze from seeping through.
If you have any questions about this process, just reach out to us at info@seattlepotterysupply.com. And you can buy our kiln wash here!
Happy firing!
