Self-Leveling Kiln Wash
If you make things out of clay, chances are you’ve seen and used some pretty atrocious kiln shelves. Almost every kiln shelf I’ve seen has the same cracked, chipped, falling off, 5 layers deep kilnwash moonscape on it that threatens to ruin whatever is fired on top of or even underneath it. Especially with the vitreous porcelain I use, it is critical that the surface I fire on be perfectly flat and decently smooth. To this end, I have been researching and perfecting my kiln wash recipe so that it will stay on the kiln shelf, never crack, and apply very smoothly. Here’s what I’ve learned…
I began with John Britt’s Ceramics Monthly article on kiln wash, which was an excellent starting point. Basically, the gist of the article is that for some reason the most widely used recipe happens to be a really bad recipe. It shrinks excessively and as a result, peels up during the drying and firing, flaking off whenever the shelf is handled. Then you just fill in the places it flaked off of, only to compound the problem, resulting in a hopelessly thick, flaked, disasterpile kiln shelf. My favorite quote from this article goes like this: “Potter’s make a significant investment in their kiln shelves but rarely take more than a few minutes to mix up two scoops of kaolin and alumina to protect them. They also spend countless hours making and perfecting their work only to suffer unnecessary breakage and loss of pots because they just don’t know that a kiln wash doesn’t have to crack or fly off into the bottoms of pots.” Britt suggests that the simplest solution is to just replace some of the EPK with calcined kaolin thereby reducing the amount of shrinkage so hopefully the flakes stop occurring. So I mixed a batch that was:
50 Alumina Hydrate
Which worked pretty well, but not perfectly. It needed better adhesion to the shelf, and small hairline cracks would occur in the valleys of the brush stokes. Also, I have a problem with brush stokes being there at all. They cause an uneven surface than can warp or even crack my pots if undue friction occurs between the shelf and the shrinking pot.
This is where I started to think that perhaps the solution would be to deflocculate, or disperse the kiln wash, making it crack less because it shrinks less (because deflocculated slurries require less water for the same viscosity), and will greatly improve the slurry’s brushing qualities allowing for a nice even coat. Have you ever noticed how kiln wash dries almost the moment it touches the kiln self, making it nearly impossible to make an even surface? Well, when you deflocculate the kiln wash, it remains a smooth, beautiful, wet puddle on the surface, almost like the shelf has been dipped in white chocolate.
Here is the recipe that I developed over the course of about 6 months and 15 or more firings. It now works perfectly and I see no need to improve upon it.
Campana Self-Leveling Kiln Wash (for all temps and atmospheres)
5000g Alumina Hydrate
4250 ml Water
25g Darvan 7
First put the water in the bucket, then add the Darvan. Mix thoroughly with a drill mixer. Add the EPK and mix that in, then the Glomax, then the Alumina Hydrate. Mix for about 5 minutes. If you dip your hand with fingers spread and quickly remove it, the wash should web across the negative space while running off the fingers. Sieve it with a 60 mesh screen. Yeah, that’s right, I sieve my kiln wash. The whole point is to get a smooth surface.
I like to check the specific gravity of the mixture to make sure things stay consistent from batch to batch. The best way to do this is to use a Syringeto draw 50ml from the bucket. Squirt it into a cup that is on a scale which has been zeroed out with the cup on it. Weigh the slip. Mine weighs 86g. To get the specific gravity, or the g/ml density, just divide the 86g by the 50ml. That would mean that the specific gravity of my kiln wash is 1.72 g/ml.
Knowing this allows me to do some quality control and to repeat my results over and over again. If the specific gravity is too low, I know it needs less water, so I can add powder dry mix. If the wash is too thick, I can weigh it and see if I need to add water or just a few drops of Darvan to make it runnier.
Logging the specific gravity, by the way, is the BEST idea for glazes. Ever get that perfect thickness worked out only to not quite be able to repeat it? Specific gravity my friend, specific gravity. Write it on the side of the bucket, check it before you dip, never mess up another pot (or kilnload of pots) with a fresh bucket of your favorite glaze.
To apply the wash, you must first take every little bit of the inferior wash off the surface of the kiln shelf however you can. I will do another post on the best tools for this job, but for now, a scraper, a rub brick, an orbital sander, and an angle grinder are the best things to use. Get it down to the bare shelf surface. I started with brand new kin shelves this time, lucky me.
First get a large sponge very wet, almost dripping, and sponge off the surface to be washed. This not only removes dust and debris allowing better adhesion, but also slows the absorption of the kiln wash, which will be important for the self-leveling.
Immediately after you sponge, brush with your hand to remove the sponge leavin’s and apply the wash. I soak a medium sized, natural fiber, soft paint brush with wash and drizzle it, numerous brushes full.
The idea is to get a continuous puddle of wash to cover the whole thing. There should be enough that the wash stays wet on the surface for several minutes. Try to vary the direction of the brush movements. You’re done when the puddle is smooth and even.
Stand the shelf up on its edge and carefully sponge every last drip off of the edges, and make sure the bottom is totally clean as well. Carelessness in this part will almost certainly ruin some work by raining little white flecks onto the glaze below.
The number one rule is to NOT do this right before or as you are loading the kiln. Allow the wash to slowly and fully dry before firing. 2-3 days at least.
So you might be thinking right now that I’ve completely lost it. That this is just kiln wash, who cares? Who has time to be that meticulous? It is my thought that being meticulous and doing things right up front may take a bit or even a lot longer in the short term but in the long term it will save a LOT of time, and some major headaches or even crying sessions/temper tantrums. No scraping and chipping between firings, only if a glaze actually runs would there be any work to do. The even and uncracked surface is more effective and can withstand some serious puddles. No more losing pots to kiln wash flakes landing in them, no more losing pots from warpage and stress cracks caused by uneven surfaces. No more losing pots because the kiln wash peeled up and stuck to the underside of your pot.
Stay tuned for my development of spackling to full in places where the glaze took the kiln wash off. Now that I got this part figured out, that is my next project.