Kiln Wash

I was just reading the Oct.3 newsletter from CM on kiln washes. John Britt's information was right on and common knowledge for the experienced (or experimentally leaning). It was Arise's comment that I found interesting. We have been wetting our shelves before applying a fresh wash for a few years. Similar to applying mortar to a brick. And the idea of grog in the wash sounds good. Be careful about what grog you are using, though. Calcining is good, but there are many materials that may require less work. Some years back, we used (and are again experimenting with) pyrophylite (nothing new, as others have mentioned it). It is a "high silica, natural alumino-silicate," (Robert Tichane- Clay Bodies, 1990). The key is the low thermal expansion. There are a number of materials that fit this description. I recall Steven Kemeneffy using kyanite, a material in the same family, in his raku body.

(I believe Soldner used talc in his body. This material is a bit more complex and the reasoning behind using it for raku is the somewhat questionable theoretical lowering of body expansion.  Don't use talc in a wash though!  In an engobe would be ok. This is for another time though.)

So, low expansion materials with a bit of binding material (like EPK or glomax if you prefer calcined clay) and your on the right path. We also use a roller rather than a brush. I picked this up while at Syracuse University. If you get the right nap, you get a textured surface with peaks and valleys, which seems to decrease the shrinkage (which is a lateral thing- but, you would think a brush stroke would align the clay particles parallel, which would be a good thing... over-think, sorry- low shrinkage is the key). Multiple applications will work.

At SUNY Cortland, I recall the notion of starting with a thin wash first and then apply thicker coats. We may have picked this up from Rhodes, I don't remember. But this means having more than one bucket of wash which is a hassle. We roll on a few coats of the same thickness, not letting the previous coat get too dry. For salt we would use silica dusted on the shelves. Straight silica is much different from the silica contained (combined) in a clay (like fire clay, stoneware, ball clay) or a clay body. The salt vapor alone just isn't enough to flux the silica. Has to do with eutectics. Anyway, we never had a problem.

The idea of employing a bit of flux is exactly what lead us to using our mix (see previous blogs) of kiln wash in the salt kiln on the insulating bricks after a few firings. The wash fused to the walls. So far, it is "sticking" quite well. But the jury is still out. We are getting a fair amount of peeling. So our experiment may prove more work than we want. I feel that some of these ideas are not necessarily "new" (few ideas really are), but are being rediscovered all the time, using "new" materials or new technology. Keeps us on our toes! The old "Ceramic Industry" magazine would publish a materials issue every January. Priceless. Now you can find it all on line very readily.

A funny story about this fusing thing. I once had an assistant (a very long time ago, in a far away place) who washed the salt kiln shelves with feldspar by mistake. All those similar white powders! It meant long hours of grinding. The idea of fusion is a good one, but...

We have some concern about the nitride bonded shelves that has nothing to do with moisture. We love them... But glaze does not peel off these shelves as described elsewhere. It seems to soak into the shelf just enough to cause problems in subsequent firings. We are finding this to be a real problem with porcelain. We are waxing the bottoms of porcelain with alumina in a water based resist to be on the safe side. In salt firings, we are dusting these shelves with alumina (oxide or hydrate) and using wadding on the bottoms of pots. Over-kill, but it works. We store n.b. shelves horizontally (with shims as spacers) between firings to keep the alumina in place (it is expensive). With care, we get a number of firings without re-dusting.

We have also applied wash (pick a recipe) thickly and used a grooved scraper to create peaks and valleys. This held up very well in the salt kiln and in some cases left a nice mark on the bottom of pots. It seems to be best, however, to dust the shelves, plain and simple.