Nan Rothwell on wed 12 mar 97
David Cuzick asked me to tell him what I know about using soft brick (k-23)
in a salt kiln. Instead of replying to him privately, I am going to reply on
the list, since my information may be of some value to others.
David asks:
"Have your actually built a kiln using the Thermo k-23 or used them only in
places in your salt kiln?" I haven't built a whole kiln from them, but have
used them successfully in the doors of salt kilns. There is only ONE type of
softbrick that worked well -- the K-23 grade that used to be sold by Babcock
and Wilcox. I had a set of B&W K-23's last for over sixty firings. (When I
say "last" I mean they resisted salt and suffered no more than the usual
breakdown of softbrick that is unstacked and restacked in a door...) During
that same period, several other types of softbrick completely decomposed
under similar treatment. (The others included AP Green k-23's & k-26's and
B&W K 26's and a handful of other brands that I picked up from friends and
suppliers...)
When I recently came back to potting after a seven-year break, my kiln
needed repair and quite a few new door-brick. I had no luck tracking down a
source for a brick that resembled Babcock and Wilcox K-23 until a friendly
Clayarter (Heather Dixon) put me onto their current manufacturer -- Thermal
Ceramics in Augusta, GA -- phone 706-796-4200. I have now fired the
Thermal-brand k-23's twice, and so far they seem to be behaving like the old
k-23's
David's other questions:
Are you using salt or soda as your source of sodium?
Salt -- firing to cone 9-10.
Have you found a good coating to protect the bricks? I have been told that a
zirconium coating is good, any comments?
I've tried several different coatings, including a thin coat of the same
kiln-wash I used to use inside kilns (two-thirds alumina and one-third china
clay -- an old recipe from the Harrow School of Art), troweled-on castables,
and special "high-alumina" materials and washes. But over a number of years
of firing salt, I have gradually stopped using any coatings and washes on
bricks, shelves etc... I found all the washes and coatings that I applied
eventually created problems and needed to be ground off. Whereas if I let
the kiln arch and walls, shelf-props, shelves, etc. simply salt up, they are
less likely to have their outside layers separate and flake off. Also, I've
come to believe that while protective coatings appear to resist salt, they
are really just masking whatever damage the salt is doing behind them, and I
prefer to see what is happening behind the surface.
Please note that my observations about brick, castable, etc. in salt kilns
are anecdotal, not scientific. Over the years, I've built up prejudices and
superstitions about my firing process -- but have never kept good notes or
run any controlled studies... Despite my occasional vows to become more
systematic and orderly about potting, I always revert to my true sloppy,
intuitive style.
Hope this helps.
Nan Rothwell in Nelson County, Virginia, where I am going to start loading my
kiln as soon as I send this message...
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