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multiple types of firing in one kiln:

updated sun 14 nov 99


I.Lewis on sat 13 nov 99

April Pauley Mink asked about the following : multiple types of firing in =
kiln : mentioning that she intended using Sodium carbonate in her firings.

I would be worried about using K-23 insulating bricks if the kiln is to be =
to top stone ware temperatures. I would go up to 26s Olsen suggests that the=
service temperature would be cone 6. I have not used the newer coatings and
relied on alumina-kaolin in my salt glaze kiln. But I had dense high alumina=
bricks. My own feeling is that although some people have successfully used
porous refractories in vapour glazing kilns, the life expectancy of a kiln =
be shortened, though to what degree, only experience would tell. Factor this
cost into your depreciation and add on to the price of your product. If you
anticipate a hundred firings before a rebuild and only get ninety, then add =
about ten percent.

Any sodium compound in excess of what goes on the pots will go up the flue =
settle in the hearth. As the hearth is the first thing to get hot, and gets =
hottest it will be the first place from which volatiles emanate. If sodium
chloride is used it will volatilise to varying degrees depending on how =
you approach it=92s boiling points. Sodium carbonate melts at 851degC =
Unless it is in contact with a silica based compound above that temperature =
only looses its carbon dioxide very slowly. It does not behave like calcium
carbonate. Heating in a platinum crucible with repeated re-weighing has
demonstrated that sodium carbonate is very stable at red heat. Soda solution
sprayed into a kiln does not vaporise but seems to become an aerosol. So I =
the effects may be less persistent than is being anticipated, if there are, =
fact, any worth noting.

Ivor Lewis