William Brouillard on thu 15 aug 96
My understanding of the salt soda vapor thing is that salt has a
shorter liquid phase and a higher vapor pressure than soda bi-carb..
When soda or salt is introduced into the kiln it must first absorb
enough heat to melt. After melting it then absorbs more heat and
turns to vapor. The vapor moves through the kiln and combines/reacts
with the clays and glazes.
The hotter the kiln the faster the melting, vaporization, and chemical
reaction with the pots. Since salt has a shorter liquid phase and
a higher vapor pressure it produces a more even salting than soda.
You can just throw some in and it works fine. Soda, ever the problem
child has to be given special handling to even come close to the
same effects. Soda can be introduced after mixing with water. It can be
sprayed intothe firebox, (never at or on the pots). My pet theory
on this method of introduction is that the steam formed from the
water acts to disperse the soda particles and speeds up the melting
and vaporization because heat works faster on small particles that
have lots of surface area and very little mass. Read Ceramic Science
for the Potter by W>G> Lawrence. There is a section on salt vapor firing.
The favored method for a low sheen orange flashing on white bodies
was to wait until someone had fired the kiln and used lots of soda
in an attempt to obtain salt like results. After the fireing the walls
of the kiln were painted with a soda solution and then the
kiln was loaded and fired with no soda being introduced. ALso see an NCCECA
paper od soda by Jeff Zamik
1011 literary road