So glad you asked.
Soda firing is a type of stoneware firing wherein soda ash and baking
soda are introduced to the flame when the kiln is nearly at its
hottest. The heat of the firing splits the sodium carbonate and sodium
bicarbonate (soda ash and baking soda, respectively) into their
component elements, sodium and carbon. The flame carries this vapor
into the firing chamber and onto the pots. The sodium interacts with
the silica and calcium carbonate in the ware (as well any slips or
glazes that have been applied to the ware) to form a layer of glass
fired to a very high temperature - between 2200 and 2400 degrees
fahrenheit - so it's very durable.
The flame deposits the sodium more heavily wherever it strikes the ware
directly, and more lightly on the lee sides of pots, making for a
delightful irregularity. This can be observed whenever one side of a
pot is very glassy and shiny (as in the butterdish above), sometimes
with the design softened or melted away entirely. In fact this is the
great appeal of soda firing for me: it allows the flame to collaborate
in the final result.
For me, the soda vapor glazing does not replace but instead supplements
applied glaze. I choose colors and carefully apply designs, and wait
with delighted anticipation to see what the flame does to them. Soda
firing is not for control freaks! But if you love spontaneity and
uniqueness and surprises, if you can see beauty in irregularity, it
might be for you.