Soda firing is done in a conventional way, often eliminating the biscuit stage. It makes the firing process slightly longer but in the end it saves in time and fuel.
Soda can be introduced in a variety of ways: spraying it dissolved in hot water, drip-feeding the solution, as small blocks of various compositions, etc.
The drip feeding is an excellent method since it only requires a short length of angled iron and achieves a good and even distribution of soda. Heavy build ups of soda obliterate colour in the yellow/orange variety and make some slips run.
As the solution breaks down in the heat of the flame, water vapour is released along with the vaporizing soda. Water vapour helps to carry the soda through the kiln chamber, enabling good glaze distribution and evidence of flame movement on the work. Water vapour also appears to assist with soda dissociation and glaze formation.
The typical technique would be raw firing with pre-heating and slow raising temperature to take the clay through it’s biscuit process. If the work is already biscuit fired then, of course, the initial stage can be quite fast.
After soaking the kiln for about one hour at around 900°C, to burn out any carbonaceous matter to avoid any possible bloating, a reducing atmosphere is necessary to convert the ferric oxide compounds in the body and in slips to the ferrous state. Depending on the clay, often around 1150°C, it could be good practice to oxidise for a few minutes in order to avoid any carbon trapping when sodium already present in the kiln is beginning to vaporise and seal the body. Then maintain only a light reduction or neutral atmosphere throughout the rest of the firing.
Soda starts to be introduced into the kiln at cone 8 through cone 10.
The use of test rings is necessary to understand how the glaze is building up.
An oxidation soak of about an hour at the end of the firing will give warmer oranges from iron bearing bodies and slips.
The kiln is then crash cooled down to about 950°C.
Ruthanne Tudball is one of the foremost ceramic artist investigating and using soda vapour glazing. She has written a book “Soda Glazing” edited by A&C Black . London and first published in 1995.