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Salt and Soda firing

Salt firing is a process where unglazed ware is fired to high temperatures and salt is introduced to produce a vapor that glazes the ware.

Key phrases linking here: salt and soda firing, salt glazing, salt-glazing, soda firing, salt fired, salt-fired - Learn more

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Plainsman P580, P600, H570 soda fired samples
These are standard mullite porcelain bodies designed to fire a little short of full density (or vitrification). They are made using #6 tile kaolin, Tennessee ball clay, nepheline syenite and silica (P580 has no #6 Tile and P600 is 50% #6 Tile). The firing process produces a range of areas having complete glaze coverage to others having no glaze and full flashing.

Salt (or soda) firing is a process where unglazed ware is fired to high temperatures (typically cone 10) where sodium-sourcing material is introduced into the kiln chamber (through the burner ports). Decomposition produces gas by-product (Cl or CO2) and Na flux vapor that reacts with the hot clay surfaces. A thin sodium alumino-silicate glaze thus develops directly on the ware surface.

Many are able to revel in the mystique of the process by simply employing the recipes and procedures of others and using their kilns. But the most successful craftspeople work to understand the chemistry and physics that produce the body and glaze character and the flashing. Of course, most important among these is experience with the kiln and its vapor path, when and how to introduce the salt or soda and how to avoid over or under saturation. However, using the right bodies and developing the right slip recipes is equally important. No conventional glaze is normally needed because the kiln atmosphere itself creates the glaze. That being said, interiors of functional ware may receive a conventional glaze for smoother sealing and better fit.

The salt glazing process has many historical roots. Salt glazed ware typically has distinctive marbled and variegated surface effects - many books showcase these. An increasing number of web pages extol the process and potters pay considerable amounts to attend workshops to learn about it and produce ware. There is, however, a bias toward describing the process in the language of art rather than science. One potter described her preference for the process saying that "the voice of the fire just does not speak to her" in electric-fired ware, with soda firing her ware "says what she wants it to say". And the historic and cultural biases, rather than logic, tend to push the methods and processes used.

Salt glazed ware can be suitable for functional use, but there are caveats. Sodium has the highest thermal example of all common oxides so glazes are almost certain to craze. The impact ware strength and functionality is obvious. Not surprisingly, the oxide formulas of salt glazes fall well outside normal glaze chemistry (a fitted liner glaze on functional pieces is an option some use). Sodium is a very active flux and the salt-glazing process is theoretically feasible for bodies vitrifying at lower temperatures.

P600 salt fired tea set, with flashing, by Jim Etzkorn

There has been quite a bit of discussion about the safety and environmental concerns of salt vs. soda firing. It does not appear to be a foregone conclusion that soda is better than salt or even that chlorine is released in salt firings (rather than HCL vapor). Some reports claim that measurements done on kilns have demonstrated that salt firing is as clean or cleaner than fuel reduction firing.

Although salt and soda firing are closely related; the kiln chemistry, surface character, and firing behavior differ enough that experienced potters treat them as two distinct processes. In salt firing, common salt (NaCl) decomposes releasing sodium as flux and chlorine as gas. In soda firing sodium carbonate (or bicarbonate) release sodium more rapidly (but without chlorine). Salt is more aggressive, its vapor penetrates strongly (especially to interiors), surfaces glaze more readily (to classic thicker, glossier orange-peel texture). However, kiln walls suffer more attack, and burners and refractories deteriorate faster with salt. Soda is thus better for localized effects and selective flashing. It produces softer surfaces, more variation, better flashing and subtler oranges, reds, yellows, greys.