Pottery is made with clay—and glass.
I made the above-pictured mug from a red clay that I dug in Idaho. You can see the natural clay on the bottom half. The top half is covered in a glaze. I threw this mug on the wheel, added a handle, and dunked the mug into a bucket of liquid glaze—a mixture of finely powdered minerals suspended in water. Those powders are left on the surface of the clay after the water has dissipated, and when it all goes into a hot kiln, the powders melt together into a glass that fuses onto the clay.
The glaze forms an impervious layer that strengthens the overall piece. Glazes can be formulated to produce a wide variety of textures, patterns, and colors. To do so requires a good amount of actual chemistry, with all the associated fiddling over measurements and particle size and molecular formulae. But there is another way.
Unlike the red mug, this cup was never dunked in a bucket of carefully formulated glaze slurry. Instead, it was put raw into a kiln, heated to over 2000°F, and then bombarded with volatized sodium, sourced from soda ash. The sodium mingled with the silica in the clay, causing the silica to melt into a glass on the surface of the pot. This process is called soda firing. Soda firing is a kind of glazing on-the-go.
Soda ash is a trade name for sodium carbonate (Na₂CO₃). It’s the sodium that we’re looking for when we put soda ash into the kiln. The sodium acts as a flux, a material that causes the silica in the clay to melt at a lower temperature than silica could melt by itself.
Could other sources of sodium be used to flux the silica in clay to form a glaze? Sure. Soda firing was developed in the 1970s, initially as an alternative to the then-common method of salt firing. Salt—sodium chloride (NaCl)—was thrown into the hot kiln to source sodium to flux the silica in clay and produce a glaze on-the-go. The effects are different between salt firing and soda firing, but that basic premise is the same. Other common sources of sodium, such as baking soda (sodium bicarbonate) do not volatize in the kiln, and so are not useful for this process.
Soda firing doesn’t produce a monochromatic or uniform glaze surface. Part of the beauty of this method is that it accentuates the clay itself by produces an effect known as flashing. Flashing refers to the colored markings left on the clay by the flames in the kiln. That’s all of the oranges and reds you see in the picture below. In a soda firing, every piece is unique because every piece documents a different interaction with the fire.
Soda firing and glazing are not mutually exclusive—you can absolutely soda fire a piece that has been dunked in glaze. That’s what I’ve done with this week’s newest pots. They are each covered in glaze, then soda fired. The soda fluxes the glaze, causing it to melt more. The soda also affects the color of the glaze, producing variations that wouldn’t occur normally. And because the soda does not hit the pots uniformly, each side of the pot looks unique.
I hope this little bit of insight into the process of soda firing can help you appreciate the beauty of a fairly esoteric method of making pottery. I know that the more I learn, the more I fall in love with it.
niiiice