Salt glazing is a technique that's continually evolving and being developed, despite being centuries old. Potters love its unpredictable, unique, and beautiful results.
All About Salt Glazing
Salt glazing began in Germany in the Rhineland region between the 13th and 14th centuries, although it didn’t become popular in England until much later in the 15th century. The Rhineland was known for its pure clay quarries, so the perfect place for innovations in ceramics to be developed. The area was also known for its busy docks that transported goods around Europe. It’s reported that the salt glazing began due to kilns in the Rhineland being packed with salt-soaked wood from the barrels that held brined food. The salt from the wood created vapors in the kiln that then reacted with the clay bodies when fired at a very high temperature. In its basic form, "salt reacts with the silica in the clay pots to produce sodium silicate." Sodium silicate is essentially a liquid glass and therefore naturally glazed the pots, using the properties from the clay. Classic salt glazed pots often had a very distinctive orange coloring. Potters soon realized the varied effects you could create by using salt in the firing process and despite the technique being discovered accidentally, it then encouraged plenty of potters experimenting with deliberately throwing salt into the firing to produce a stronger salt glaze effect.
What Is Soda Glazing?
Soda glazing came much later than salt glazing, although still uses the same technique of adding soda into the kiln at the optimum temperature. The difference is not in its usage or its technique but in its properties, as soda is less toxic than salt. Both salt and soda firing create sodium vapor, but sodium carbonates don’t contain chloride and are therefore less toxic.
How to Do a Salt or Soda Firing
Essentially salt glazing is salt thrown into a wood-fired kiln at the rough temperature the silica starts melting; this should be around 2372 F/1300 C. Note that wares should be bisque fired first before you start the salt glazing process.
To achieve the glaze, you’ll need to carefully add the salt to the firebox (slowly, using a steel angle, so it has enough time to vaporize before hitting the firebox floor). Some alternative methods potters use are to add sodium carbonate to water and spray it into the firebox.
The amount of salt you add is very dependent on the effect you want to achieve, but to get the "orange peel" look, which often characterizes salt firing, you’ll need to add roughly "a pound of salt per cubic foot of kiln volume."
Once the silica and salt have created the vapor and sodium silicate (liquid glass), they’ll start running down the pot. These distinctive runs are how you pretty much always identify a salt-glazed pot. Potters usually have a separate wood-fired kiln they do their salt firings in, as residue from salt vapors can build up on the inside of the kiln and affect other firings.
You’ll also need to be aware that the salt reacting with the sulfur can drop down onto your kiln shelves, and they’ll need proper cleaning after each firing. You’ll need to be very careful when doing a salt or soda glaze firing due to the chemicals that are created. So, make sure to wear protective clothing, goggles, and gloves throughout the process.
The Benefits of Doing a Salt Firing
Salt and soda firings can really affect any underglazes or slips you use on your ware, and the results can be very varied and interesting. Salt glazing also adds a brilliant texture to the ceramics, from the building up of layers to the running of salt vapors.
Potters Who've Used the Salt Firing Technique
One of the most famous studio potters to embrace salt firing as a technique was the innovative Bernard Leach. Down at his studio in St. Ives, Cornwall, he embraced the technique and one of his most loved "accidentally salt glazed" pieces sits in the Victoria and Albert Museum in London’s Kensington.
Michigan born Gail Nichols now lives in Australia and has been creating ceramics with soda vapor glazing since 1989. Her incredible wares are inspired by nature, and she’s written an informative and inspiring book called Soda, Clay, and Fire.
American potter Chris Baskin also creates and sells some beautiful soda fired wares.