View Larger Image Bowl, 6 in. (16 cm) in height, high-alumina body containing 10% red clay, soda glazed at Cone 9–10, reduction cooled, 2003.

April 14, 2007

Gail Nichols: 'Soda, Clay and Fire'

by Ceramics Monthly | Read Comments (0)

After years of research, one of the foremost practitioners of soda glazing shares her expertise in a new book.

The following are excerpts from the book “Soda, Clay and Fire,” by Gail Nichols, published by Ceramic Publications Company, a subsidiary of The American Ceramic Society. Read more about “Soda, Clay and Fire” in the Ceramic Arts  Bookstore.

Soda glazing was once hailed as an alternative to salt glazing, but has proven to be much more than that. The choice of vapor glazing is now primarily one of aesthetics, with soda’s potential extending far beyond that of imitation salt. A contemporary challenge is to explore what soda has to offer in its own right, and to set aesthetic directions for this new ceramic process.

What is Soda?

Soda, or sodium oxide (Na2O), is an active ceramic flux known for its bright color response in glazes. It is chemically related to two other alkaline oxides: potassia (K2O) and lithia (Li2O). Soda becomes unstable above 2192°F (1200°C), making it impractical as the sole flux in high temperature glazes, unless introduced in feldspathic form. However, its ability to volatilize at high temperature makes soda highly suited to vapor glazing. When sodium chloride or carbonates are introduced into a kiln at high temperature, they produce sodium oxide in vapor form. Where that vapor comes in contact with clay surfaces, it produces a glaze with a very simple composition: soda, alumina, silica.

Common sources of soda for vapor glazing include sodium chloride (salt) and sodium carbonates (soda ash, sodium bicarbonate), although sodium hydroxide (NaOH) has also been used. Borax (Na2B4O7•10H2O) is often used as a supplementary source of soda.

The use of salt in vapor glazing was developed in Germany during the twelfth to fifteenth centuries. Through its long history and widespread industrial use, the term “salt glazing” became nearly synonymous with vapor glazing. Industrial salt glazing declined in the mid-twentieth century, in favor of more efficient, economical and environmentally acceptable production methods. But the aesthetic merits of salt glazing continued to be valued and pursued by studio potters. During the 1970s, many potters seeking chloride-free alternatives to salt glazing began experimenting with sodium carbonates. For lack of a better name, this new form of vapor glazing became known as “soda glazing.” Technically, both salt and soda glaze are formed by clay materials being exposed to soda vapor.

How Is Soda Introduced to a Kiln?

At Alfred University in 1973–74, Jeff Zamek investigated three methods of sodium carbonate introduction:

1. Dry sodium carbonate/bicarbonate can be dropped into the firebox using a piece of steel angle. Frequent introduction of small amounts of material, dropped from the highest point above the firebox, gives the best results. This allows time for the soda compound to vaporize during its fall before landing on the firebox floor. Zamek noted that the firebox must be designed to cope with a buildup of molten soda.

2. Sodium carbonates can be introduced to the kiln through a burner-blower unit. This proved to be a highly efficient method of dispersing sodium carbonate vapor, but Zamek noted some dark patches on white clay bodies, which he blamed on corrosion of the burner/blower unit.

3. Sodium carbonate can be dissolved in water and sprayed into the firebox. This spray method proved to have good results, but excess sodium carbonate in the solution would cause the spray nozzle to clog up. A solution of 6 lb. of sodium carbonate to 12 quarts (.24 kg/L) of water was satisfactory (Zamek 1974, 2–3; 1999, 166–171).

In Rhode Island, U.S.A., Jay Lacouture developed a portable soda/sawdust injector that would introduce both materials to the kiln simultaneously. He claimed, “I could now be an urban wood/salt potter without using either material and be politically correct at the same time!” (Lacouture 1993, 29).

In Montana, Rick Pope mixed rock salt with soda to make it volatilize faster. He wrote, “This combination reacts nicely in the kiln and creates a much softer and less obviously salt-glazed surface than straight sodium chloride” (Pope 1993, 29).

In yet another approach, Richard Behrens suggested dispersing alkaline carbonates in a sizeable quantity of calcium carbonate (Behrens 1974, 44). Separating the soda particles in such a nonreactive medium prevents them from melting together and promotes volatilization. It also enables soda introduction as a solid mixture, rather than as a sprayed solution.

Using Behrens’ proposal to increase the efficiency of vaporization, I tried introducing a dry mixture of sodium carbonate, sodium bicarbonate and calcium carbonate into the fireboxes [burner ports] of my gas-fired kiln. The soda did vaporize, but failed to disperse through the kiln chamber. Pots nearest the fireboxes were over-endowed with runny glassy glaze, while the rest of the pots remained dry and unglazed.

The discovery of my current soda introduction method occurred in 1992, a fortuitous accident that occurred while I was firing the soda kiln in my backyard studio in Sydney, Australia. I mistakenly spilled a quart of water into a container of dry calcium/sodium mix, and watched the unexpected setting process take place. After a few minutes, I was left with a bucketful of a hard white substance that looked like plaster. Faced with the question of what to do with it, I decided to try introducing pieces of it into the hot kiln. It went into the fireboxes quietly, no obvious drama taking place, but the draw rings began to show glaze buildup, so I repeated the process. The real moment of discovery occurred a few days later when I opened the cooled kiln. Acceptable quantities of glaze had formed on nearly all the pots, and there were some exciting flashing marks where pots had been packed closely together. Following many months of effort, this was my first soda firing that could be deemed a success. It was indeed a dandy firing, and even the need to clean kiln shelves was cause for celebration. Here was a new and simple solution to the problem of soda introduction and glaze distribution. It opened my eyes to soda’s unexplored potential, and set the stage for further discoveries.

Clay

Soda tends to deposit large quantities of glaze on the pots it strikes first, leaving others unglazed or lightly flashed. A clay body that reacts with soda vapor too readily will result in runny, glassy, colorless glazes, particularly along the path of the flame where soda attacks the clay surfaces most heavily.

Coating pots with high-alumina kaolin slips to give them a more soda resistant surface proved a workable solution. Such slips act as effective barriers between the clay body and soda vapor, limiting glaze formation on heavily exposed areas while causing colorful flashing in lightly glazed areas (see Basic Soda Slip below).

The importance of the clay body in soda glazing cannot be overstated. In the absence of an applied glaze or slip, the clay body provides two of the major glaze components: silica and alumina (and to a lesser degree, some flux material). Understanding the roles of clay body components and their responses to soda vapor is fundamental to vapor-glaze work.

Using commercial bodies along with applied slips and glazes is a simple, practical approach to starting out with soda glazing, as it allows time and energy to be focused on mastering the mechanics of the firing process. Most potters who prepare their own clays for soda glazing also apply glazes and/or slips to the surfaces of their work.

 

Recipes
Basic Soda Slip Nepheline Syenite………………….....10 Kaolin………………………………..80 Silica (Flint) ………………………….10                                                           100%	Gail Nichols Soda Mix Light Soda Ash……………..……….....20 Sodium Bicarbonate (baking soda).….......30 Calcium Carbonate..................................50                                                             100%
Add: Bentonite…………………..................5% Blue: Cobalt Carbonate…...….......0.25–1% Red/Brown: Iron oxide..... ..................1–5% Purple/Black: Manganese Dioxide.......1–5% Gold: Rutile.......................................1–5% Turquoise/Green/Red: Copper Car......2–10% Commercial Stains...........................5–15%	Add 9 U.S. fl. oz. of water per 1 lb. (600 ml of water per 1 kg) of dry mix. Wearing gloves, mix the dry ingredients thoroughly, then add the water all at once. Stir until the mixture begins to set, then break it into small pieces.   As this mixture breaks down in the heat of the flame, water vapor is released along with the vaporizing soda. Water vapor helps to carry the soda through the kiln chamber, enabling good glaze distribution and evidence of flame movement on the work. Water vapor also appears to assist with soda dissociation and glaze formation.
Health and Safety Precautions   Soda ash dust is an irritant to the nose, throat, and lungs. In combination with lime, it will form sodium hydroxide (caustic soda), which can cause alkaline burns. Wear impervious rubber gloves and a NIOSH (National Institutes for Occupational Safety and Health) approved respirator mask. Chemical safety goggles are recommended for eye protection, and long sleeves and trousers should be worn. These precautions apply to all preparation and handling stages for the calcium/sodium mix, including handling the firebox residue, which is high in sodium hydroxide.