by Paul Soldner
I am often asked why there isn't any written information on how to do low-fire-salt fuming. Despite the fact that it has been practiced for more than 20 years, I don't know of any books or articles giving specific directions. The following are concepts and methods I have learned mostly through trial and error.
In the beginning, it should be expected that there will be even more accidental effects from low-fire salting than ever found in raku. Perhaps this is the reason that so little information is available. Nevertheless, with experience accumulated from each firing, potters can discover what works best in their own kilns. And, yes, soda can also be fumed in the same way as salt.
The Clay Body
Almost any clay can be used in low-fire-salt fuming, but if orange-flashing effects are desired, then the body should include some iron oxide. If slips, terra sigillata or stains are to be applied to the surface, the clay body can be any stoneware, porcelain or a raku blend.
My favorite low-fire-salt body is a mixture of equal parts plastic fireclay, Kentucky ball clay (OM 4), red clay and sand (20 to 60 mesh). Note that there is an absence of flux; however, salt vapor fluxes the body, making it harder than regular bisqueware even at Cone 010.
Either an updraft or downdraft fuel-burning kiln can be used. Excellent results can be obtained with hardbrick, softbrick, even fiber kilns, but the burner ports must enter horizontally. Kilns with bottom burners cannot be used because salt cannot be volatilized anywhere in the kiln except in the burner flame itself.
A salting port should be located directly above each burner so that salt can be dropped into the flame. Because it is important that salt fall into the flame, each burner port should be no higher than the kiln floor. If it is higher, build a salting platform level with each burner. The kiln also needs a peephole near the bottom of the door so that the quality of the atmosphere can be inspected during the firing. Finally, there must be a primary-air control on each burner.
Effects of low-fire salting can be compared to high-temperature wood firings. Variation is enhanced by the flame moving through the ware; there is also a flame-resist effect when work is tightly stacked and touching. In some ways, flame movement is similar to the beautiful patterns produced by a river flowing under, over and through rocks. In this case, the flame is the river and the pots are the rocks.
Specific patterns can be achieved by masking surfaces with thin (approximately 1/8-inch-thick) pancakes of clay. Several layers of pages from glossy magazines or even thin slices of wood if placed under a clay pancake will produce dark gray patterns. Keep in mind that the shapes of the masking objects that touch the surfaces will have an effect on the patterns they leave.
Other patterns can be achieved by embedding rock salt into the clay pancake. A few large grains of salt will leave a beauty spot! Metal oxides or organic materials like seaweed can also be used.
The kiln, properly stacked for low-fire-salt fuming, will look like a disaster area to the uniformed observer. And it will necessarily be completely full. Shelves are not only unecessary, but actually nonproductive, as glazes are not used and the temperature is so low.
If there is a small amount of iron in the clay body, spectacular oranges, yellows and brown flashing can be expected. Raku slips will also have a positive reaction to the process. A raku white slip containing 1 part Gerstley borate, 2 parts flint and 3 parts kaolin will often flash a beautiful pink from copper added to the salt or fumed off other copper-decorated ware. The same slip with 3%-5% copper carbonate added will be even more reactive.
Colors (from copper in particular) are often quite varied because of the complex stacking, which results in reduction-oxidation and neutral flames licking over the object simultaneously. Similar variation in flashing can be anticipated from the use of terra sigillatas; however, the sigillata should be applied very thinly and/or fired to a higher temperature (possibly from Cone 06 to Cone 01) to prevent cracking. Applying the sigillata to damp bisqueware also seems to help. Remember, less planned decoration is better, and none is often good enough.
The firing cycle is approximately the same length as a bisque firing. Although stacking, surface preparation and body composition are important, it is the quality and the quantity of the flame that make low-fire-salt fuming so different from other firings. To begin, the primary air on each burner is reduced to make a long, dirty, soft yellow flame for the entire firing. Oxidation and reduction cycles of glaze firings are of little significance in the low-fire salting; however, to pull the flame through the ware, dampers need to be open throughout the entire firing.
Of utmost importance is the need to fire the kiln with excess fuel. This is determined by observing the pressure at the bottom peephole. Above 1300 degrees Fahrenheit, visible flames should be exiting constantly from the peephole. If this state cannot be maintained, increase the gas, add extra burners or drill out the burner orifices until flames are obtained.
Of course, this is a reducing atmosphere, except that it is achieved with the dampers opened and the kiln drawing. Curiously, cones will change their melting point and are therefore not an exact indication of the actual temperature, but are close enough to warrant their use.
Before loading the kiln, place salt in the flame path of each burner. A mound the size of a large orange is a good amount to start with.
When the kiln turns dull red, at about 1000 degrees Fahrenheit, add more salt to each burner. For convenience, the salt can be wrapped in newspaper to form a salt "burrito," then pushed through the salting port into the flame. Additional salting every hour should be enough to achieve the desired effects.
A small amount of copper carbonate added to the salt may be helpful in encouraging a pink blush on the otherwise white slip surfaces. Many other oxides may be used to modify the fuming effects, but none as dramatically.
Oversalting may dull the surface color, so be conservative. Salting at the end of the firing is optional. Personal experience will determine its importance or not. Also, experiment with closing the dampers during the salting cycle, but only for a few minutes.
Work decorated with slips can be fired from Cone 010 to Cone 06. Terra-sigillata surfaces are better fired higher, from Cone 08 to Cone 3. Because there is no glaze to melt, precise temperature control is not a problem.
After the cones have melted and the last salt burrito has been added, the kiln can be shut off and cooled in the usual manner; no other treatment is needed or helpful. A good rule is to cool the kiln in the same amount of time it took to fire it.
In order to protect the somewhat soft surfaces, apply one or two coats of acrylic floor wax (such as Futura) diluted about half and half with water to both the inside and out. If the result is too shiny when dry, dilute the wax a little more. The coating will preserve the colors and allow the work to be cleaned by washing with water from time to time.
|Ceramics Monthly, April 1995, pp. 41-44.|