Share
Salt, by virtue of its wide distribution and relative cheapness, is the traditional source of sodium in vapor glazing. Until comparatively recently it was the only feasible material available for such purposes. A brief look at the material itself may be of interest. Common salt (NaCI) is often known as halite, or rock salt, to distinguish it from a class of chemical compounds known as "salts." It is essential to the health of humans and animals. Table salt is finely granulated, and, being hygroscopic (moisture-attracting), contains additives to keep it free-flowing. Small quantities of sodium aluminosilicate, tricalcium phosphate, or magnesium silicate are added for this purpose. Pure rock salt usually contains none of these additives.
Salt is used in the manufacture of sodium bicarbonate (baking soda), sodium hydroxide (caustic soda), hydrochloric acid, chlorine, and many other chemicals, as well as in the food-processing and meatpacking industries. It is widely used in cold climates to melt ice, and it is employed in water softeners to remove magnesium and calcium compounds.
The Greeks and Romans often used salt as an offering in religious rituals, where its characteristics as preservative led to its symbolic representation of enduring qualities. Arabs used the expression, "There is salt [fidelity] between us," and, in English, an individual is respected for being the "salt of the earth." Cakes of salt have been used as money in Africa and as stipend in the Roman armies. The word "salary" derives from the Roman “salarium"—an allowance of money for salt.
Most salt comes from mining rock-salt deposits, such as those occurring along the United States Gulf Coast, by evaporating sea water—which contains salt in a ratio of 3.5 to every 100 parts—or by processing natural brines, which occur in Great Britain and the eastern United States.
SALT AS A SOURCE OF SODIUM VAPOR
It should interest any ceramist to know that salt melts at 1472°F. (800°C.)—far lower than the temperature at which most salt glazing is accomplished—and can be used in vapor glazing at raku or earthenware temperatures. As long as the clay body is mature, glaze will form as a surface coating and will influence slips and glazes by fluxing them.
To observe the process described above during a salt firing, one should wear glasses designed to protect the eyes from ultraviolet rays. Toss a tablespoon or two of salt into the firebox of the kiln when the maturing point of the clay is being reached.
Individual grains hitting the hot bricks will be seen to liquify instantaneously, forming a vapor which will follow the paths of convection and draft in the kiln. Small quantities of salt may be tossed or blown directly onto the objects being fired, if care is taken to avoid creating unnecessarily runny effects, which can adhere the piece to a shelf or the floor of the kiln. In passing from a solid to a gas, salt liquifies. In this state it is highly corrosive to refractories, especially those containing silica, which it attacks, causing spalling.
Small amounts of salt introduced rather frequently—at 10 to 20 minute intervals— will cut down on refractory wear. By comparison, larger quantities of salt thrown in at one time may be less effective, since the charges volatilize more slowly, producing greater quantities of liquid salt. In older kilns, especially, large salt charges may cause a molten flow to leak from the kiln, which will solidify on contact with air. From 1 to 2 c. (1/4 to 1/2 I.) of salt per charge may be considered average. Much more than this may choke off the rate of heat increase by lowering the temperature of the fireboxes.
Granulated salt, being of a fine texture, vaporizes readily as it enters the kiln, in some cases being sprinkled directly on the ware, as in groundhog kilns. The fine crystals are well suited to being blown into the chamber through a pipe attached to a vacuum cleaner motor, as well. Dendritic salt is the finest type manufactured, volatilizes extremely rapidly, and may be ordered through firms purchasing salt in large quantities.
Rock salt has a tendency to snap and pop when introduced into a hot kiln, at times creating dangerous projectiles, which may fly from the firebox with some force as far as 6 ft. For this reason, the use of goggles during salting is urged, especially when using rock salt. The larger crystals present more surface area to the hot atmosphere, and, containing more moisture than smaller granules, disperse with greater force.
Block salt, used to feed livestock, can be used if broken into manageable chunks for insertion into the kiln; but large pieces, especially if damp, could explode from the release of steam within a chunk, possible endangering the ware, bagwalls, and individuals nearby.
State of the Salt upon Introduction.
The question of whether to use dry, damp, or wet salt seems to provoke much discussion. Those who favor dry salt point to the ease of handling the material, the lack of corrosive liquids near metal burners and gas pipes, and the simplicity of eliminating yet another step in the process. The damp salt proponents point to the notion that more vapors in the kiln assist in the dispersion of sodium, itself in vapor form, promoting better results. This variable is simply one more that must be explored and tested individually to determine its applicability. Wet salt certainly increases pollution from the kiln.
Before leaving the matter entirely, however, it can be said for certain that the introduction of water into any heat-containing structure is potentially dangerous. Water can volatilize, forming steam with explosive force, damaging a kiln, and creating an extremely dangerous situation for all concerned. If salt is to be dampened, it is best contained in paper packets when introduced into the kiln.
Trace Minerals in the Salt.
The presence or absence of trace minerals in any form of salt used in glazing is largely a matter of personal preference. The purity of the material is generally sought after, even though trace minerals such as magnesium and calcium do occur and are bound to have slight but noticeable effects on the quality of the glaze. They may either flux or inhibit the melting of the sodium-alumina-silicate glaze, depending on their concentration in the salt and the degree to which they are already present in the clay. So many types of clay are in use, and so many varieties of salt available, that experimentation can be conducted quite easily. While personal preferences for one kind of salt or another may develop, the differences among them will probably not be radical. Drawbacks of using Salt. The chief drawback to using salt is the liberation- of chlorine gas, which accompanies the breakdown of sodium chloride at high temperatures. Water vapor, in the form of highly visible fog, is another drawback, especially where it may be mistaken for smoke from any uncontrolled fire.
SODIUM COMPOUNDS OTHER THAN SALT
Several sodium-bearing compounds other than salt are: sodium bicarbonate (NaHCO.,), Sal soda(Na2C03), known also as sodium carbonate, washing soda, or soda ash, and monosodium glutamate (NaOOCCH2 CH2CHNH2), used in the food industry. At the present writing all have been used with varying degrees of success, either as substitutes for salt or in combination with it. Of the many sodium compounds these are among the cheapest and most available.
Sodium bicarbonate is perhaps the least expensive and most readily available salt substitute. It can be purchased in bulk and is commonly referred to as baking soda. The tendency of sodium bicarbonate to dissipate slowly in comparison to salt has been a problem, but the process can be hastened by spraying or blowing it into the kilns. The glaze produced with sodium bicarbonate, while not identical to that made from salt, can be handsome, though the surface tends to be somewhat more "dry." The addition of 3%-10% borax to the baking soda may brighten the surface considerably, and if wood is used as a primary or secondary fuel, effects virtually indistinguishable from those obtained from salt vapors may be obtained.
The attractiveness of sodium bicarbonate as a salt substitute in glazing stems from the elimination of chlorine gas as a byproduct in the process. With sodium bicarbonate, carbon dioxide, a relatively harmless gas, is produced, along with some water vapor. Salt and sodium bicarbonate may be combined, which helps cut back on the less desirable effects of each. Sodium carbonate (soda ash) may also be combined with sodium bicarbonate, since in combination they disperse well, especially if blown into the kiln.. Experiments with various sodium compounds should be conducted in a new kiln where salt has. not been used, or at least new bricks should be installed in the firebox-bagwall areas, which retain most residual sodium from previous salt firings. Residual sodium from salt may cause corrosive build-ups when sodium carbonate or soda bicarbonate are used. Generally speaking, similar amounts of these compounds could be used, but smaller quantities may be efficient if the damper is shut for a few minutes after the introduction of the material.
In using any salt substitute, the tendency is to want to reproduce the effects of “salt glazing," at the risk of bypassing visual and t a c t i l e qualities which might be best exploited for their own unique characteristics. One of the most inhibiting factors in the glazing of any type of ceramics is preconceiving desired effects. Comparatively few individuals seem willing to try such materials as sodium bicarbonate and soda ash because the surfaces are different from those they had anticipated, whereas the effects might well be used to aesthetic benefit. This is an area where much more experimentation is needed and may be demanded as air-quality standards become more stringent.
AMOUNT OF SODIUM COMPOUND
Several factors influence this decision, and can be posed as questions:
What Type of Surface ls Desired? Assuming the kiln.is fired to a.temperature sufficient to mature the body, glaze accumulation will be directly proportional to the amount of glazing material introduced. In a small kiln—30 cu.. ft. (.9 cu.nn.)—coated with alumina, or composed of high-alumina brinks, as little as 1 to.3 lb (:5 to 1.5 kg.) of salt-may be necessary to produce well glazed objects. However, the same kiln made of unprotected" brick may require much more salt,-.especially for first firings, since,a comparatively small proportion of vapors ends up on the objects intended to be glazed.
What Type of Clay Will Be Used? Some clays are much more receptive to vapor glaze than others and require less sodium agent to be added to the kiln. However, a typical high-alumina clay with little free silica may take as much as three to four times the amount of agent to produce the same effect. The temperature at which glazing takes place is a further variable. Some clays may require much more sodium to produce a glaze at, say, cone 7-8, than they do at cone 10, when the silica may combine more readily with a smaller amount of vaporous sodium. Naturally, clay body experimentation will help determine this factor.
What Firing Methods Will Be Employed? Many variations in firing are practiced among contemporary ceramists. Some work with extremely tight kilns, using close damper control, and, in effect, are conservative in the way they retain sodium vapors in the kiln. Others use comparatively large amounts of glazing agents and give little heed to the obvious escape of great quantities of potential glaze-producing vapors. To a large degree, much of the "bad press” about salt glazing is due to such individuals. Simply stated, glazing operations frequently do not have to be as obvious as they often appear.
How Can Glaze Accumulation Be Measured? The best way during a firing is to use from 3 to 5 draw trials—rings of clay which are removed in sequence after specific amounts of glazing agent have been introduced. Since the draw trials are removed and cooled readily (they may be dunked in water immediately), they are invariably poor indicators of clay color but do show quite accurately the depth of glaze build-up.
What Is an Average Amount of Glaze Agent to Consider Introducing? About 10 to 20 oz. (300 to 600 ml.) per cu. ft,(.03 cu.m.) of kiln space.
METHODS OF INTRODUCTION
As with the decision of how much sodium compound to use, this is an area open to a wide range of possibilities. (1) The simplest method is to toss the glazing agent into the firebox area through the port made for the same purpose (although the traditional German and groundhog kilns are salted through ports in the roof of the kiln, directly onto the wares). The compound can be loose, in which case it may scatter around outside the kiln with corrosive effects on most metals, or it may be made up into packets of newspaper or paper cups to be thrown or dropped into the firebox area. Plastic bags should never be used since they liberate extremely dangerous hydrocarbons when they burn. (2) A simple metal pipe and plunger arrangement can be made by fitting a wooden handle onto a plug which slides piston-fashion in the pipe, pushing a charge of glazing agent out the end and into the firebox. A piece of angle iron about 3 by 3” (7.6 by 7.6 cm.) also works well, inserted in the port and tipped to one side to spill the charge into the firebox.
(3) The material can be blown into the kiln through a pipe inserted into the ports and connected to a compressed-air source or a vacuum-cleaner type of blower. (4) Sodium brine solution may be dripped into the kiln if care is taken to regulate the flow evenly. Too much water vapor entering the kiln has explosive potential and must be guarded against. A gentle dripping of brine may be an effective means of producing a foggy vapor in the kiln, but would mean devising a noncorrosive holding container and flow regulator, along with a feeding pipe or tube directed into a firebox or other area. I personally would never use such a system due to the complications and potential safety hazard.
(5) Brine-soaked wood thrown into the kiln is yet another means of introducing the glazing agent. Salt and other compounds can be dissolved in a crock or bucket of hot water, stirring the liquid until saturated. Sticks of dry wood or wood shavings can then be soaked and introduced after they are dried (wet wood ignites slowly and may cause the kiln to lose heat). Dry wood ignites rapidly, dispersing sodium vapors along flame paths, often flashing objects to advantage. Long slivers and sticks of such wood may be painted with soluble colorants and pushed into the kiln to flash nearby pieces when they ignite. Porcelain clay is especially receptive to such random effects. A few sticks of wood about 1 by 1 by 12" (2.5 by 2.5 by 30.5 cm), stoked sparingly during the salting, should show their effects on the ware.
The glaze produced in this manner may exhibit characteristics normally attributed to fortuitous "accidents" in firing and warrants exploration. Seaweed, salt-saturated sawdust, and most soft, porous woods will be found to work best. Such combustible agents can be made up some time in advance of the firing and dried to be used as needed. While large amounts of such supplementary fuel might be needed to add all the sodium in this manner, it should be tried by anyone willing to depart from a regular firing schedule, since it is an alternative with considerable potential for certain effects unobtainable by other means.
(6) Another method, long in use in central France, is that of placing salt in small—1A to 1/2 c. (.025 to .05 I )—clay containers throughout the kiln among the ware. As the temperature increases, sodium vapors permeate the kiln, flashing the pieces. Pottery made in the vicinity of LaBorne, near Henrichemont, south of Bourges, is an example of the effectiveness of this technique. Here the kiln reaches approximately cone 12; the ware is vitreous, and sodium vapors do not produce an orange peel effect, but show up as subtle highlights, often with pleasing shifts in clay tonality. No additional salt is thrown into the kiln during the firing.
(7) An experimental technique in recent use consists of placing one or more objects to be vapor glazed in a sagger, together with a cup containing salt and/or various combinations of borax, sodium carbonate, and sodium bicarbonate. As the soda compound volatizes, the objects in the container become glazed, and, since they are in a confined space, very little glazing agent is required.
The inside of the sagger may be coated with alumina hydrate or other sodium-resistant wash if desired. Small cups may be thrown, bisqued, and used to hold 1 or 2 tablespoons of say, 90% salt and 10% borax, or 50% sodium carbonate and 50% sodium bicarbonate. Pots in such saggers should be placed on wads in case the glaze-producing material flows out of the cup. Although this method needs more investigation, it remains a tantalizing alternative to conventional salt glazing because of the possibility of producing sodium-glazed objects without the need for a special kiln. Since vapors would be contained in the sagger, virtually no atmospheric effluents would come from the process. Several promising trials along these lines were done at the NCECA Conference at Louisiana State University and at the Penland School of Crafts in the summer of 1976. A surprisingly small amount of vapor agent was sufficientto produce obvious results, although some substances such as borax, which assists vaporization, left undesirable residual deposits in the saggers.