The Use of Barium in Clay Bodies
MSDS sheets indicate that barium (CAS: 7440-39-3) is poison and toxic if ingested. MSDS sheets also claim that barium is an irritant for inhalation and skin contact. Excessive amounts can cause violent diarrhea, convulsive tremors, and muscular paralysis. Barium is known to affect the heart and nervous system.
In ceramics barium is most commonly inhaled as a fine dust or absorbed through cuts or lesions on the hands of people who handle compounds containing the material. However, some authorities quote exposure limits that are less restrictive than those for silica (e.g. ACGIH lists a Threshold Limit Value at 0.5 mg/cubic meter for barium while quartz is 0.05). OSHA, on the other hand, lists quartz as 10.0 and barium as 0.5. (see the links to learn more about TLV, PEL).
Almost all raw clay materials (including kaolins) contain soluble impurities (i.e. iron stained calcium/magnesium sulphates). When a raw piece of clay is dried, these impurities come to the surface with the water as it evaporates. During firing this often iron stained precipitated scum left on the surface forms a glassy discoloration and in some cases can even creates a glaze-like surface.
Soluble salts are highly prized in sculpture bodies because they highlight surface textures and contours. This is especially so if the salts are iron stained and create variations in coloration. However, in functional pottery and structural products these salts are not wanted. Even after a clay is fired, these soluble salts can come to the surface if the clay is not vitreous and it is rewetted often (this is often the case with red terra cotta brick, the brick industry calls this phenomenon Efflorescence). These surface deposits often affect adherence to the clay and melting patterns of overlying glaze layers. Fused solubles on the surface can also stick ware together or to kiln shelves and they can seal the surface causing premature bloating during firing.
Clay body and material supply companies commonly add barium carbonate to clay bodies in small percentages (0.1-0.8%) to solve this problem (heavy clay industries use more barium, others strive to reduce it to a minimum, often for production reasons). The barium chemically reacts with the sulphates to precipitate insoluble products. In the reaction, very slightly soluble barium carbonate (in contrast with barium sulfate used in medical x-rays which it is insoluble, inert, and non-radioactive) and soluble calcium sulphate convert to insoluble barium sulphate and calcium carbonate. Thus they both remain within the body and do not concentrate on the surface during drying. The ceramic industry has found this strategy to be effective and has not found a viable substitute. It is possible to measure the amount of solubles in a clay body and calculate the minimum amount of barium needed to precipitate them. However in practical terms this does not work because different types and even shipments of barium vary in their effectiveness and the natural salt content within the clays varies greatly. It is far more effective to simply observe the effect of various additions and adjust periodically so that there is just enough to remove the solubles (barium is expensive and industry obviously does not want to use more than is necessary).
Controversy Surrounding Toxicity
The presence of barium in clay bodies is sometimes thought to be a health hazard for hypersensitive people. Barium is so very slightly soluble that it is difficult to demonstrate solubility of any material in a simple lab experiment (MSDS sheets even list it as insoluble). Nevertheless, the precipitation of dissolved calcium and magnesium salts in the body can only occur if dissolved barium is present, this demonstrates some solubility. You need to add alot of barium carbonate (up to 0.5%) to a body to generate the small amount of dissolved barium needed for the reaction. The undissolved barium remains in the carbonate form. It must be remembered that quartz is completely pervasive in almost all ceramic glazes and clay bodies and any balanced view of safety should put primary emphasis on it. In addition lab tests to deduce barium carbonate content in aged pugged clay bodies have shown a much lesser quantity of unreacted barium than the slight solubility suggests is possible. This means the issue cannot be fully understood by considering only the slight solubility of barium. Further, as noted above, permissible limits for barium can be higher than those for quartz, thus the conclusion that clay containing barium is a highly toxic material is questionable since typical clays can contain 20% or more quartz and yet only a tiny fraction of 1% of soluble barium. An MSDS from BassTech International classifies barium as moderately toxic on inhalation, non-toxic on contact or skin absorption, an eye irritant and toxic if ingested at 57mg per kg of body weight (this would be over 1 kg of pugged clay containing the highest proportion of barium normally used).
Leaching Hazard When Used in Clay Bodies?
The practice of adding barium to clay bodies is also questioned by some as posing a leaching hazard in functional ware made from barium-containing clay. However one must consider some factors that indicate otherwise:
The US Food and Drug administration does not have a leaching requirement for barium. However, the US EPA has a standard for drinking water of 2 mg/liter or less. This standard was set to protect the population most at risk for heart effects - adult men. Testing can be done at Elemental Research Lab, 309-267 West Esplanade, North Vancouver, B.C. Canada V7M 1A5 604-985-0445 (they test many metals). Also at Kirby Health Center Lab, 71 North Franklin Street, Wilkes-Barre, PA 18701 717-822-4278.
A typical DFAC drying disk of an iron stoneware clay
The center portion was covered and so it lagged behind during drying, setting up stresses that caused the disk to crack. This test is such that most pottery clays will exhibit a crack. The severity of the crack becomes a way to compare drying performances. Notice the test also shows soluble salts concentrating around the outer perimeter, they migrated there from the center section because it was not exposed to the air.
Soluble salts on cone 04 terra cotta clay bodies
Low temperature clays are far more likely to have this issue. And if present, it is more likely to be unsightly. The salt-free specimens have 0.35% added barium carbonate.
The magic of a small barium carbonate addition to a clay body
Two bisqued terracotta mugs. The clay on the right has 0.35% added barium carbonate (it precipitates salts dissolved in the clay to prevent them coming to the surface with the water and being left there during drying). The process is called efflorescence and is the bane of the brick industry. The one on the left is the natural clay. The unsightly appearance is fingerprints from handling the piece in the leather-hard state, the salts have concentrated in these areas (the other piece was also handled, but has very little marking).
How bad can efflorescence of soluble salts be?
Like this! This terra cotta clay vitrifies here at 1957F (cone 03). This problem is common in many terra cotta materials but can also surface in others. Barium carbonate can be used to precipitate the salts inside the clay matrix so they do not come to the surface on drying.
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