Barium Carbonate

CAS: 7440-39-3

MSDS sheets indicate that barium 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.

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, standards for exposure are less restrictive than those for silica! The TLV (threshold limit value) in milligrams per cubic meter for barium is in the order of 0.5 whereas silica is 0.05-0.1.

Barium sulfate is used in medical x-ray exams; and in contrast to barium carbonate, it is insoluble, inert, and non-radioactive substance.

The Use of Barium in Clay Bodies

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 move to the surface with the water as it evaporates. During firing the 'scum' left on the surface forms a glassy discoloration and in some cases can even create 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 can cause unwanted aesthetic effects. 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.

Clay body and material supply companies commonly add barium carbonate to clay bodies in small percentages (0.1-0.8%) to solve this problem. The barium chemically reacts with the sulphates to precipitate insoluble products. In the reaction, slightly soluble barium carbonate 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 extremely effective and has not yet found a viable substitute.

While this practice is questioned by some as posing a possible leaching hazard in functional ware made from barium-containing clay, one must consider several factors that indicate otherwise:

-The barium is distributed within the clay matrix and a very small fraction of that which is added actually presents itself at the surface.

-The barium reacts with comparatively abundant fluxes and silica in vitreous clay bodies to form insoluble silicate glasses within the body.

-Functional ware is covered with glaze completely isolating the barium from food surfaces.

-Highly refined clays and minerals used in non-native clay bodies often do not require barium, however many are subjected to a wide variety of chemical processes that have a greater bearing on functional safety.

-The soluble portion of the barium converts to insoluble and harmless barium sulphate after the body is wetted during preparation.

-Barium carbonate makes it possible to make bodies using native natural materials with minimal processing.

-In the dissolution and precipitation process, a portion of the barium carbonate should, according to some theorists, remain unreacted. However, according to lab tests we have had performed, all barium has reacted.

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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.

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