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Barium in Materials and Fired Glazes

Raw barium carbonate powder is well known as a poisonous substance. Barium sulfate powder is insoluble and non-toxic. Many frits contain barium and are considered relatively safe to handle as powdered materials. Barium carbonate performs certain physical and oxide sourcing functions in ceramic glazes and bodies for which substitutes can be either very difficult or expensive to implement. Determining the threat of barium is not as simple as simply saying "all uses of barium are dangerous". Barium sourcing materials, like barium carbonate, supply BaO to glazes which may or may not be soluble in acids that come into contact with the glaze surface. This solubility depends on the chemical balance of the glaze, that is, the degree to which it locks the BaO molecules into the glass structure. In bodies, minute percentages of barium carbonate precipitate soluble salts that cause surface scumming in fired ware and no other material as effectively performs this function. Thus discussions of barium toxicity relate to the actual use of the powder in the studio or plant and the degree of solubility of ware made using barium compounds.

In cases where barium carbonate is employed for chemistry reasons, that is, to specifically supply BaO to the fired glaze, hazards related to its raw form can be minimized by using a frit that sources barium oxide. This involves doing chemistry calculations to introduce the frit without changing the overall chemistry of the glaze.

Hazards related to the quality of ware produced are not connected to the material sourcing the BaO, but to the availability of BaO ions to solubility. This can be a complex issue and tradeoffs must be considered. Where small percentages are involved, the likelihood of danger is also small if the glaze is balanced (e.g. well melted and having adequate silica and alumina). But at the same time small amounts are likely candidates for substitution with other fluxing oxides without significant affecting physical properties or appearance of the glaze. Where larger amounts are involved successful substitution is much less likely. Crystalline matte surfaces done with barium, for example, often have a character that is difficult to duplicate with other oxides. There are many frits intended for the production of matte glazes that employ BaO as the mechanism. One alternative is to develop a base matte glaze using another mechanism (i.e. high CaO, SrO) and substitute the colorants, opacifiers and variegators into it. However, the presence of barium is key to the development of certain colors (e.g. barium blue) and substitution of other oxides often will not work.

It is possible to achieve barium-unique effects and still have a glaze that has low solubility. However most would agree to the ethical position that such glazes should not be used on food surfaces without additional leach testing on over and underfired ware. Even then, the possibility that a glaze has a fragile chemical balance that brings leachability within acceptable limits still presents the possibility that small variations in materials could disproportionately increase solubility rates.

One final note: Barium is considered dangerous when swallowed, however it is much less toxic as dust or in skin contact. The author has worked in a local ceramic industry that has routinely used barium carbonate in tile, brick and pottery bodies for 90 years and the silicosis threat from quartz in the clay has always completely overshadowed toxicity ingestion or inhalation issues surrounding barium. Here is another interesting comment we received: "The EPA in the US allows up to 2 mg per liter of drinking water, and that's far below the lowest level at which anyone has actually observed toxicity. I think the lowest acute dose that can sometimes result in death is about 800 mg, and at that level it can be treated by administration of potassium to counteract the muscular effects of hypokalemia."

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By Tony Hansen

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