Section: Materials, Subsection: Toxicity

Description

Toxicity concerns about the use of firebrick or alumina brick to make ovens used for baking bread

Article

Many people have expressed concern about the toxicity of bricks that come into contact with food (e.g. as in bread ovens). Companies that supply these bricks are hesitant to discuss such things both because of lack of knowledge in this area and legal liabilities. This article is presented as a conceptual overview that might help you to ask the right questions.

For toxic metals to be leached into food or drink they need to be present and loosely bound in the crystal or glass matrix of the ceramic. Where complete melting has occurred there is a better chance that metals are a secure part of the matrix but only if the glass has a balanced chemistry and is not itself toxic (e.g. lead glass). However in the case of fire bricks, very little if any glass development has happened, only sintering (the bonding of the matrix by changes in the mineralogy (and therefore shape, size and surface) of particles associated with the heat treatment). Most toxic metals are also fluxes, (e.g. barium, lead) or colorants (e.g. cobalt, manganese) and many are both (e.g. cadmium, selenium). These are thus not desired in firebrick since they impede its performance. In theory a typical brick should be on no more concern than the
average rock, the kiln is basically a device to create metamorphic square rocks!

However there is one area of concern I might mention: Barium carbonate is often used in structural clays to precipitate soluble salts in the clay so they are not brought to the surface with the water during drying (these solubles create an unwanted glassy scum on the surface of the brick). A reaction takes place that converts the slightly soluble barium into non-soluble barium sulphate. Additions of 0.3% are typical. Particle size of the barium is also a factor, if fine enough it should react during firing at the high temperatures necessary (barium is a flux) to bond with neighboring particles and thus be insoluble. Also, very little of the barium would be exposed at the surface of the brick.

Also, bricks are made from clay and other minerals, that by their nature contain every element on the periodic table in minute amounts. Pottery is the same. However the ceramic industry has not considered this a factory in judging potential toxicity, at least no that I am aware of.

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