In face brick, soundness and color are the prime requisites. In pav ing brick, toughness alone is the prime requisite. If a clay contains sul phide of iron (pyrites) scattered throughout the mass, local slagged spots scattered all through the brick will be formed in burning. These slagged spots will be spongy or vesicular, i. e., full of cavities, just as is the black warty mass that appears on the face of a brick made from a pyritiferous clay. The local fused spots are detrimental to the tough ness of the brick, not only because they are spongy but also because they are fused glassy ferrous silicates, which are generally- very brittle and have no property in common with the tough, stony matrix which makes up the body of the brick.
As in weathering, the first step in the oxidation of iron pyrites is the separation of the iron and sulphur. In the kiln, however, sufficient length of time cannot be allowed to drive off more than one of the two atoms of sulphur. The first atom is expelled early in the oxidation period and passes off in the waste gases as sulphurous and sulphuric acid gases. The remaining atom of sulphur is not expelled readily, in fact either a very long time or much higher heat is required for its expulsion. In the customary heat treatment in kilns, this last atom of sulphur probably remains with the atom of iron until a temperature is reached that will cause fusion between the iron and silicates, form ing the black slag mentioned in preceding paragraphs. It can be said, therefore, that under the usual kiln treatment, iron pyrites is oxidized as follows: (1) One atom of sulphur is oxidized and expelled during the oxidation period. (2) The other atom of sulphur is oxidized and
expelled only by long heat treatment or higher temperatures. (3) The atom of iron, under the long heat treatment, will oxidize to the higher oxide forms before slagging begins, but under the usual heat treatment, in which sufficient time is not allowed for the oxidation and expulsion of the last atom of sulphur at a low temperature (about 500°C), the iron is not freed from its sulphur radical and oxidized to Fe0 until a temperature has been attained that would cause this Fe0 to flux with silicates.
From these discussions of oxidation it is evident that a good paver cannot be made from a pyritiferous clay unless it either be thoroughly weathered, or an unusually long time be given to thoroughly oxidize the iron and sulphur before fusion is allowed to take place.
Other Substances—There are many substances other than carbon and iron that suffer oxidation, but inasmuch as their oxidation is not at tended with serious difficulties and is, therefore, of little consequence to the paving brick manufacturer, they will not be discussed. The gases given off from clay wares under oxidizing and reducing conditions are just now being studied by ceramists. By these studies many phen omena in fusion, discoloration, etc., of pottery wares are being explained and the potter is receiving much benefit. It is not necessary, at this time, to discuss the results of these studies.