The practical lessons to be learned from these two experiments, the first with granite dust and the second with iron free as an oxide and combined as a silicate, are : (1) that when combined as a ferrous sili cate the maintenance of strictly oxidizing conditions in a kiln will not result in oxidation of the iron; (2) that iron oxide uncombined is not only easily reduced but- will form ferrous compounds when fused with silicates; (3) that when iron is combined as a ferric compound with alumina and silica, it will retain its ferric condition against the re ducing influence of fusion and hence is very apt to retain its "ic" form even under reducing conditions. This latter statement is an assump tion, for no direct evidence bearing on the point is at hand, but deduc tion from known data seem to leave no doubt as to the validity of the assumption.
In clays we have iron combined with silicates in a large variety of mineral forms and compounds. If these compounds are stable when heated, the iron will retain its form of combination against oxidizing and reducing influences. Iron when combined as a silicate, therefore, will not be affected during the oxidation period. In this connection, however, chemical analysis of a given clay will not show exactly how much of the iron is combined or in what form it is combined with the silicates.
Structure of clay ware—Orton and Griffin have shown that the more porous the brick the more readily can it be oxidized. Soft-mud bricks by actual porosity determinations were found to be the most porous, dry-press somewhat more dense, and the stiff-mud bricks the most dense. Our experiments have shown also, that clay issuing from the machine die in as dry. or "stiff" a condition as is compatible with form ation of a perfect bar, will produce a denser brick than when the bar is permitted to issue in a softer condition. While maximum density in unburned bricks means minimum toughness that can be produced with a particular clay, it means also maximum difficulty in oxidation. This, however, is a minor factor in the problem of oxidation of clay wares, for an easily oxidized clay will still be easily oxidized and a difficultly oxidized clay will be difficultly oxidized, no matter how dense the wares may be in either case.
The thickness of ware and consequently the manner of setting is a more important factor than density of the clay body. Hollow goods, where the walls are thin, would be completely oxidized under conditions that would not permit the complete oxidization of bricks manufactured from the same clay. Depth to which oxygen must penetrate is ob viously the effective factor in these cases.
Temperature as a Factor in Oxidation—Quite obviously, the higher the temperature the more rapid will be the combustion of the carbon.
In the case of clays free or practically free from iron, or where the iron is in a stable silicate combination, rapid combustion at high tem peratures would have no attending evils and would materially shorten the oxidation period. When the iron is present in the "ous" condition, or where it can be easily reduced to the "ous" form, combustion of the carbon at temperatures above 1000° C. would result in partial slagging of the iron with the silicates forming a dark gray mass that cannot, without expenditure of excessive time, be reoxidized. Such action would cause premature fusion of the clay mass, especially near the bag walls of the kiln, and, as a consequence, careening of the whole "setting," or at least ,a falling over and fusing together of the bricks near the bags.
Oxidation at too high temperatures is frequently shown by a per manently discolored center or core, in which vitrification has progressed further than in the outside shell of the brick. Orton and Griffin found that 800° C was the safest temperature at which to oxidize the average clay. In some rare cases, like the clay found at Loraine, Ohio, which Orton and Griffin cited, even 800° C would be too high for safe oxida tion.
Moisture as a Factor in Delaying Oxidation—In the majority of yards which were visited by the writer evidence could be found of incomplete oxidation of a few bricks in an otherwise thoroughly oxidized kiln of brick. Inquiry developed the fact that in most instances, in the rush to make a' day's work, the setters would set the bricks as they came from the dryer, no matter how wet or dry they may have been. In variably either the head setter or superintendent would recall that a carload or two of wet bricks were set in the particular place where the unoxidized bricks were found on "drawing the kiln" It is evident, therefore, that moisture in the bricks has an influence on oxidation of the clay.
Theoretical calculations, laboratory experiments and factory observa tions have proved that wet brick set in a kiln of dry bricks, are de layed in heating up by the fact that the heat, which in case of the dry bricks is sufficient to carry it well into the oxidizing period, is spent in evaporating the water from the wet bricks, thus delaying their "heat ing up" process. Bricks thus delayed will not be heated much more than is sufficient to cause the beginning of oxidation, when in the bulk of the bricks oxidation is completed and fusion begun. Under these conditions the bricks that were wet will pass through the oxidizing period (450 to 1000° C) too rapidly to permit their complete oxidation. Water, therefore, indirectly delays oxidation.