Singer' has shown that the acid radical (CO2) is expelled from fer ous carbonate at temperature below 430 C. The basic radical (FeO) would thus be given ample time to become thoroughly oxidized to Fe3O4 or Fe203 before the temperature could be raised sufficiently to cause fusion between the ferrous iron and the silicates. Under normal kiln treatment complete oxidation of the iron would be effected, provided the clay mass contained but a small amount of carbon. In the almost total absence of carbon, our experiment with the concretionary mass proved that the iron could be quite readily oxidized. As the carbon content increased, the difficulty in oxidizing a given amount of ferrous iron would increase, for between carbon and oxygen there is a stronger affinity than between iron and oxygen. In case there is a high content of both carbon and ferrous carbonate, time would have to be allowed in burning to com pletely burn out the carbon before the heat is raised. If this should not be done the ferrous oxide would flux with the silicates causing an early fushion in the unoxidized portion of the brick.
In case the carbon is easily ignited and burns freely it has been found that the fires in the furnaces have to be drawn, all air supply shut off and the carbon allowed to smolder until completely burned out. If these precautions are not taken in such cases, the heat from the burning carbon will raise the temperature in the kiln to the point where the fer rous iron will be slagged with the silicates. In fact, the iron that was originally in an oxidized condition would be reduced, and the whole iron content thus be brought to its most active fluxing condition.
Where the carbon is less inflammable, a longer time would have to be allowed for its complete combustion, but such stringent precautions would not have to be taken as in the case where the clay contained more inflammable carbon.
The chemical explanation of these cases is that although the CO. radical is expelled from ferrous carbonate at an early stage in burning, the basic radical (FeO) cannot receive the oxygen required to con vert it to its less active fluxing form, i. e., to Fc_O3 as long as there is carbon left in the clay mass. Carbon having a greater affinity for oxy gen than the ferrous iron will withhold it from the iron. If a clay con tains insufficient carbon of an easily inflammable variety, or, if the car bon, even though present in quantity, is difficultly inflammable, time must be allowed to permit the oxygen to penetrate the brick, for oxida tion proceeds from the exterior towards the interior in a manner similar w the oxidation of shale in the bank from the outcrop downward.
High content of ferrous carbonate does not in itself mean that trouble will be experienced in oxidation, nor does a high content of thoroughly ,,xidized iron considered alone indicate immunity from oxidation troubles. The substance that controls the wanner in which the oxida tion period of burning clay wares must. he conducted is c:u•bon. Burn ing carbon not only will prevent oxidation of the ferrous iron, but will reduce the iron that may have originally been in a thoroughly oxidized condition. It depends, therefore, upon the amount and form of carbon ,resent in a given case; as to whether in burning there must be allowed a short or long oxidizing period.
Ferrous Sulphide—This very frequently occurs in clays as bright yellow or white crystals. The first of these forms is often mistaken for gold because of its similarity in color. It is commonly known as "fool's gold." Mineralogically it is known as iron pyrites or marcasite, depending upon its crystalline form.
If clay containing pyrites is loosened and allowed to weather, the pyrites will be desulphurized. The iron will, in the dry, oxidize to the hematite (Fe203), or, if moigture is present, to limonite (2 Fe=03 3H20). The sulphur will at the same time oxidize to sulphurous or sulphuric acid. By weathering, therefore, iron pyrites can he thoroughly oxidized and the sulphurous and sulphuric acid removed in solution by percolating waters. 'These reactions require time, especially under dry conditions. Brick manufacturers cannot, under the existing trade con ditions, weather their clay. The face brick manufacturer, therefore, must allow as little time as possible to elapse from the time that his clay is mined until it is under fire in the kiln if he wishes to avoid that bane of the face brick manufacturer, scumming, which results from the formation of soluble salts by the sulphurous and sulphuric acid fruit iron pyrites.
To the front brick manufacturer, the presence of iron pyrites is not, aside from the question of scumming, a serious disadvantage, for the black-slagged specks resulting from ferrous iron from the pyrites fluxing with the silicates is not objectionable to architects. If, however, a clean buff brick is resired or if, for any reason, the smoother and more uni formly distributed black specking by the use of pyrolusite (1n02) is needed, then a clay practically free from iron pyrites must be used.