SUBSTANCES IN CLAY THAT ARE AFFECTED BY OXIDATION.
In general terms, the oxidizable substances in clays are carbon com pounds, carbonates, nitrates, sulphites, etc. The most noteworthy ox idizable substances in clays are : Carbon and the carbon compounds, ferrous carbonate and ferrous sulphide.
Carbon and the Carbon Compoands—Carbon is present in practically all of the secondary clays in forms ranging from unaltered vegetable matter, humus and its compounds, to the highly metamorphic carbon graphite in graphitic shales. The least altered carbon ignites and ox idizes most easily and the highly metamorphosed carbon most difficulty. To the decomposing carbon compounds and their by-products, the or ganic acids, are due many of the physical properties of clays. It has been shown in earlier pages that organic acids are the main agents that cause defiocculation, a condition that must exist before plasticity can be developed. It could be readily shown that humic acid (C2H2O.) with its peculiar properties of absorbing and holding heat, moisture and soluble salts, is a very active agent in promoting chemical changes in the mineral ingredients of clay, thus altering the physical condition of the mass. Unaltered carbon compounds and their by-products are, therefore, not only easily oxidized in burning, but have been highly ben eficial in that they have promoted the development of those physical properties which, if the carbon is not in excess, permit of easy manufac ture into wares.
The more metamorphosed the carbon compounds the less active they are in promoting physical and chemical alterations in the clay mass and the more difficult are they to oxidize in the kiln. For these reasons fire clays and clay shales in which the carbons compounds have been com pletely converted to graphite are—within small areas—more constant in their properties, thus being more constant in their working and burning behavior, and at the same time, more difficult to burn.
Ferrous Carbonate—Ferrous carbonate occurs in clay in various phy sical conditons and sizes of grain. Large concretions—"nigger heads"—
which are often composed mainly of ferrous carbonates, are to be seen in most shale banks. Ranging in size from 12 to 18 inches in diameter, down to minute, almost microscopic particles, these concretionary and globular forms of ferrous carbonate play a role in burning clay wares which, while most peculiar, is but little understood. The ferrous car bonates that exist as finely precipitated powder surrounding the other mineral grains must, in burning, pass through the same chemical alter ations as the ferrous carbonate in lump form, but under such different conditions that distinction must be made between its behavior when in these two conditions of aggregation.
One of these large ferrous carbonate concretions pulverized, pressed into brick form and burned under the same heat treatment required to produce pavers from the shale in which the concretion was found, produced a brigh red brick which possessed a toughness that was equal to that of the brick made from shale. This experiment proved that the clay mass which is bound together by ferrous carbonate in a mass so hard as to wreck ordinary crushing machines like dry pans, and contain ing a comparatively large quantity of ferrous carbonate, can be burned as safely and into just as good brick as the softer shale containing but a small quantity of ferrous carbonate (3 per cent of total ferrous iron.) In this brick made from the crushed concretion there was practically no carbon, while the shale contained three quarters of one per cent While it is true that the carbon content of the shale is so small that no difficulty is experienced in thoroughly oxidizing the mass under the time temperature schedule required to raise heat uniformly in a large kiln, yet it is a significant fact that occasionally unoxidized brick are drawn from the kilns,. and that the mass containing a large amount of ferrous carbonate was perfectly oxidized under similar kiln treatment.