PHYSICAL AND CIIEMICAL EFFECTS OF INCOMPLETE OXIDATION.
Usual effects—Reduction of iron and the consequent early fusion of the unoxidized portion of a brick results in the formation of a par tially fused glass surrounded by a shell that has as a rule just begun to vitrify. Entrapped in this glass is some burned carbon which when partially oxidized is converted into a gas. Aside from the CO2 formed by the oxidation of the entrapped carbon, there are salts that are volatilized into vapors at this heat. These gases and vapors expand on heating, causing the black unoxidized core of the brick to swell up until, in extreme cases, the brick is twice its normal size and will float in water: Inasmuch as the oxidized shell is thickest on the edges and thinnest of the faces, the swelling core will bulge out the faces of the brick until it approximates the shape of a cylinder.
It is obvious at once that bricks which have swollen centers will not be fit for pavers. It follows also that the toughness of a brick is lessened in proportion to the extent that its center is reduced and rendered vesicular. It is imperative, therefore, that ample time be given at the oxidizing period (red heat) to insure complete combustion of the carbon and oxidation of the iron.
Exceptional Effects—In the case of H, 23, oxidation had not pro gressed very far at the end of 24 hours exposure at 650°, and the un oxidized portion of the briquettes vitrified on further heating to as hard hard and dense a mass as did the outer oxidized portions. No swelling or distortion of the brick due to the oxidation of the carbon and ferrous iron was noted. In fact, the shrinkage and rate of de crease in porosity was not abnormal in any respect. In Fig. 25 are shown the volume-shrinkage, porosity, and specific gravity curves for this clay.
In this figure, the specific gravity, porosity and volume of the bricks burned at different temperatures are calculated in terms of the per centage of increase or decrease over those of the unburned bricks. In other words, the raw factors are considered as a basis from which the "burned" factors are calculated as increase or decrease. Zero, there fore; represents the data obtained from the unburned bricks.
The percentage of increase of the burned ware over that of the un burned is shown above the datum line on the ordinate, and the per centage of decrease is shown below the datum line. On the abscissa is shown the actual percentage of porosity of the burned brick.
Points on the same ordinate represent a single brick. Data from all the bricks studied in this test have not been plotted, but only those in which the percentage of porosity differed sufficiently to fix points on the curves that would show a comparative increase or decrease in the several factors.
The fact that the actual percentage of porosity of the burned brick was taken in each case as a point on the abscissa, without regard to the porosity of the unburned brick, will account for the irregularity in the curves.
Notwithstanding the fact that the black unoxidized core remained, even when the whole exhibited a porosity of only 2 per cent, the brick continued to shrink normally with each increase of temperature, and the specific gravity of the brick decreased less than in the ease of many normally burned paving brick shales. This steady decrease in volume and comparatively slight increase in specific gravity gives evidence of a thermo-physical behavior that is opposite to that of the majority of clays containing carbon.