BURNING BRICK WITH NATURAL GAS.* The process of burning with gas is less simple than that of drying, and to be successfully accomplished requires a slight knowledge of the nature of natural gas, its action during com bustion and the appearance of the brick whilst hot. The pre dominant element of natural gas differs from that of wood or coal. The analysis of most gases gives about 75 per cent. hydrogen, 15 per cent. carbon, and a small percentage each of nitrogen and oxygen. Naturally the products of combustion differ considerably from those of wood or coal ; when the heated brick are observed through this atmosphere, the shade of color seems entirely different from brick at the same temperature in a kiln where other fuel is used. To this fact more than any other may be attributed the failure of several good coal-burn ers to succeed with gas. The greater affinity or uniting power which oxygen possesses for hydrogen over carbon is another stone over which many have stumbled. If from any cause there is a greater proportion of gas to air, the brick will be injured, as we shall see in relation to water-smoking and the early stage of firing. In water-smoking with gas, the process will probably require a longer period than with coal, in order to preserve the brick in their natural color and original form. If haste is at tempted, the gas will not be thoroughly consumed, and the oxygen taking up the hydrogen frees the carbon, which being in minute particles seems to enter the pores of the clay. When the kiln becomes hotter, these particles are consumed, and act as if bituminous coal-dust had been mixed with the clay. The brick become spongy, blackened and run together in a mass. Where this has occurred, I have seen ordinary sized brick in the bench misshapen and extended to a length of 15 or 16 inches. Often, when the draft is not good, the carbon will be deposited in considerable quantities in the form of coke in the bottom of the arch furnace. This usually occurs from the im proper admission of the air, which should enter the arch in such a manner as to thoroughly mix with and consume all the gas before coming in contact with the brick. In fact, in the proper admission and control of the air-current entering the furnace or arch is contained the secret of successfully burning natural gas. In ordinary kilns, and without the aid of costly burners and by utilizing only the most simple and inexpensive contrivances, the brick can be burned thoroughly, without dis coloring, and from wall to wall and from bottom to top, except the last course. Certainly to do this requires careful watching
and delicate manipulation of the air-currents from the moment the water-smoke starts until the kiln is closed, but the waste and soft brick in such a kiln are a very small percentage when the undertaking is completed, which with ordinary clays is about three days. One day should be used in heating the kiln throughout a perceptible red, another in settling the sides, a third the middle, and the last in bringing to terms any spots that may have remained. Seven days are thus occupied from lighting to closing, but some difficult clays, or clays that de mand a tender handling, will require a longer time, whilst others can be done in a much shorter period. As an example of the difference in clays, not long since I secured a sample of bluish clay from Haverstraw, N. Y., took another from the light, loamy clay near Pittsburgh, and had them both worked up and burnt beside the strong plastic clay of the Wittmer Brick Co. After keeping them at a moderate heat for some time they were allowed to gradually cool, and when taken out we found the Haverstraw clay closely fused and distorted in shape ; the loamy clay was somewhat past color, having a bluish tinge ; whilst the Wittmer clay, though withstanding a greater resistance before fracture, was only a fair salmon. Where color and uniformity of shade are an object, a longer time will give better results, the heating should be more gradual, and the settling less rapid. When to cease firing must be judged al most entirely by the degree of heat. There is no blue smoke with gas, and the settle is not always a safe guide. I have seen clays with 18 inches settle having seven courses of soft brick, whilst in other burns the same clay with 18 inches settle gave good front brick in the second course from the top. The best indication in the up-draft kiln is the intensity of the heat at the top. With a little experience and a knowledge of the clay, the burner can judge from this to a nicety. In down-draft kilns, of course, the heat is judged by aid of the peep holes and test brick.