As will be seen on reference to the analyses, one great reason for the superior quality of the Mount Savage clay is its un usual freedom from potash, one of the worst constituents of fire-clay ; the valuable properties of fire-clay being reduced in ratio to the larger percentage of potash which the clay may contain.
Robert Anderson Cook, A. M., who was employed by the Union Mining Company, of Mount Savage, Md., in making such tests as seemed to be desirable to keep the brick up to the best form for any change which might take place in the market, says : " It was not intended that other clays should be brought to mix with those found here ; and from tests made here of brick from other places, and calculations from analyses of other clays, it is doubtful if any could be procured which would be of any advantage in the general run of brick work.
"For the calculations in getting at the value of a fire-clay from its analysis, the formula used was one given by a German chemist, Dr. Carl Bischoff, who is a recognized authority on the subject of refractory materials, and whose investigations on the subject have been carried out and verified to some extent in this country. He divides the clay into two parts, the silica and alumina constituting the refractory part, and the impurities the fluxing part. Dr. Bischoff, in this formula, uses the im purities as a whole, but in another he divides them according to their relative strength as fluxing agents.
"Taking the alumina divided by the total impurities as a dividend, and the result of the silica divided by the alumina as a divisor, the quotient will be a measure of the refractoriness of the clay as compared with that of another clay treated in the same way. Calling RO the impurities, the formula will be as follows : SiO, RO "As small a difference as o.05 between the quotients thus obtained for two different clays indicates a difference in refrac tory quality which, other things being equal, will show itself in a furnace test.
" These calculations are of great use in the comparison of different clays, but the result one might expect from them may be entirely changed when the clays are made into brick.' The
physical qualities of all clays must be tested before an abso lutely perfect comparison can be made. A sample of the same clay being used by two different brick-makers, yet the one brick made from it may not be as refractory as the other, though the sample may have been thoroughly mixed ; for if in one case the clay be coarsely, and in the other finely ground, the coarse one will stand a great deal more heat than the other before it vitrifies to a homogeneous mass.
" This has been observed before ; and the writer has found it perfectly true as regards this clay, that the more finely it is ground, the less refractory it becomes. At the same time, the more finely it is ground, the stronger and harder the brick be comes, the more abrasion it will stand; and the less likelihood there is of its being broken in handling. Though refractoriness is an essential of fire-brick, yet it is not the only one.
" For the various positions in which the brick are placed, and the duties they are expected to perform, from the upper part of a blast furnace, where the heat is low, and the abrasion of stock is the greatest element in the destruction of brick, to the ports of an open-hearth steel furnace, where intense heat is the most destructive element, particular mixtures of clays should be made to get the best results from raw materials.
" The greatest trouble of a brick manufacturer is that he can not be sure for what purpose the brick will be used, or in what position in the furnace they will be placed. Another trouble is to find out where the fault lies, when complaint is made. This is almost impossible. It may be in the construction of the furnace, or in bad bricklaying, or the grade of the brick, or that the brick were not hard burned. And if a sample lot of brick is sent to a mill to be tested, the chances are that when the superintendent is asked how the brick stood the test he will have forgotten all about them. The only way for a manufac turer to test the brick is to build a furnace and test them him self, and to do this under, as nearly as possible, the same conditions as those under which they will be used in practice.