In many fire-clay mines streaks or spots come in at irregular intervals, confined to no particular line in the vein, of foreign matter very injurious to the clay, or sometimes clay strongly impregnated with calcareous or ferruginous matter, that need great care in order to separate them ; hence none but experi enced men should be employed for the work, men who by long practice can tell good clay from bad as if by second nature.
The Glenboig Union Fire Clay Company, Limited, of Glas gow, Scotland, mines its fire-clay near Glenboig. On making the descent of the shaft, the total depth of which is about seven teen fathoms, while in transverse section it measures 13 by 4X feet, we find the clay at the working face to be of a very hard texture, and requiring to be blasted. The method of working is that which is called in Scotland the " stoop-and-room " system—Anglice, " pillar-and-stall." Some of the pillars or stoops are 3o feet square ; others are 6o feet, and those im mediately around the bottom of the shaft are about 70 feet square. There is an excellent roof to the workings—generally about three feet of rough, hard sandstone. The pumping is ef fected by means of a double action steam-pump. It has a con tinuous water discharge, which is delivered by means of a four inch pipe to a tank situated at the back of the works, and at such an elevation that the water can flow by gravitation to the steam boilers, condenser, etc. The pit is worked by a hori zontal fourteen-inch cylinder high-pressure engine, and winding gear embodying all the latest improvements.
The fire-clay, which is got in irregular masses, frequently weighing one or two-hundred weight or more, is raised to a height of about 35 feet above the pit mouth, and run out on a tramway either to a clay " bing " or direct to the crushing mill. When the clay is exposed to the weather for some time, it be comes physically disintegrated to such an extent that the sub sequent crushing operation is very easily effected. By weather ing, the clay is also rendered much more suitable for the pro duction of such goods as require a fine surface texture.
The following tables of analyses made in the laboratory of the Geological Survey of New Jersey, are here presented as a convenient arrangement of facts for reference. In the first col umn the silica which is in combination is given ; in the sec ond, the alumina ; in the third, the water of combination ; in the fourth, the sum of these three constituents, or the essential elements of a clay ; in the fifth and sixth, the titanic acid and quartz sand appear ; in the seventh, the sum of these two ; in the eight, ninth, tenth, eleventh, and twelfth are given the pot ash, soda, lime, magnesia, and sesquioxide of iron ; the thir teenth gives the sum of these; the fourteenth has the hygro scopic water (moisture), fluxing agents. In the last column the total of the constituents determined is placed.