At a few places in New Jersey the extraction of the clay has been by underground work or mining. This consists, on side hills, in cutting short drifts, or tunnels, in the clay bed, timber ing them so as to hold up, temporarily, the superincumbent earth, and when the work is done, allowing it to fall in. By a series of drifts side by side most of the bed is in this way worked out.
There is some loss of material in the clay which has to be left at the bottom as a floor, and at the top as a roof to hold up the overlying sand or other loose material, and to keep out the water. These drifts are inclined a little, if the bed allows any inclination, to let any water which may get in them by accident run out. They are narrow, being only wide enough for the passage of men with their barrows or carts. The timbering consists of upright posts set at the sides, at varying distances apart, sometimes close together, and at others a foot and a half or two feet apart. Upon these, cross-beams or sleepers are laid to support the roof. These drifts are seldom more than too feet long. Wherever the beds of clay are uniformly thick, the bearing heavy, and the clays of superior quality and value, it may be practicable and more economical than the ordinary mode of stripping off the top and pitting the clay. It is costly and attended with risks ; and these objections must be consid ered in its application to any locality. It is believed that the scarcity of clay at easily accessible depths for open working will in the future compel the attention of clay-miners to it as prac ticable, and the only way in which some of the New Jersey clay territory can ever be made available and productive.
The digging of fire-sand, kaolin, and feldspar is carried on very much like that of the clays. As the strata are not imper vious to water the pits are generally smaller, so that the length of time in sinking one is seldom more than a day. The quan tity of water to be raised is commonly much greater, and in some cases it is so large that it is scarcely possible or practi cable to get to the bottom. In working the strata of these ma terials there is more loss than in digging clay. More of the bed is left in the ground. In digging these the gouge spade is rarely used, but ordinary shovels and spades, aided occasion ally by picks where the material may be more firm or too hard for spading. The loading is generally direct from the pit or
the side platform into carts or cars, and there are fewer grades, rarely more than number one and number two.
Nearly all of the clays, and all of the feldspar, kaolin, and fire-sand, are sent into market in a crude state. They are shipped in bulk, either in boats or cars. With some varieties, as the paper and ware clays, more care is taken in keeping them clean and free from admixture with inferior grades. Formerly the paper-clays were shipped in barrels, but at the present time they are generally transported in bulk.
The mines from which fire-clays are taken should be kept in first-class condition, so as to be able to meet all demands made upon them for raw material, and nothing but pure clean ma terials should be sent to the factory to be made into fire-brick.
In mining fire-clay more care is required than in mining iron ores, or even coal. It is considered good ore that produces fifty per cent. metallic iron ; the other half consists of foreign in gredients that are run off from the furnace as slag. An acci dental or careless increase of this foreign matter results in no more serious consequences than the lessening of the percentage of metallic iron produced, thus causing a slight increase in the cost of the pig-iron. The case is very different with fire-clay. Foreign matter carelessly mixed in with fire-clay may easily spoil a large quantity of brick. Fire- clay often runs in veins contiguous to iron ore or limestone, or even closely associated with them, and it takes but little of either of them to ruin a large quantity of clay. The writer remembers a case in point where a part of the clay supply came from a mine where the clay bed was directly under the limestone. The latter was first got out, going to the furnace for flux ; afterward the fire-clay was taken out. Some slight changes among the hands em ployed at the mines resulted in a quantity of the small lime stone chippings getting mixed with the fire-clay. As they were both the same color, it passed unnoticed until some thousands of brick were spoiled.