SLATE-WORKING. Referring to Shave, in Nal.. Hisr. Dry., for an account of the geological formation and structure of this valuable rock, we here add a few words concerning its practical working.
The quarrying of slate is comparatively easy, seeing that the lamel lated structure enables the substance to split easily int r layers. The usual processes are adopted for sawing and smoothing the slabs ; but improved machines for these purposes have been introduced within the last few years. One machine has been invented for hollowing out thick blocks of slate into sinks and vessels, by means of cutters screwed to the ends of revolving shafts. By the use of peculiar tools. slate is now turned in the lathe. Mr. Mathews's apparatus for cutting and dressing slate consists of a frame, provided with arms, cutters, levers, toothed wheels. &c., in such a way that the cutters may be raised by a lever, and let fall with a sudden blow ; and this in such a manner as to work the slate either into plain or fancy surfaces. Ordinary slate cannot very well be polished like other kinds of hard atone ; it is rubbed smooth with an iron plate. sand, or gritstone, and water. Within the last quarter of a century, there has been a tendency to use slate for many purposes for which other substances used to be em ployed. The slate top of a billiard table is perhaps the best kind of work in this material. The best tables measure 12 feet by 6 feet ; the
top consists of four slabs of slate 6 feet by 3 feet, and one inch thick, ground on the lower surface. planed on the upper, and adjusted edge to edge with the most scrupulous care. Pavements, cisterns, walls, partitions. &c., are now largely made in this material " Enamelled slate" is used for table tops, chimney pieces, wash-stand tops, columns, pilasters, door furniture, monuments, mural tablets, &c.
What are called elate pencils are simply narrow slips of a soft kind of slate. Some of them are made to slide in wooden tubes or holders, by a propelling apparatus similar in character to that of certain of the ever-pointed pencils Artificial slate pencils have been introduced, made of a mixture of alumina, French chalk, and soapstone.
The largest slate quarries in the United Kingdom are the Penrhyn, belonging to Colonel Pennant. These are briefly noticed under BANGOR, in (=soo.
M. Raphael Cannana has recently proposed the use- of slate as a substitute for boxwood in wood engraving. He states that it is easily penetrated by the graver ; that the finest lines are producible ; and that slate is more durable than box under the action of the printing press.