The common blue and red slates consist of clay and silex in about equal parts; the largest slate quarries, perhaps in the world, are at Bangor in Wales. The blocks when quarried are split into sheets, sometimes exceeding eight feet by four, by means of long, wide, and thin chisels,applied on the edge, parallel with the laminae, and struck with a mallet or hammer. The sheets are sawn into rectangular pieces and slabs, by ordinary circular saws with teeth, moved rather slowly; and these are afterwards planed for billiard-tables, etc, in machines nearly resembling the engineer's planing machines for metal, but with tools applied at about an angle of thirty degrees with the perpendicular, †
Slate is also turned in the lathe with the heel or hook tools used for iron, and also with ordinary tools, used with or without the slide rest, which are however rapidly blunted when applied superficially: it is much tougher at the ends or edges of the laminae than at the flat sides. Slate has been recently worked into chimney-pieces, and a variety of objects for internal decoration, which are ornamented by a patent process,‡ in the manner of papier mache and china; imitations of marbles and granite are thus made at about one-third the prices of marble. Some of the substances known to mineralogists as slates are exceedingly hard, and vary from the hardness 2 1/2, to that of flint or 7. Many varieties, including the Turkey oilstones, are used for sharpening tools; and this family also includes the touchstones formerly used in assaying gold.*
* In Sir John Soane's museum there is a magnificent sarcophagus of stalagmite, purchased of Belzoni for one thousand guineas; and there are also fine specimens in the Egyptian Gallery in the British Museum.
† The process of sawing slate appears rather crushing than cutting, or a trial of strength between the tool and the slate, as the latter is carried up to the saw by machinery, and cannot recede from the instrument; the saw is sharpened about four times a day, and is worn out in about two months. The planing tools for common slabs are six inches wide, and when made of the best cast-steel and pro-parly tempered, they last a day and a half without being sharpened; the jambs for chimney-pieces and other mouldings, not exceeding about six inches wide, are planed with figured tools of the full width.
‡ Invented by Messrs. Magnus ft Co.. Pimlico Slate Works.