Page:The American Cyclopædia (1879) Volume XV.djvu/96

 gfl SLATE pine timber, and into sheets of any desired thin- ness The faces should be perfectly smooth and parallel, without any curvatures or irregulari- ties There should be no lines of cross fracture that should prevent their breaking in any one direction more than another. When one is bal- anced on the finger and struck with a hammer, it should give a clear ringing sound ; and after being dried in an oven and immersed in water, it should absorb but little, as may be ascer- t .-lined by weighing it before and after immer- sion. This is an excellent test of the compar- ative values of different slates. The powder of slates is light gray, and when a pointed piece is rubbed upon a smooth slate surface a portion of the powder remains behind, leav- ing a plain mark that is easily wiped or washed off. It is this property which renders the slates serviceable for drawing and writing upon. Ar- gillaceous slates, like the clays which they ori- ginally were, are essentially composed of silex and alumina, and the following is the result of the analysis of a common Scotch variety : silex, 50 parts in 100 ; alumina, 27 ; oxide and sul- phate of iron, 11; potash, 4; magnesia, 1; water, 7; carbon, a trace. The slates are found often in beds of great extent, associated with other beds of similar character ; and this singular feature is observed in the structure of the rocks, that the cleavage, or lines along which the slates naturally separate, has no relation to the lines of stratification. However nru-h the beds themselves may be contorted and follow irregular waving planes, each sys- tem of cleavage lines, in case there are more than one, as sometimes occurs, maintains its own direction and rarely coincides with the plane of dip. It is evident that the cleavage seams must have been produced subsequently to the time when the beds acquired their final position. This structure is what is known as slaty cleavage ; and sometimes when the strata are themselves thinly bedded and the stratifi- cation is regular over extended areas, it is not easy to distinguish immediately the two sets <>f planes one from the other. Slates are quar- ried either by blasting out large slabs, or, when practicable, splitting them off with gads and large wedges. The slabs from a foot to a foot and a half thick, and it may be 8 or 10 ft. long and 1 or 2 ft. wide, are set on edge, and grooves are cut across the top and down the sides to determine the lines of fracture for separating thorn into rectangular blocks, which is done l.y blows from u wooden beetle directed upon the top near the furrow. The splitting is effected by driving wide, thin chisels between the lamina), and the sizes of the slates are reduced whenever desirable by cutting cross grooves and then breaking the pieces with the dii-n-l. When n-diu-fd to the required thinness, the slates are roughly dressed over the edge of a him -k i if wood by the blows of a sort of chop- I>in_' knife called a sack, sax, or zax. On the back of this tool is a sharp tapering steel point, with which the workman when preparing roof- SLATER ing slates pecks two holes through the slates near what is to be the head or upper edge, for the nails which are to hold it down to the roof. In Vermont machines have been applied to cutting grooves in the slate in the ledge to fa- cilitate the quarrying, and the cutting and trim- ming are also done by machinery. It is impor- tant that all this work should be done while the blocks are fresh from the quarry, as in dry- ing they are apt to lose their property of split- ting freely, though freezing may restore this ; but a succession of frosts and thaws has the effect of thorough seasoning. Slabs for inter- nal decoration, as mantelpieces, and for articles of furniture, as table tops, billiard tables, sinks, &c., are cut by circular saws which are made to revolve slowly. The sheets when thus squared to suitable sizes are planed in machines similar to those used for planing metals ; and pieces for mouldings are shaped by tools of the desired figure. Various ornamental arti- cles are prepared of slate in imitation of mar- bles, granites, and other stones, by the appli- cation of colors, which are baked in, varnished, and polished, the applications being several times repeated. (See ENAMELLING, vol. vi., p. 591.) SLATE PENCILS are made from argilla- ceous slate rock, sometimes from talcose slate, and sometimes from various materials ground together and compressed. Near the town of Castleton, and near one extremity of the west- ern Vermont slate belt, is found an argillaceous slate from which the finest pencils are made. The stone is sawed into blocks 7 in. long by 6 in. wide, and split into slabs a little more than a quarter of an inch thick. These are then planed and placed in a machine, in which a series of grooved knives cut through one half the thickness of the slab, when it is placed in a second machine having a bed with grooves corresponding to the sides of the pencils cut, and a cutter like the one in the first machine completes the operation. The pencils are then counted and put up in boxes of 100 each, and packed in cases of 10,000. There are three sizes, 6, 5, and 4 in. in length. The waste of this slate has been utilized by grinding it into flour and making it into artificial pencils. SLATER, Samuel, an American manufacturer, born at Helper, Derbyshire, England, June 9, 1768, died at Webster, Mass., April 21, 1885. He was apprenticed to cotton spinning under Jedidiah Strutt, partner of Arkvvright, and was a favorite with his master. He aided Mr. Strntt in making improvements in his mills, and gain- ed a thorough mastery of the theory and prac- tice of the new manufacture. In 1789 con- gress passed its first act for the encouragement of manufactures, and the Pennsylvania legis- lature offered a bounty for the introduction of the Arkwright patents. These laws met the eye of young Slater in an English journal, and he believed himself able to carry the Ark- wright cotton manufacture across the Atlantic without drawings or models, the export being forbidden under severe penalties. He arrived