SLATING, is employed, in architecture, in sundry ways, the principal of which refers to the covering of the roofs of buildings, but such has been lately the perfection of working ill slate, that it is now wrought and fitted into many useful utensils, as well as made up into balconies, chimney-pieces, casings to walls, skirtings, staircases, &c.
The slate principally used in London is brought from Wales, taken from quarries on Lord l'enrhvn's estate at Bangor, Caer narvonshire, whence it is forwarded to all parts of the United Kingdom. There are also some other kinds of slate in use, the best sort of which is brought from Kendal, in Westmore land, and is called Westmoreland slate. These are of a fine pale bluish-green colour, and are most esteemed by archi tects. They are not of a large size, but of good substance, and well calculated to give a neat appearance to a roof. The Scottish slate is nearly similar in size and quality to a slate from Wales called ladies, but they are very little sought after.
French slates, which were very much in use many years since, are small in size, most commonly not larger than the Welsh doubles, extremely thin, and, consequently, light ; but their composition has been found to be not well adapted to this climate, where the atmosphere contains an excess of moisture. By analysis, this slate is ascertained to contain of manganese, besides other matters, such as iron, &c., the excessive affinity of which for oxygen soon shivers the stony portion of the slate, when employed as a covering, in this country.
Slaters class the Welsh slates after the following order and designations, viz.
The doubles, so called from the smallness of their size, are made from fragments of the larger qualities as they are sorted.
The ladies are similarly obtained, but in pieces that will square up to the size of such description of slate.
Countesses are a gradation above ladies ; and duchesses still larger.
The slate is extracted from the quarries, as other stony substances usually are, that is, by making perforations between its beds, into which gunpowder is placed and fused. This opens and divides the beds of the slate, which the quarrymen remove in blocks of very considerable size. These
blocks are aftewards split by driving iron wedges between their layers, which separates them into scantlings of from four to nine inches in thickness, and as long and wide as may be required. Such of the scantling as is intended for expor tation is sawn Co the sizes ordered.
For the purpose of sawing the slate, the works in Wales are provided with abundance of ingenious machinery, some of which are put in motion by steam, and others by water, which keep in action a vast number of saws, all cutting the scantlings into pieces adapted to their several purposes.
The imperial slating for roofs is uncommonly neat ; and is known by having its lower edge sawn, whereas all the other slates, used for covering, are chipped square on their edges only.
The patent slate is so called, among the slaters, from the mode adopted to lay it on roofs, as no patent was ever obtained for such a mode of slating. It was first brought into use by Mr. Wyatt, the architect. It allows of being mid on a rafter of much less elevation than any other kind of slate, and is considerably lighter by reason of the laps being less than is necessary for the common sort of slating. This slating was originally made from that description of slates known as Welsh rags. The slitters now frequently make it of imperials, which renders it still lighter, and somewhat neater in its appearance. Experiments have been instituted on the Westmoreland slate by the Bishop of Llandaff, from which there appears very little difference in its natural composition from that obtained from Wales.
Thirteen loads of the finest sort of Westmoreland slate will cover 42 square yards of roofing, and 18 loads of the coarsest will cover the same quantity ; so that there is half a ton less weight put upon 42 square yards of roof when the finest sort of slate is used, than if it were covered with the coarsest kind, and the difference of expense is very trifling. it must be remarked, that it owes its lightness, not so much to any diversity in the component parts of the stone from which it is split-, as to the thinness to which the workmen reduce it ; it is therefore not so well calculated to resist violent winds as that which is heavier.