Hardness And Toughness

A good slate should be both hard and tough.

If it is too soft it will absorb moisture, the nail holes will become enlarged, and the slate will be loose.

If it be brittle it will fly to pieces in the process of squaring and holing, or at any rate will break on the roof if any one walks over it, which is often necessary when the roof is being repaired.

A good slate should give out a sharp metallic ring when struck with the knuckles - it should not splinter under the slater's zax - should be easily "holed" without danger of fracture, and should not be tender or friable at the edges.

Colour

The colour is not much guide to the quality of a slate. Some people think, however, that the black varieties absorb moisture, and decay.

The colours of slates vary greatly. Those most frequently met with are dark blue, bluish-black, purple, grey, and green.

Red, and even cream-coloured slates have been found.

Some slates are marked with bands or patches of a different colour - e.g., dark purple slates often have large spots of light green upon them. These are generally considered not to injure the durability of the slate, but they lower its quality by spoiling its appearance.

Absorption

A good slate should not absorb water to any perceptible extent.

The amount of absorption may be ascertained by the test given at page 28.

Grain

A good slate should have a very fine grain.

The grain of the rock is easily seen, and the slates are cut so that the grain is in the direction of their length, in order that if a slate breaks when on the roof it will not become detached, but will divide into longitudinal pieces, which will still be held by the nails.

"Veins are dark marks running through some slates. They are always objectionable, but particularly when they run in the direction of the length of the slate, for it will be very liable to split along the vein.

Pyrites

Crystals of iron pyrites are often found in slates, especially in those from Scotland, etc.

They are often considered objectionable. It should, however, be borne in mind that there are two varieties of pyrites, of the same chemical composition but of different crystalline form, and very different in their resistance to atmospheric influence.

Ordinary Iron Pyrites, consisting of yellow brassy crystals, generally cubical, weathers well. The crystals have been found perfectly bright and firm in their places in roofs 100 years old, even in the atmosphere of Glasgow.

White Iron Pyrites (or marcasite), on the other hand, is easily decomposed, and slates containing it ought to be rejected. This form of pyrites is generally dull and wanting in lustre, and ie therefore not easily seen.

Sizes

The slates sent to the market are squared in the quarry - sometimes roughly by hand, sometimes by machinery - to certain sizes, which are distinguished by different names,1 as shown in the following Table.

In buying and selling slates in this country, a " thousand " is generally understood to mean 1200 or 1260.

The Table shows the weight and cost of 1200 slates of each description; it also shows the number of slates required per square (100 superficial feet) of roofing, and the weight pei square.

1 These names are used in the building trade, but not much in the quarries.

The following Table of the Different Slates in use has been taken from the list of the Oakeley Slate Quarries Company (Jan. 1887) for slates supplied at Portmadoc. The rates vary of course according to the state of the market, though the prices show the relative value of the different kinds.

Hexagon, Gothic, Diamond, Round, or other Fancy Slates made to order at the above prices in First Quality only. Queens - First Quality. - 24, 26, 28, 30, 32, and 34 inches long and various breadths, assorted as made, without specifying quantities of each, at per ton of 20 cwt. of 112 lbs., 82/6 and upwards. Medium Quality. - As above, 65/ and upwards. Second Quality. - Do. 50/ do.

The weights given are for Portmadoc slates. Slates from Bangor and Penrhyn are somewhat heavier, and Westmoreland slates heavier still. Rags vary in size, but average 36 x 24 inches. Imperials 30 x 24 inches. Queens, Rags, and Imperials are sold by weight.

Quality

The market qualities of slates are classed in the quarries according to their straightness, smoothness of surface, fair even thickness, and in the Cambrian quarries according to the presence or absence of discoloration.

Slates are generally divided into 1st and 2d qualities; in some cases a " medium quality " is quoted.

All slates for good work should be hard, tough, non-absorbent, of uniform colour, free from patches, from veins, iron, cross-grain, and with smooth and even surfaces.

Thickness

The thickness increases with the area of the slate, and the rule for the proportionate thickness varies in different quarries but for Welsh slates is somewhat as follows: - 1

The best qualities of Welsh slates generally split easily into even sheets with smooth surfaces, and holding their thickness close up to their edges, even after being squared.

Irish and Scotch slates are often of very uneven thickness, being thicker in the middle than near the edges, and very much stouter and more substantial than Welsh slates of the same area.

Slates are sometimes split too thin, so that they are not strong enough for roofing purposes. The Ffestiniog quarries have produced (for exhibition as specimens of perfection of cleavage) slates 5 feet to 10 feet long, 6 inches to 12 inches broad, only -^ inch thick.2

Tests

The following rough tests are generally recommended, but they are not of a practical character, nor can they be relied upon. Experience is required to judge of a slate by the eye.