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.
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.
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.
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.