The Story of Slate - 1923 - Page 6
The finished slates are piled in racks according to size, and when holes are punched in them for nailing they are ready to be placed on a roof.
Slate used for other purposes is usually termed "structural slate" or "milled stock."
Mills equipped with various machines are required for the manufacture of structural slate products.
The first milling process is most surprising.
You have all at some time or other observed a circular saw cutting logs into lumber or boards into shorter pieces.
You may have wondered at the ease with which a circular saw eats its way through a hard piece of timber, but it seems even more remarkable that a circular saw can be used to cut a piece of rock.
Such is the case, however, for the first operation in the mill is to place the block of slate on a heavy traveling bed that carries it against the teeth of an especially designed circular saw.
The bed travels very slowly, and the saw rotates at much slower speed than a wood saw, but a mass of slate 1 foot thick and 4 feet wide may be cut across in a very few minutes.
The blocks obtained may be split to the desired thickness for blackboards.
If structural slabs are desired the blocks are placed on a second traveling bed and passed repeatedly beneath a heavy blade which scrapes the surface smooth. This machine is called a "planer."
The surfaces may be sand-rubbed and polished by other machines.
Edges may be trimmed or bevels cut with carborundum wheels.
In a properly arranged slate mill the rough block enters at one end and passes from one machine to another in a regular order until it is prepared for shipment.
An interesting feature of slate-working is the importance of keeping the block moist until final splitting is accomplished.
As the block lies in its natural bed it contains what is called "quarry water."