Slate.
A metamorphic clay rock, frequently fossiliferous.
It is readily divisible into thin plates, and, being easily worked and smoothed, is much employed for roofing and in the manufacture of mantels,
billiard-tables, and other similar objects.
In the quarry, the direction of these cleavage planes is usually vertical or nearly so, but never coincident with those of the beds and joints.
The masses are, therefore, removed by cutting trenches in the side of the hill and splitting the rock in vertical layers.
As the perpendicular breast becomes too high for convenient working, say 40 feet, a second trench is cut above the first; then a third, and so on.
In the great slate quarries of Ybron, six miles southeast of
Bangor in
N. Wales, sixteen of these stages are in progress together, the lower ones being gradually widened by the getting of the slates as the upper ones are advanced.
In the upper part of the quarry the slates are removed with crow-bars; but the slates become harder as they are lower from the surface, and require the use of gunpowder to detach the main masses.
The miners engaged in drilling the holes for the powder are suspended by ropes from the upper parts of the rock, and are liable to many and severe accidents.
After the slates are detached by powder or otherwise, they consume considerable labor in splitting them with wedges and mallets into marketable sizes, and reducing them to the several grades required for roofing and other purposes.
Slate adapted for ordinary economic uses is not very common.
A number of varieties are, however, found in
Cornwall,
Wales,
Scotland, and
Ireland, and also on the Continent of
Europe.
Those from the
Ardennes, from Angers on the Loire, and from
Nassau, are largely exported.
In this country,
Vermont furnishes slates of unsurpassed quality and beauty.
Their quarrying and manufacture are beginning to constitute an important feature of national industry, promising large increase in the future.
Aluminous slate yields alum.
Adhesive slate is porous and adheres readily to the tongue.
Bituminous slate yields
coal-oil.
Whet slate has a fine grain and makes hones.
A tough kind,
hornblende slate, is used for flagging and sidewalks.
A soft kind, containing carbon, —
drawing slate or
graphic slate, — is used for pencils.
Polishing slate has a peculiarly fine grain, and is found in Bohemia.
It is used in slips and in powder.
Slate clay consists of alumina and silica, and, from the absence of fluxes, makes a refractory
fire-brick.
[
2200]
2. A thin riven slab of slate used in roofing.
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“Countess” slating. |
The upper surface of a slate is called its
back, the under surface the
bed, the lower edge the
tail, the upper edge the
head. The part of each course of slates exposed to view is called the
margin of the course, and the width of the margin is called the
gage. The portion hidden from view is the
cover.
The
bond or
lap is the distance which the lower edge of any course overlaps the slates of the second course below, measuring from the nail-hole, and may be from 2 to 4 inches.
In preparing slates for use, the sides and bottom edges are trimmed and the nail-holes punched as near the head as can be done without risk of breaking the slate, and at a uniform distance from the tail, regard being had to the spring of the laths.
Slates are laid on
laths, battens, or sheathing, and must break joint.
The nails are of copper, zinc, or tinned iron.
In
England, 1,200 slates constitute a thousand, common sizes, as follows:—
| description. | Size. |
| Length. | Breadth. | Average Gage in of inches. | No.
of Squares 1,200 will cover. | Weight per 1,200 in Tons. | No.
required to cover one Square. | No.
of Nails required to one Square. |
| Ft. | In. | Ft. | In. |
| Doubles | 1 | 1 | 0 | 6 | 5 1/2 | 2 | 3/4 | 480 | 480 |
| Ladies | 1 | 4 | 0 | 8 | 7 | 4 1/2 | 1 1/4 | 280 | 280 |
| Countesses | 1 | 8 | 0 | 10 | 9 | 7 | 2 | 176 | 352 |
| Duchesses | 2 | 0 | 1 | 0 | 10 1/2 | 10 | 3 | 127 | 254 |
| Imperials | 2 | 6 | 2 | 0 | A ton will cover 2 1/4 to 2 1/2 squares. |
| Rags and Queens | 3 | 0 | 2 | 0 |
Other denominations and sizes are —
| Ins. | | Ins. |
| Small | 11 × 7 | Viscountess | 18 × 10 |
| Plantations | 12 × 10 | Marchioness | 22 × 12 |
| 13 × 10 |
| 14 × 12 |
A
square of slate or slating is 100 superficial feet.
The
pitch of a slate roof should not be less than 1 in hight to 4 of length.
Dimensions of Slates (
American.)
| Ins. | Ins. | Ins. | Ins. | Ins. | Ins. |
| 14 × 7 | 16 × 8 | 18 × 10 | 20 × 11 | 22 × 12 | 24 × 13 |
| 14 × 8 | 16 × 9 | 18 × 11 | 20 × 12 | 22 × 13 | 24 × 14 |
| 14 × 9 | 16 × 10 | 18 × 12 | 22 × 11 | 24 × 12 | 24 × 16 |
| 14 × 10 | 18 × 9 | 20 × 10 |
The thickness of slates ranges from 3/16 to 5/16 of an inch, and their weight varies from 2.6 to 4.53 pounds per square foot.
3. A tablet for writing upon, formed of slate, or of an imitation of slate.
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Slate. |
School-slates are made from a fine and soft quality of slate.
The great demand for them has led to various improvements in the manner of making and uniting the frames, and to the invention of special machines for this purpose.
Slate-frames are now generally made with rounded angles, and one invention consists in securing the parts together more firmly by wires entering grooves at the corners, and having bent ends, which are inserted into holes in the side and end pieces.
Artificial slates are prepared by coating the surface of wood or paper with a fine gritty substance, as pulverized emery or pumice, mixed with black size or paint; or the surface is painted black, and dusted with the powder before it becomes dry.
