Slate is, as we know, merely a variety of argillite. Slate quarries
are found in England, Switzerland and Italy, but it is in France
especially that the industry has been most extensively developed by
reason of the large deposits that underlie its surface, particularly
in the province of Anjou, where they extend from Trelaze to Avrille, a
distance of six miles, and in the department of Ardennes, at Remogne,
Fumay, etc.
Normandy, Brittany, Dauphiny and Marne likewise possess quarries,
although they are not so productive.
The exploitation is commonly done in open quarry. After the vegetable
mould (which in this case is called "cover") has been removed, we meet
with a solid slate which it is difficult to split into laminæ, and it
is not until a depth of at least fifteen feet is reached that we find
a material that is fit to be exploited. All the best beds of slate, in
fact, improve in quality in proportion as they lie deeper under the
surface, near to which they have little value. Without entering into
details as to the exploitation of this product, let us say that the
blocks have to be divided in the quarry, since, in the open air, they
rapidly lose the property of readily splitting into thin, even laminæ.
[Illustration: SLATE STORE-VATS FOR BREWERIES.]
Slate has but slight affinity for water, and, moreover, resists
atmospheric influences, humidity and heat pretty well.
This property renders it valuable for a large number of domestic
purposes.
There is no certain proof, it is true, that it was employed by the
ancients, but it is, nevertheless, extremely probable that it was used
in mass at an early period for stair heads, pillars for buildings and
as a material for fencing.
The exploitation of the material became especially active at the
period when the idea occurred to some one to use slate for the rooting
of houses. It was employed for this purpose along with tiles as far
back as the eleventh century in the majority of schistose districts.
It is well known, for example, that Fumay (Ardennes) at this period
had a brotherhood of slate quarrymen.
A method of getting out the material and cutting it regularly was
found toward the end of the twelfth century, and it was not till then
that it became of general application. Moreover, with the advent of
the Gothic period slate became indispensable for castle roofs, which
have a conical form.
The best slate for roofing purposes is hard, heavy and of a bluish
gray color. A good slate should readily split into even laminæ; it
should not be absorbent of water either on its face or endwise, a
property evinced by its not increasing perceptibly in weight after
immersion in water; and it should be sound, compact and not apt to
disintegrate in the air.
For a long time past there have been used in schools slate tablets
upon which the pupils write with a pencil made of soft gray schist.
This application, which is capable of rendering services in a host of
details of domestic economy, has given rise to artificial slates,
which, made by a process of moulding a composition analogous to
cardboard pulp, present the same advantages as ordinary slate, while
being much lighter.
Along about 1834 an Englishman of the name of Magnus utilized the
property that slate possesses of taking a fine polish in the invention
of what are called enameled slates. These products are used especially
in the manufacture of table tops, mantelpieces, altars, etc. They very
closely imitate the most expensive marbles, and their properties,
along with their low price, have been the cause of their introduction
into the houses of all classes of the English population, as well as
into those of entire Europe and America.
The ease with which slate is obtained in slabs of large dimensions has
greatly contributed in recent times toward still further increasing
its applications. One of the first of such applications was the
substitution of it in urinals for cast iron plates, which very rapidly
oxidize and become impregnated with nauseous odors that necessitate a
frequent cleaning and constitute a permanent source of infection.
For a few years past, too, slate has been used, in the manufacture of
vats designed for breweries. These vats, of which we show in the
accompanying figure a model of the installation employed in the Ivry
Brewery, are each 6½ feet square and 5 feet in depth. For leading the
beer, which, upon coming from the brewing apparatus, must rest for a
few days, they are connected by a system of pipes. A second system of
pipes, which in our figure is seen running along the cellar vault,
serves as a cooling apparatus and maintains a temperature of 5° C.
above zero in the vats arranged in two rows to the right and left.
The details or even a simple enumeration of the new applications of
slate would, in order to be anywhere nearly complete, necessitate a
lengthy article. Let us say in conclusion that slate is substituted
for wood, which is too easily attackable, and for marble, which is
much more costly, in our laboratories and amphitheaters and everywhere
where the manipulation and stay of easily corrupted liquids and solids
require the greatest cleanliness in the material of construction.--La
Science en Famille.
* * * * *