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 laminae, 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 laminae.
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 laminae; 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.