A Visit to the Slate Quarries of Angers (France)
Scientific American - Supplement No. 974, Munn & Co., New York, 1894
Among the excursions, often very picturesque, that can be made into the subterranean world, one of the most original, and about the most exciting, is a descent into the slate quarries of Angers. The special character of this mine visit is connected with the method of exploitation by immense chambers that has been applied at Angers from time immemorial. But (and this is what gives the subject a sort of actuality) it will be well just now to make haste, if one desires to enjoy this spectacle, since the ancient method of work, which was attended with serious danger to the laborers, will perhaps soon disappear. The evolution, begun a few years ago, may be precipitated some day or other in the train of legislative prescriptions, and we shall then no longer see any method employed in the quarries except the one that is probably safer, and certainly much more curious, called the ascending.
The argillaceous schists of Angers form, in the midst of the Silurian formation to which they belong, several almost vertical strata (called veins in the country) of variable thickness, exceeding 300 feet in places. It is these so-called veins, and more especially their homogeneous and fissile portions that it is a question of extracting from the earth in great blocks in order to split and shape them-the large pieces for billiard tables, mantelpieces, etc., and the smaller ones for the covering of roofs. As the price of the large pieces goes as high as about a dollar per square foot, while the slates for roofing have descended to less than twenty cents a thousand, it is very important that the dimensions of the blocks shall be reduced as little as possible, in taking into account their natural joints. This leads to the brilliant lighting of the working points by electricity and contributes to the peculiar character of the operations.
These joints and fractures of the schists, of which one constantly hears spoken in the Angers quarries, bear a series of old and more or less strange names (torsins, cordes de chat, chefs, bavures, crusses, etc.), as to the precise signification of which one is not always absolutely agreed, but which have nevertheless played a great part in all the discussions relative to the changing of the method of work. These joints have not only an influence, which would be of secondary interest, upon the division of the blocks and their utilization, but they also have the effect, if care be not taken, of leading to the sudden sliding of huge masses of slates, and sometimes, as a too frequent consequence, to the disastrous burying of a whole brigade of laborers.
The methods of exploitation applied at Angers are three in number: (1) The method by cuttings in the open air used from the twelfth century up to 1832, but now more and more restricted in measure as one is led, through the exhaustion of the outcroppings, to exploit the deeper and deeper parts of the deposit. (2) The method called descending, by large chambers lighted with electricity, fully 200 feet in length and 325 in height, and the first application of which was made in 1832 at the Grands-Carreaux, upon the advice of Mr. Le Chatelier. (3) Finally, the new method, called the ascending, in which the excavations are filled in in measure as the work progresses, as is generally done in coal mining.
We shall say but a few words as to these two latter methods, of which the two accompanying figures give the aspect and arrangement. The open cuttings of from 130 to 160 feet in depth, which generally strike visitors ignorant of the art of mining with astonishment, have often been described, and, moreover, present nothing more extraordinary than do numerous other exploitations of the same nature in metalliferous deposits; Dannemora in Sweden, Rio Tonto in Spain, Mokta-el-Hadid in Algeria, etc. The first engraving represents a chamber of subterranean exploitation of the Hermitage slate quarry. This chamber, which is being deepened more and more at the base, may be, perhaps, 300 feet in height, with a rectangular section of 100 by 180 feet. The very great resistance of the schist permits of forming these immense and nearly vertical walls without very serious danger and of preserving as a vault, during the entire duration of the exploitation, one and the same horizontal rock surface. But, in order to prevent the falling of blocks from such a height upon the laborers at work below, it is necessary to keep a close watch over these slate surfaces. This is the object of the flying bridges that are seen crossing the chamber lengthwise and breadthwise near the ceiling, and which are suspended in space by iron rods fixed in the slate, and are reached through vertical ladders inclosed here and there in wooden cases that form a sort of landing places.
Walking over these frail bridges, suspended by rods that oscillate at every step, cannot be done without some perturbation by those who are subject to vertigo. When blasts are fired below, one feels the mass of slate vibrating violently and for a long time, and if he is not yet hardened to this sort of impression he may imagine that the rods, which are perfectly solid, are about to give way and precipitate him from a height of over three hundred feet.
The view obtained from any one of these bridges is very curious. At the back of the immense chamber of shining black walls illuminated by the theatrical light of the electric beacons, one distinguishes through a vague cloud of dust the pygmy laborers, who by pick and jumper are getting out blocks of slate by stopes of from 10 to 13 feet in height, in starting, to the right and left, from a trench previously excavated below in the axis. In a corner of the ceiling, a round aperture, like a cellar window, allows of the entrance of a wide beam of daylight in which operates the hauling box that carries up the slate or serves for the ascent and descent by the laborers.
It is through this aperture, the bottom of a shaft four hundred feet in depth, that one enters the quarry, as if through the neck of a bottle, and this descent would of itself pay for the visit. The hauling box is a sort of round bucket suspended by three chains from a pulley which is itself attached to a steel cable and rolls over a guide cable that extends from the exterior surface to the bottom of the chamber. Three men take their place in a standing position in the box, in balancing themselves and holding each of the suspension chains with the hand. Then the apparatus begins to move, and, in the first place, descends four hundred feet in the moss-covered shaft, at the bottom of which, in describing a curve of wide radius due to the flexibility of the guide cable, it suddenly enters the subterranean chamber. A person then has the sensation, which he never experiences in an ordinary mine shaft, of being absolutely lost in space above a strongly lighted abyss where he observes men in motion three hundred feet beneath him. Meanwhile the box is sliding down the undulating guide cable, and, in a few minutes, one is at the bottom, surrounded by laborers who have a load of slate all ready to be sent up by the same route.
This box is a dangerous device, both by reason of the breakages that may occur in the carrying cable, which is ever ready to wind around the guide cable, and the liability of fragments of schist to fall upon the head of the laborers at the bottom during the ascent of the box.
In a general way, moreover, the exploitation of slate quarries is one of the most murderous of the mining industries. In 1892, a year that was not marked by any great accident, six men out of the 794 employed at the bottom were killed, say a proportion of 7.56 per 1,000, while in the same year the proportion in coal mines was but 1.18 per 1,000 (112 killed out of 95,000 laborers at the bottom), and, in all the mines and quarries of France, 1.09 per 1,000 (287 killed out of 262,348 laborers).
This proportion explains why it is that so earnest an endeavor has been made to find a less dangerous method of exploitation. The system selected, which was at the beginning very vigorously discussed and is still rejected by the principal body of quarrymen called the Commission of the Slate Quarries of Angers, is the one called the ascending method.
In the preceding method of work it will be seen that the laborers constantly extract the slate under their feet in chambers that deepen indefinitely. In the ascending method, a beginning is made, on the contrary, by proceeding means of a shaft to the lowest point that it is desired to exploit, and from there rising progressively toward the surface in extracting the slate and gradually filling in the space below with material let down from the surface for that purpose.
One will at once see the advantage of the system, which is the suppressing of immense walls difficult of surveillance, especially vertical ones, which are really much more dangerous than the horizontal ceiling, since, if there be some concealed joint therein, the whole mass of superposed slate may some fine day slide over such polished joint, and advancing into space, fall into the chamber. At the same time, the hauling box is suppressed; but, per contra (and this is what has created very resolute adversaries of the new method), instead of the well known and well watched fixed ceiling, we have one that is incessantly variable and shaken by blasts; and, for the convenient exploitation under foot is substituted the difficult work above upon movable scaffoldings, so that very sound minds, while favoring the extension of this system, are still fearful that the number of great accidents will be diminished only to singularly increase that of the minor ones. Experience, nevertheless, seems to pronounce rather in favor of the ascending method, which is applied with large chambers of feeble height at the Grand'Maison and with small chambers previously surrounded by a system of galleries at the Foret.
Fig. 2 (insert in Fig. 1) shows how the operation is performed at the Foret, in excavating successively, starting from the bottom and for the entire width of the chamber, a series of trenches 12 feet in depth. Above, upon a platform suspended from the slate roof by iron rods, men are seen boring blast holes. Beneath them may be seen the front of the 12 foot cutting, which gradually recedes in measure as the work goes on. Lower still, a dark band represents the upper part of the space created by the exploitation of the lower section already removed, the rest of the space having been filled in with material that forms the floor, and upon this material there rests to the right a large block of slat that, after being divided into sufficiently small pieces, is to be carried away upon a small platform car moving upon rails. Here, again, the lighting is done by electricity, through incandescent lamps suspended by a cord and that are lifted to the ceiling by pulleys before a blast is fired.
After the blocks of slate have been extracted by any of these methods and raised to the surface, it remains to shape them.
The working of the slate for roofing must be done in situ before the material has dried, and, in losing its water, has lost also its fissility. The block is first slit into thick plates by means of a chisel. Then the slitter, seated under a wind screen, his feet incased in thick sabots and his legs wrapped with rags to prevent him from wounding himself, places the fragment of schist between his legs, and, provided with a very thin chisel and a mallet, strikes the edge in order to detach sheets from it. All that has to be done then is to give this sheet its proper form and size. This is done by means of a plane iron-very heavy iron knife, whose extremity is shored into a ring and which turns down against the edge of a block.
The laborers are so used to this work that they perform it very rapidly, and at the rate of from 700 to 800 slates per day. But the more the dimensions of the slate are increased (as is now the tendency in order to compete with machine-made tiles), the more delicate the operation becomes; and this question of large sized slates, called the "large English," is a source of frequent conflicts between bosses and workmen. It is estimated that an outside laborer earns from fifty-five to seventy cents a day, and a pitman from sixty to eighty cents.
There might be still much to say about the curious working costumes that have been perpetuated at Angers from the middle ages up to recent years, and are but slowly disappearing one by one; but this is a subject that we do not wish to touch upon here, and we shall content ourselves with a few statistical figures for finishing this curious study.
In 1893, the slate quarries of Maine-et-Loire gave employment to 1,034 inside and 2,185 outside laborers-say altogether 3,219, of whom 1,511 were slitters. In 1891 a total of 2,749 was divided into 2,121 for the center of Angers (Trelaze), 109 for the Foret and 135 for Misengrain. During the year these 2,749 laborers produced 159,820,047 slates, worth $710,650.
The figures of 1893 correspond to a certain revival in this industry, which for some years previous had undergone a crisis due especially to the competition of machine-made tiles. As a sequel to the reduction in the production, it has been possible to raise the price slightly. -
Additional Sketch of Quarries Near Angers
(Not in the Original Article)