1850.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



i87 



second mass contains fossil remains of fish, without any other traces 

 of animal life; the marls also contain at times kidney-shaped 

 nodules of the sulpliate of strontian. In the first mass are found 

 the numerous remains of extinct birds, animals, plants, and shells, 

 iiliich render these formations 90 celebrated in a geological point of 

 view. On the nortli of Paris they are preserved in the gypsum 

 itself, and they retain a considerable degree of consistence, being 

 only surrounded by a thin coat of marl. On the south of Paris, 

 however, they are often found in the marl beds, and are then veiy 

 friable. The fossils of mammalia are exclusively confined to the 

 first mass, and in no instance are they met with in any of the 

 lower divisions. In the lowest, fossil tress have been found, and 

 fresh-water shells in remarkable abundance. Cuvier gives a list of 

 fourteen extinct species of mammalia, three or four birds, three 

 reptiles, and three or four species of fish: Lyell gives a much 

 greater number. 



Now, the immense development of these gypseous formations, 

 and the total absence of any traces of salt throughout the whole 

 extent, as well as the nature of tlie fossils they enclose, lead us to 

 believe that they must have been deposited under different circum- 

 stances from those which gave rise to the saliferous gypsums. An 

 examination of the phenomena connected with their probable 

 geological history would lead us into discussions which might be 

 considered out of place here. Those who may be desirous of 

 studying the question more thoroughly are referred to Sir C. 

 Lyell's ' Principles of Geology.' In the chapter upon the eocene 

 formations of the Paris basin the question is fully treated, with the 

 elegance, eloquence, the power of grouping facts, of adorning 

 details, which in Sir C. Lyell's case gives to science all the charm 

 of romance. Suffice it to say, that the present theory of geologists 

 leads them to regard the great mass of gypsum, in this district, 

 " as a purely fresh water deposit, produced by a river whose waters 

 were highly charged with the sulphate of lime, somewhat like 

 La Frume Salso, in Sicily." 



The method of raising the plaster stone differs, of course, with 

 the circumstances under wliich it is found; that is to say, it is 

 sometimes got by means of open cuttings, or by galleries, worked 

 either from the hill side or by wells. The peculiarly abrupt man- 

 ner in which the spurs of gypsum terminate upon the heights round 

 Paris, renders the mode of working from galleries driven into the 

 hill face the most usual. At Montmartre, Triel, and Belleville, 

 the quarries are all worked in that manner. Tlie regulation of the 

 quarries is, like everything else in France, subject to a very scien- 

 tific and inquisitorial supervision on the part of the government. 

 The service of the mines is under the control of a special body of 

 engineers, called "Les Ingenieurs des Mines," who are charged to 

 insure the public safety and the lives of the workmen, which might 

 otherwise be compromised by the mining operations; to defend the 

 rights of the state to the discovery of the precious metals; and 

 subsidiarily to ascertain all geological facta which might influence 

 the national wealth. The consequence of this organisation is, 

 that the statistics of French geology, if such a term be allowed, 

 are classified in the most wonderful manner; an instance of which, 

 by the way, is to be found in the geological map and explanation 

 published under the direction of M. Elie de Beaumont. How- 

 ever, quarries in open cutting are worked by the proprietors of 

 the land, without any control on the part of the engineers of the 

 mines; and they are simply under the control of the police. When 

 they are under ground, the quarries are under the special control 

 of the engineers, and the principles which regulate their working 

 are those laid down by a decree of Napoleon's, dated March 2nd, 

 1813. Rigorously, the stone or gypsum quarries ought to be 

 worked with something like the regularity of a chessboard; the 

 galleries being 15 metres, or about 50 feet wide, with piers at 

 equal distances of 10 metres, or 33 feet square. In practice this 

 mathematical precision is neglected; but it may be considered as 

 the average manner of working. The quarr\'-cap of the gypsum 

 does not admit of being left with so wide a bearing as 50 feet, as 

 might naturally be supposed. A small heading is then driven in 

 the bed, called the "souchet," by a man lying flat on liis back, for 

 tlie bed is only 1 ft. 8 in. deep, who leaves the upper bed, "le banc 

 be grand abattage," unsupported in this manner, for a width of 8 

 feet. For this very painful work the miner, called in this case the 

 "caveur," is paid at about the rate of Is. per foot forward; he find- 

 ing his own picks, the proprietor the candles. The other beds are 

 then raised by wedges, bars, or gunpowder, as may be required. A 

 good quarryman can raise about 9 yards cube per day, of the first 

 mass, and about 53 yards of the two lower masses, when the work- 

 ings are in galler)'. 



\Ve have before seen that the quality of the gypsum is not the 



same through the whole thickness of the different masses. Great 

 care is then required in mixing the different sorts of stone, so 

 as to secure an uniformity in the plaster obtained by the burning. 

 Some of the beds are reserved for special uses; the hard beds, in 

 tlie remaining portions, require to be mixed with the softer ones. 

 As might naturally be expected this variety introduces a compli- 

 cation in the manufacture, which frequently gives rise to improper 

 fabrication, and opens the door to much fraud. Indeed, the fabri- 

 cation of plaster near Paris, still more in the departments, is liable 

 to all the reproaches we so unsparingly address to our own cement 

 manufactures. Such must always be the result of unlimited compe- 

 tition, and as long as price is made of more importance than quality 

 such they will remain. 



The mode of burning usually adopted is very rude. It consists 

 simply in building, within three walls, covered with a rough fixed 

 roof, a series of arches 1 ft. 8 in. wide by 2 ft. 4. in. high, with piers 

 formed of gypseous stones, as are also the arches. These are then 

 fiUed up to a height of 13 feet with stones, so arranged that the 

 largest are at the bottom, the smallest at the top. The arches are 

 filled in with fire-wood, wliich is set light to, and the fire kept up 

 so as to maintain the baking for twenty-four hours. The dimen- 

 sions of these kilns are such as to enable them to hold from seventy 

 to seventy-five tons. In some of the quarries a more rational style 

 of burning is adopted, which consists in passing the already pul- 

 verised stone through cylinders, which revolve in an open fire. I 

 have, also, in one of Mr. Weale's Treatises, mentioned an applica- 

 tion of over-heated steam to the same purpose; but the inquiries I 

 made in Paris, about a month since, lead me to believe that it has 

 not yet been fairly tried. 



Indeed, there is always a difficulty in introducing any new pro- 

 cess in the ordinary arts of life, such, for instance, as the one 

 which meets us on the threshold in the use of the French plaster. 

 Near Paris, the workmen have always been accustomed to employ 

 plaster burnt in immediate contact with the wood. In that process 

 the brees become necessarily mingled with it, and we find now that 

 the men have come to consider the grey colour they communicate 

 as an indication of a superior quality. The Paris workmen, in 

 fact, do precisely the reverse to what our workmen do; upon the 

 same principle, nevertheless, viz. — from an irreflective habit. They 

 dislike a white plaster; we attach far too much importance to it. 

 Truth, as in most cases, lies in the mean. The absence of the brees 

 certainly does not diminish the value of the plaster; the extreme 

 whiteness we contend for in London is for the most part obtained 

 by the use of a softer description of stone, or by the admixture of 

 some extraneous ingredient. 



The operation of burning the plaster stone, is, after all, only 

 effected for the purpose of dehydrising, or driving off the water of 

 crystallisation from the gypsum. Before this is done, the stone is 

 hard; afterwards, it becomes pulverulent and floury. The ra- 

 tionale of its use is, simply to present such a quantity of water 

 as is necessary to restore it to the original .state, when it resumes 

 its natural hardness, with a commencement of a confused crystalli- 

 sation. Now this action may be, and is, carried on irrespective of 

 colour; that is to say, at least, the presence of the wood ashes, 

 which gives rise to the grey tint the Paris workmen require, does 

 not affect the combination with the water. Our own very white 

 plasters owe their beautiful colour to the absence of the carbonates 

 of lime, or the marls, which, in fact, communicate the very supe- 

 rior qualities to the stones yielding plaster less purely white. 



To secure a good quality of plaster it is advisable to apply a 

 moderate heat in the beginning, which is to be augmented gradually 

 ^V^hen the plaster is not sufficiently burned, it becomes dry and 

 sandy; in this state it does not set with any degree of hardness. 

 AV^hen it is overburnt, it also loses its adhesive properties; it ceases 

 to have what the workmen call "de I'amour;" it will not cling to 

 the fingers, nor has it the rich unctuous quality which characterises 

 the well-burnt plaster. As soon as it is burnt, it should be 

 ground, and employed as soon as possible after the manipulation 

 is completed. 



Fourcroy believetl that the carbonate of lime contained in the 

 Paris gypsum, became converted into quick lime during the burn- 

 ing; and that the superiority of that plaster was to be attributed 

 to that change. Guy Lussac, however, held that the carbonate 

 could not be aft'ected by the moderate heat called into action (it is 

 only absolutely required to be about 2T0 Fah.) He attributes the 

 superiority rather to the great hardness of the stone; and really 

 there does not appear to be any other explanation. We are aware 

 that, cceteiis parilmx, the law exists, that the limestones yield 

 limes producing mortars whose degree of hardness, when set, is in 

 the ratio of the hardness of the stone. Nor does there appear to 



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