SLATE. 



ferves to line the ciftems in which olive oil is preferved. 

 The canton of Claris, in Switzerland, is the only one in 

 which roof-flate is procured. Roof-flate occurs in Saxony, 

 and in various mountainous diilrifts in the north of Europe; 

 it is found alfo on the continent of North America ; and as 

 it is only a modification of clay-flate, which is an abundant 

 rock, it is probable that its localities are much more nu- 

 merous than are at prefent known in alpine diltrifts in 

 every part of the globe. 



As this fubftance forms the moft light, elegant, and 

 durable covering for houfes, and is, of courfe, of confi- 

 derable value ; it is rather furprifmg that fo much indifference 

 prevails refpefting the fearch for it, in thofe diflrifts where 

 common flate, or clay-Jlate, abounds. We believe all the 

 roof-flate quarries at prefent worked are thofe which acci- 

 dent has difcovered. This negleft is the more remarkable, 

 when we confider the great expence frequently incurred in 

 fearching for coal, a fubltance of much lefs value in propor- 

 tion to the weight. 



All the bed beds of roof-flate with which we are acquaint- 

 ed, improve as they fink deeper into the earth ; and few, 

 if any, are of a good quality near the furface, or are indeed 

 fuitable for the purpofe of roofing. There cannot be a 

 doubt that many beds of flate, which appear fliattered and 

 unfit for architeftural ufe, would be found of a good qua- 

 lity a few yards under the furface ; for the befl; (late, in 

 many quarries, lofes its property of fplitting into thin 

 laminae by expofure to the air. Notwithttanding the value 

 of flate, few quarries are worked to a very great depth, or 

 have fubterranean galleries, like mines. The quarry or flate- 

 mine at Rimagne, four leagues to the well of Charlevillc, 

 on the Meufe, in France, is an exception. The mouth of 

 the mine is near the fummit of a hill : the bed inclines forty 

 degrees to the horizon : it is about fixty feet in thicknefs, 

 but its extent and depth are unknown. It has been pur- 

 fued, by a principal gallery, to the depth of 400 feet ; and 

 they have driven many lateral galleries, which extend about 

 200 feet on the fide of the main gallery, where 26 ladders 

 are placed in fucceflion, for the pafl'age of the workmen and 

 the carriage of the flate. In this bed, which is 60 feet in 

 thicknefs, there are only forty feet of good flate, the other 

 being mixed with quartz. They cut the flate into blocks 

 of about 20olbs., which they callyai.r .• every workman, in 

 his turn, carries them on his back to the very mouth of the 

 pit, mounting the 26 ladders, or a part of them, accord- 

 ing to the depth of the bed where hti is working. When 

 brought to day, thefe blocks are firit fplit into thick tables, 

 which are called tepartons. The workman holds the block 

 between his legs, puts a chifl'el on the fide, and divides it 

 with 3 blow of a mallet. The repartons are divided in a fimi- 

 lar manner into roof-flates. Thefe operations muft be per- 

 formed foon after the blocks are drawn from the quarry ; for 

 if the (lone has time to dry, it would no longer be poilible 

 to fplit It. Some of the (late-galleries pafs under the river 

 Meufe. 



There are few places in Great Britain where flate is 

 worked as a mine under ground ; moft of the quarries are 

 open to the day ; and the covering of other rocks, or of 

 coarfe flate, which requires removing, greatly increafes the 

 expence. There is one flate-quarry worked as a mine by 

 penetratmg the interior of the mountain at Place-Fell, on 

 the head of the lake of Uilfwater, in Cumberland. Slate 

 IS alfo worked under ground on the weftern fide of York- 

 fliire, adjoining Weftmoreland. In many other fituations it 

 IS probable that flate might be worked to advantage, in fub- 

 terranean galleries, fimilar to thofe defcribed in the quarries 

 at Charlevillc ; for as this mineral is generally of a better 



quality at a confiderable depth, the expence of procuring it 

 by mining would be much lefs than that of removing the load 

 of upper rocks, and working it in open quarries, as at pre- 

 fent ; at leaft the molt valuable flate might be purfued to a 

 far greater depth than is prafticable in the eommon method 

 in open quarries. 



Slate, to be of a good quality for roofing, fliould have 

 the property of fplitting into thin even laminae : it fliould 

 refill the abforption of water; to prove which, it (hould 

 be kept fome time immerfed in water, being weighed before 

 and after the immerfion, wiping the furface dry ; it is ob- 

 vious, that the flate which gains the leaft weight by this pro- 

 cefs is the leaft abforbent. It (hould refift the proceis of 

 natural decompofition by air and moifture : this depends on 

 its chemical compofition and compaiSlnefs, and is (hewn by 

 its refiiting the procefs of vegetation. That flate which is the 

 moft liable to decay, will be the fooneft covered with lichens 

 and modes. The flate from Chamwood foreft, in Leicef- 

 ter(hire, refembles, in colour and appearance, the Weftmore- 

 land flate, but it will not fplit fo thin, nor would this be de- 

 firable, for the vegetation which takes place upon it re- 

 quires the roofs to be frequently fcraped ; this carries away 

 a portion of the furface : a fecond growth of moffes pro- 

 duces a further wearing of the flate : hence a flate of a cer- 

 tain thicknefs from this part of England is not fo durable as 

 that from North Wales, Cornwall, or Weftmoreland. This 

 decay is probably owing to the greater quantity of alumine 

 in its compofition ; it does not contain pyrites, nor have any 

 organic remains been difcovered in it. 



Few flate-rocks have been accurately analyfed ; a reddi(h- 

 purple flate from North Wales contained, according to 

 Kirwan, .38 filex, .26 alumine, .8 magnefia, .4 lime, and 

 .14 parts iron; but as there is in this analyfis a lofs of 

 10 per cent; it cannot be confidered as very accurate. As 

 the hardnefs of flate arifes principally from the filex it con- 

 tains, which is of all the earths the leaft favourable to vege- 

 tation, thofe flates which are the hardeft when firft taken 

 from the quarry, and which have the leaft fpecific gravity, 

 are to be preferred ; for the increafe of weight in flates is 

 owing to the prefence of iron, either in pyrites, or a ftate 

 of oxyd. To the prefence of iron, many kinds of ftone and 

 flate alfo owe their tendency to decompofition. The py- 

 rites being decompofed by moifture, and the iron admitting 

 a ftiU higher degree of oxygenation, the furface of the ftone 

 fwells and peels off, or falls into an ochrey powder. 



According to Dr. Watfon, (bifliop of LandafF,) the fpecific 

 gravity of the Weftmoreland flate varies in different quarries, 

 from 2797 to 2732 ounces the cubic foot. The effeft of froli 

 is very fenfible on tiled houfes, but is fcarcely felt on flated 

 houfes ; for good flate imbibes very little water. According 

 to an experiment made by Dr. Watfon on Weftmoreland 

 flate, compared with tile, in which two pieces of each, about 

 30 inches fquare, were immerfed in water 10 minutes, and then 

 taken out and weighed as foon as they ceafed to drop ; the 

 tile had imbibed about ith of its weight of water, and 

 the flate had not abforbed the xj^rth part of its weight : 

 indeed the wetting of the flate was merely fuperficial. 

 When placed before the fire, in a quarter of an hour the 

 flate was of the fame weight as it had been before it was put 

 into the water ; but the tile had only loft about 12 grains 

 of its moifture, which was as near as could be expetled to 

 the quantity which had been fpread over its furface ; for it 

 was the amount gained by the flate, the furface of which was 

 equal to that of the tile. The tile was left to dry fix days, 

 in a room heated to fixty degrees, but did not lofe all the 

 water it had imbibed till the end of that time. 



The flate in Weftmoreland is blafted from the quarry in 

 1 1 large 



