The division and preparation of the softest of the former materials, namely clay, can be accomplished by the hands alone; in others, as alabaster and slate, with the ordinary toothed saws; and for those of a harder nature, the stone-saw fed with sand and water, is an economical mode of dividing them with great exactness and little waste, from their original forms to those in which they are ultimately required, and which is greatly facilitated by the structure of such as occur in stratified beds; but the use of the stone-saw may be considered to cease with the sandstones.
Different and far more troublesome methods of working are necessary with those materials now to be considered, that are much harder, and in which the existence of stratification is considered but rarely and imperfectly to exist; namely, in the compact and cemented porphyries, principally from Egypt and Sweden; the crystalline granites, abundant in Devonshire, Cornwall, and Aberdeenshire, and the elvans of Cornwall, which latter, and some other varieties, appear to merge from the porphyries to the granites, are used for similar purposes, worked by the same means, and ask for an intermediate position.*
"In converting the rude masses of granite to their intended forms, the line of the proposed division is first marked, and holes from two to three inches deep, and four to six inches asunder, are bored upon this line, by means of an iron rod terminating at each end in chisel-formed edges of hardened steel, with a bulb in the middle to add weight; this tool, called a jumper, is made
* The harder crystalline rocks last referred to, rarely occur in stratified beds, although a remarkable case has of late years been found to exist in the Foggintor Quarry, worked by the Haytor Granite Company, on Dartmoor, the subjoined particulars of which have been kindly communicated to me by William Johnson, Esq., who read a paper before the British Association at Plymouth, 1841, on the Railways and Machinery connected with this interesting work. See the Report in the Athenaeum,p. 654.
"The distinct beds of the Foggintor Quarry follow pretty nearly the natural slope of the hill, and are thicker and more nearly horizontal the deeper they are situated. The beds are intersected by heads or natural joints, which are nearly perpendicular fissures, (sometimes solid seams,) of unknown depth, that separate the horizontal floor of the quarry into irregular figures, and the continuity of the beds is often broken at the different headings. The quarry, in some parts 100 feet high, is worked in benches, or like a huge irregular flight of steps.
"In detaching the masses of granite rock from their natural beds, the points of least resistance are first determined by an experienced eye, and holes are sunk at those points, vertically or inclined as circumstances may require: the diameters of these holes vary according to the mass and the amount of resistance; and their depths according to the thickness of the blocks to be detached.
"The holes are made with an iron rod, terminating at foot in a chisel-formed edge of hardened steel; the tool is held by one man, who changes its position at every blow received from sledge-hammers worked by other men who stand around. When the holes are thus made sufficiently deep, they are charged with gunpowder in order to effect a separation of the mass by blasting, the ordinary process of tamping confines the powder, and the fuse communicates the blast. The art of the quarryman consists in placing the blast, (or shot,) where the smallest amount of powder will remove the largest mass of rock with the least breakage, simply dislodging or turning it over ready for converting. Three and even five thousand cubic feet have been removed by one discharge." to fall on one spot, it rebounds, and is partially twitted round to present the edge continually in A different angular position , in this manner a very expert workman will bore about a hun-dred holes in a day. Every one of the holes is then filled with two half-round pieces of iron called feathers, with an iron pointed wedge between them; the wedges arc progressively and equally driven until the stone splits, and the fissure will be in general moderately flat, even should the mass be four or six feet thick, although in such cases the holes are sometimes continued round the ends also."
"The scouters, the next class of men, employ the jumpers' feathers and wedges for removing any large projections, by boring holes sideways, and thus casting off large flakes; the spalders employ heavy axe-formed or muckle-hammers, for spalling or scaling off smaller flakes; and the scabbiers use heavy pointed picks, and complete the conversion, so far as it is effected at the quarry, ready for the masons employed in erecting the buildings for which the blocks are used, who complete their formation on the spot." All these materials are likewise used in the ornamental arts.
Porphyry is worked in the lathe with remarkable perfection, and many excellent specimens from Sweden, of vases, slabs, pestles and mortars, and bearings intended for the gudgeons of heavy machinery may be seen in London.* I learn that these objects are first worked as nearly as possible to the required forms with the pick, are then mounted in lathes driven by water power, and finished by grinding them with other lumps of porphyry, supplied with emery and water; the machinery is kept going day and night, and the gangs of men relieve one another at certain intervals.
Granite is incapable of being turned in the lathe: it is therefore treated like porphyry, that is, shaped with heavy picks, and finally with smaller points used with a hammer; it is afterwards ground with circular or reciprocating motion, according to the figure, by means of iron plates fed with sharp sand, next with emery, progressively finer and finer, upon wooden rubbers, the endways of the grain; and lastly, the polish is perfected with felt rubbers and crocus. The process is tedious and difficult from the unequal hardness of the particles; in this respect granite is inferior to porphyry.
* Various works in porphyry are contained in the Polytechnic Exhibition, Regent Street. Amongst them is a table five feet ten inches diameter, tastefully inlaid with a great number of pieces of various colours; the pedestal is fluted with great exactness. apparently by mechanism: it is said the construction of the table occupied fire men during seven years. The sum asked for it is 500l.
The Emperor of Russia had four splendid rases of porphyry fire or six feet high, two of which he gave to the Duke of Devonshire; these are at Chatsworth House. This manufacture has in a very few years converted the barren valley in which the porphyry is found, to a scene busy with the hum of industry.
The Saloon of Egyptian Antiquities, British Museum, contains various sculptures in porphyry, the work of almost inconceivable labour and perseverance.
Of late years numerous vases and other circular and orna-mental objects have been admirably executed in polished granites and elvans, which occur of various colours and degrees of hardness; when decomposed they are friable, and furnish the china stones extensively used as one of the materials for porcelain and china, and also for making very refractory crucibles.*