Limestones, as a whole, are inferior as building material to sandstones generally, though Portland may be considered as excepted from that category. They consist of almost pure carbonate of lime, and are, of course, affected by the sulphurous and smoky atmosphere of large manufacturing towns, which gradually eats away the stone. Compared with sandstones, as a building stone, they are less durable; they weigh from 116 to 150 lbs. per foot cube, withstand a pressure of about 250 tons per square foot, and are much more easily worked, especially when freshly quarried and green with quarry sap.
The best of the limestones are dense and uniform, both in structure and composition, with a fine even grain, and crystalline texture; and moreover, they will weather well, and, especially in some atmospheres, are the most economical stone, being preferred for their uniform colour and facility of working. They are often coated with other substances, to preserve them, as will hereinafter be explained.
A good limestone should have most of the qualities enumerated above, absorb no more than 17 per cent of water, and be free from any earthy or dull appearance. There are several classes of limestone, including the Compact, the Granular, the Shelly, and the Magnesian. The first of these is chiefly used for road material, pavings, etc.; wherefore it is unnecessary to refer to it, not being a building material.
Granular limestones, as the name implies, consist of grains of carbonate of lime cemented together by the same substance; and they belong to the Oolite formation, those with large grains being called Pisolites or Pea stones, and those with small grains Roe stones, because they resemble the roe of a fish in appearance. They are of a brown or yellow colour, and, from their nature, rather absorbent, and liable to the attack of acid in a town atmosphere; though some of them are less so than others, and will weather well in ordinary positions. Among their number must be included several very good and commonly used building stones, viz.: -
Bath Stone is perhaps the best-known building stone in the country, being a free-working oolite, containing 95 per cent, of pure carbonate of lime, and of various kinds - including Box Ground, Coombedown, Monks Park, Corngrit, Corsham Down, Farleigh Down, Westwood Ground, and Winsley Ground - all similar in general appearance, and only to be correctly distinguished from one another by a skilled and practised eye. St. Aldhelm Box is the best weathering stone of the lot; and Corsham or Monks Park is the most suitable for external work, other than weatherings; while the other varieties are only fit for interior work, though they are good soft suitable stones for inside purposes.
The Bath Stone quarries are really stone mines, because they are worked partly underground, the depth of stone varying from 6 to 20 feet, running in fairly level beds, increasing in depth the lower they get The quarrying is done by the use of pick and saw, without any explosive, a horizontal groove about 10 inches high and 5 feet deep being picked out next the ceiling of the mine, into which groove the quarryman puts a saw, working downwards to the bottom of the bed, nearly 4 feet down. Another cut is then made at a distance of 3 feet from the last, and directed towards it at the back, making the stone-block narrower at the back than in front. The block is then got by .means of wedges inserted under the bed, at the bottom, and into the saw cuts at the side, and these being driven home, they burst the stone off at the back, and the block is brought out. The quarrymen then have a start into that bed or beds, the other stones being easily procured afterwards, and trimmed with axes and saws into the required sizes of blocks.
The colour of Bath Stone may be said to be light brown, creamy, or stone colour. It is of a nice, even nature, and good, when free from yellow or earthy beds, easily worked, and suitable; in one or other of its varieties, for most purposes - especially when coated with the fluate which the "Bath Stone Firms Limited" recommend as a means of hardening and preserving its face - so that, taking all things into consideration, it is a most useful building stone.
Portland, containing 95 per cent, of pure carbonate of lime, is, in very many ways, a splendid stone, of the Oolite formation, found in the island of Portland, in quarries with generally three different beds, in use for different purposes, as follows: -
The roach bed, the first good one found in the quarry, is a mass of fossils, thoroughly cemented together with pure carbonate of lime. The fossils are like a corkscrew in form, and only appear in this bed, which is of a light brown colour, very tough and strong, and of a very high resistance to the action alike of air and water; so that it is in great requisition for engineering works of all kinds, though not for ordinary building purposes, on account of the fossils and the holes in it.
The whit bed, of either a white or brown tint, comes next to the roach, and is a fine, open, well-cemented, hard, and crystalline stone, suitable for the best buildings, and capable of taking a good arris and smooth face, while it weathers excellently well. It is "roe"-like in appearance under a microscope, and hard to distinguish from the " base bed," an inferior quality found lower down in the strata.
Next to the whit bed comes the bastard roach; distinct from the true roach, which it resembles in appearance, though it never contains the "screw" fossil among its mass of fossils, and its cementing matter is so bad that it is altogether inferior; in fact, not fit to use at all for any purpose.
The base bed, coming next, is a similar stone to the whit bed, but not so good, being softer and not able to withstand the weather; though it is often substituted for the whit bed, from which it can hardly be distinguished.
Caen is a soft, creamy-coloured stone, used only for internal work, carving, etc., and is found in Normandy.
Chi/mark, Tisbury, or Wardour stone is more of a sandy nature, found near Salisbury, and considered to be a good, durable, fair-working stone, which weathers well, and can, from certain beds, be adapted for internal staircases, not exposed to extraordinary wear, besides being suitable for general purposes.
Ketton is another good, useful stone of this formation, and one which shows the "roe-stone" grains very plainly. It is found near Stamford in three qualities, the hard, medium, and soft; the best of which is very durable, free-working, and suitable for all kinds of dressings and stairs.
Of the Shelly limestones, the only one of any note used for building purposes is the Hopton Wood (found in Derbyshire), which is a very hard stone, looking well on account of the fossils it contains. It is capable of taking a polish, and is suitable for steps, staircases, chimneypieces, and other ornamental work.
The Magnesian limestone formation supplies us with a few good stones, which are called Dolomites (as previously mentioned), the chief variety being Bolsaver stone, found in Derbyshire, of a light yellow-brown colour, and of great durability. It is crystalline, and very even in texture, and was extensively used in the erection of the present Houses of Parliament.