In consequence of the difficulty which exists in many localities of obtaining durable natural stone at a moderate cost, many processes have been invented for the manufacture of artificial stone.

Some of these processes are successful in producing artificial stones which compare favourably in all their qualities with natural stones having a high character.

The expense of artificial stone is a bar to its extensive use for ordinary blocks, but the facility with which it can be moulded to the most intricate forms makes it very economical when it is required to take the place of carvings or other enrichments in natural stone.

A few of the best known artificial stones will now be described. Some of them are merely forms of concrete, and will be mentioned in the chapter devoted to that material.

Ransome's Artificial Stone is made by mixing artificially-dried sand with silicate of soda (dissolved flint) and a small proportion of powdered stone or chalk. These are thoroughly incorporated in a pug or mortar mill, and forced by hand into moulds.

The blocks turned out have a cold solution of chloride of calcium poured over them, and are then immersed in a boiling solution of the same, sometimes under pressure, so that the pores of the material are entirely filled with the solution, after which it is found to be as hard as most building stones. The excess of chloride of sodium is then washed off, otherwise it is apt to cause efflorescence.

It will be seen that the above process depends upon the double decomposition of the silicate of soda and chloride of calcium. The chlorine and soda combine to form chloride of sodium, which is washed out, and the silica attacking the calcium forms silicate of lime, a strong and durable cement which binds the particles of the stone together.

Characteristics

This stone has a fine homogeneous structure, so that it can, if necessary, be worked and carved like the best building stones.

The great advantage that it possesses is the facility with which it may be moulded into any form required.

Several experiments have been made upon this material.

It absorbs about 6.5 per cent of water.

Its tensile strength is about 360 lbs. per inch.

Its resistance to crushing about 2 tons per inch.

It weighs about 120 lbs. per cubic foot.

Of course these figures vary according to the nature of the material used in making the stone, the age of the specimen, etc.

The composition of this stone indicates that it will weather well, and some experiments made by Professor Frankland show that its resistance to acids was fully equal to Portland, Anston, Parkspring, and other of the best building stones.

Details of the experiments made by different observers will be found collected in Gwilt's Encyclopaedia of Ardritecture, page 485.

Uses

This stone is well adapted for all purposes for which natural sandstones and limestones are used. It can, however, he most economically employed for dressings (especially for those of an ornamental character), and for imitation carved work, though its use for this purpose has been condemned from an artistic point of view.

This stone is also used for caissons or hollow blocks for foundations, for grindstones, filters, etc.; and by substituting grains of corundum and oxide of iron for the sand, a substance called solid emery is produced, which is formed into wheels for sharpening tools, polishing metal surfaces, etc.

Ransome's stone has been used at St. Thomas's Hospital, the India Office, the London Docks, the Brighton Aquarium, the Albert Bridge, and in several other buildings both at home and abroad.

Apcenite is a variety of Ransome's stone, made with 5 parts of sand, 1 of Farn-ham rock, 1 1/4 of Portland cement, with the same proportion of silicate of soda.

It can be made more quickly, and is considered superior to the other.1

Moreover, it has the great advantage that it can be made on the works where it is to be placed in position.

It is used for steps, balustrades, cylinder foundations, etc.

It weighs about 137 lbs. per cubic foot, and absorbs in 24 hours about 5 1/2 per cent of its weight of water.