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.
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.
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 Encyclopedia of Architecture, page 485.
This stone is well adapted for all purposes for which natural sandstones and limestones are used. It can, however, be 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, 11/4 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 51/2 per cent of its weight of water.
Victoria Stone consists of washed, finely-powdered granite, bound together with the strongest Portland cement, and then hardened by immersion in silicate of soda.
The silicate is formed by boiling ground Farnham stone in cream caustic soda.
A mixture of four parts of crushed granite with one of Portland cement is allowed to set for three days or more into a hard block moulded to the required shape. It is then immersed in the silicate of soda for some seven or eight weeks.
The lime in the cement combines with the silicate, the whole mass beincr indurated by the silicate of lime thus formed.
This artificial stone is used chiefly for paving, which is said to be more durable, to be cheaper, and to stand a greater crushing force, than Yorkshire flags. It is used also for window sills, coping stones, caps for piers, stairs, landings, troughs, tanks, sinks, etc.
It weighs from 140 to 160 lbs. per cubic foot, and absorbs from 2 to 6 per cent of its weight of water in 24 hours. The thinner flags are less compact and more absorptive than the thicker ones.2
"The white colour, semi-transparency, and extreme hardness of this oxy-chloride, as well as the small quantity which is required for binding together a considerable mass of any material, facilitate the production of imitations of any description of stone, and render it highly probable that it will play an important part in the future history of artificial stone." 1
Where used. - This stone has been used for the whole of the external stonework, except the cornice, at Fresh Wharf, London Bridge.
Also for the panels in the tower, and for the chimney shafts at Messrs. Peek and Frean's biscuit manufactory at Bermondsey, and for paving in many parts of London.
Silicated Stone is made in the same way as Victoria stone, and is used for paving slabs and drain pipes.
Sorel Stone is so called after M. Sorel, a French chemist.
Native carbonate of magnesia, or magnesite, is calcined and mixed with sand or powdered marble. It is then wetted with waste liquor from salt works containing a large proportion of magnesium chloride, pugged, and then rammed or stamped into iron, wooden, or plaster moulds.
It hardens rapidly, setting throughout its mass like ordinary hydraulic cement. In 24 hours it is hard enough to remove from the moulds, and the blocks will bear handling in three or four days.
The proportion of magnesia to the inert material bound together varies from 3 to 15 per cent.
Chance's Artificial Stone is made by melting the Rowley Rag, a basaltic rock found in Staffordshire, and then casting it into the shapes required for different architectural ornaments.
Greenstone, whinstone, or any similar rock, may be treated in the same way.
The moulds are of sand in iron boxes, and are at a red heat when they receive the melted stone. They should cool slowly, in order to obtain a hard material like the original stone; if allowed to cool too quickly the material becomes brittle and glassy.1
Rust's Vitrified Marble is produced by fusing together a mixture of glass and sand. "The soft pasty mass is taken out of the pot on the end of an iron rod, and placed in a small metal mould of any required shape or design. The large proportion of sand used prevents the mass, when thus suddenly cooled, from acquiring such a high state of tension as to be liable to fly to pieces, which would be the case with glass alone. The material, when cool, is either used in the form in which it is cast, or it is broken up into small pieces by the stroke of a light hammer, to be used in the construction of mosaics for pavements, or other purposes.
"Any colour can be given to the mass when in a semi-fluid state by mixing with it the oxides of iron, chromium, cobalt, or such other colouring materials as are usually employed for fired ware. This vitrified marble has been used for the bosses and coloured portions of the string course which extends round the Home and Colonial Offices, and also at the Albert Memorial in Hyde Park." 2
Other artificial marbles are made, which partake of the character of plasters, and will be noticed in Chapter III (Masonry. Walling).
Artificial Paving Slabs and Paving Stones of many kinds are in the market. They are often composed of Portland cement concrete (see p. 210),very carefully made, with hard aggregates and the very best cement. It is said that the very finely ground German cement (see p. 16 4) is used for this purpose. Silicates are sometimes added to give hardness to the mass.