Magnesian limestones are composed of lime and magnesia carbonate in variable proportions, together with a small quantity of silica, iron, and alumina. Many limestones contain magnesia carbonate, but those with less than 15 per cent. do not come into the class now under consideration. The better varieties are those in which there is at least 40 per cent. of magnesia carbonate, with 4 or 5 per cent. of silica. When the magnesia is present in the proportion of 1 molecule of magnesia carbonate to 1 of lime carbonate (i. e. 54.18 and 45.82), the stone is called a dolomite. Prof. Daniel states that the nearer a magnesian limestone approaches dolomite in composition, the more durable it is likely to be. It is not merely the nature of the constituents or their mechanical mixture that gives dolomite its good qualities; there is some peculiarity in the crystallization which is all-important. Some peculiar combination takes place between the molecules of each substance; they possess some inherent power, by which the invisible or minutest particles intermix and unite with each other so intimately as to be inseparable by mechanical means.

On examining with a highly magnifying power a specimen of genuine mngnesian limestone, such as that of Bolsover Moor, it will be found not composed of 2 sorts of crystals, some firmed of lime carbonate, others of magnesia carbonate, but the entire mass of stone is made up of rhomboids, each of which contains both the earths homogeneously crystallized together. When this is the case, the stone is extremely durable. Some magnesian limestones contain sand, in which case their weathering qualities are greatly injured; while some are peculiarly subject to the attacks of sulphuric acid, which forms a soluble sulphate of magnesia easily washed away.

Preserving

Many processes for preserving stone from decay are successful in the laboratory of the chemist; but none is likely to be of use in practical execution which is not economically applicable on a large scale. Any preservative solution, to be of practical value, must be capable of application to the surface to be protected by means of a brush.

One of the most common methods of preserving the surface of stone is to paint it. This is effectual for a time, but the paint is destroyed by atmospheric influence in the course of a few years. In London the time hardly amounts to 3 years, even under favourable circumstances. Morever, it cannot well be used in important buildings, where appearance has to be considered. Oil has also been used as a coating; it fills the pores of the stone and keeps out the air for a time, but it discolours the stone to which it is applied. Paraffin is more lasting than oil, but is open to the same objection as regards discoloration of the stone. Soft-soap dissolved in water 3/4 lb. soap per gal.), followed by a solution of alum (1/2 lb. alum per gal.), has been frequently employed. Paraffin dissolved in naphtha, 1 1/2 lb. paraffin to 1 gal. coal-tar naphtha, and applied warm, is perhaps superior to the 2 preceding for this purpose. There is, however, no evidence to show that any methods such as these are likely to be successful in affording permanent protection to stone. Beeswax dissolved in coal-tar naphtha has also been proposed, or, when the natural colour of the stone is to be preserved, white wax dissolved in double distilled camphine.

Another plan is to melt 2 parts wax in 8 of pure essence of turpentine. The surface should be cleaned with water dashed with hydrochloric acid, but should be perfectly dry, the solution applied hot and thin.

There is a large class of preparations whose preservative influences depend upon the presence of soluble silica, which combines with substances contained in, or added to the stone under treatment. By this means insoluble silicates are formed, which not only preserve the stone from the attacks of the atmosphere, but also add considerably to its hardness. Unfortunately the use of these substances sometimes causes efflorescence on the face of the wall to which they are applied. The soluble alkaline salts left in the pores of the stone are drawn to the surface; these crystallize in the form of white powder, and disfigure, or in some cases injure, the wall. The soluble silica is sometimes found in the natural state. A large proportion may be obtained from the Farnham rock, or from the lower chalk beds of Surrey and Hampshire by merely boiling with an alkali in an open vessel. Ordinary silica in the form of flints may be dissolved by digesting with caustic soda, or potash, under pressure. If a piece of porous limestone or chalk be dipped into this solution, part of the silica in solution separates from the alkali in which it was dissolved, and combines with the lime, forming a hard insoluble lime silicate; part of it remains in the pores and becomes hard.

Kuhlmann's process consists in coating the surface of stone to be preserved with a solution of potash or soda silicate. The hardening of the surface is due to the decomposition of the silicate. If the material operated upon be a limestone, potash carbonate, lime silicio-carbonate, and silica will be deposited; besides which the carbonic acid in the air will combine with some of the potash, causing an efflorescence on the surface, which will eventually disappear. When applied to lime sulphate, crystallization takes place, which disintegrates the surface. In order to correct the discoloration of stone sometimes produced by the application of preservative solutions, Kuhlmann proposed that the surfaces should be coloured. Surfaces that are too light may be darkened by treatment with a durable manganese and potash silicate. Those that are too dark may be made lighter by adding baryta sulphate to the siliceous solutions. By introducing the iron, copper, and manganese sulphates, he obtained reddish-brown, green, and brown colours.

Ransome's indurating solutions consist of soda or potash silicate, and calcium or barium chloride. The surface of the stone is made thoroughly clean and dry, all decayed parts being cut out and replaced by good. The silicate is then diluted with 1 to 3 parts of soft water until it is thin enough to be absorbed by the stone freely. The less water used the better, so long as the stone is thoroughly penetrated by the solution. This is applied with an ordinary whitewash brush. After say a dozen brushings over, the silicate will be found to enter very slowly. When it ceases to go in, but remains on the surface glistening, although dry to the touch, it is a sign that the brick or stone is sufficiently charged; the brushing on should just stop short of this appearance. No excess must on any account be allowed to remain upon the face. After the silicate has become perfectly dry, the solution of calcium chloride is applied freely (but brushed on lightly without making it froth) so as to be absorbed with the silicate into the structure of the stone.

The effect of using these two solutions in succession is that a double decomposition takes place, and insoluble lime silicate is formed, which fills the pores of the stone and binds its particles together, thus increasing both its strength and weathering qualities. In some cases it may be desirable to repeat the operation, and as the lime silicate is white or colourless, in the second dressing the prepared calcium chloride may be tinted so as to produce a colour harmonizing with the natural colour of the stone. Before applying this second process, the stone should be well washed with rain-water and allowed to dry again. Special care must be taken not to allow either of the solutions to be splashed upon the windows or upon painted work, as they cannot afterwards be removed therefrom. Upon no account use any brush or jet for the calcium that has previously been used for the silicate, or vice versa. Under ordinary circumstances about 4 gal. of each solution will be required for every 100 yd. of surface, but this will depend upon the porosity of the material coated. This material has been used with success not only for the preservation of stone from decay, but also to keep out damp.

It is applicable both to stone and brick surfaces, as well as to those rendered with cement or lime plaster.

Szerelmey's stone liquid is stated by Prof. Ansted to be a combination of Kuhl-mann's process with a temporary wash of some bituminous substance. The wall being made perfectly dry and clean, the liquid is applied in 2 or 3 coats with a painters' brush until a slight glaze appears upon the surface. This composition was used with some success in arresting for a time the decay of the stone in the Houses of Parliament. The stone liquid is transparent and colourless, but Szerelmey's stone paint is opaque and of different colours, and is applied like ordinary paint.

The petrifying liquid of the Silicate Paint Company is stated in their circular to be a solution of silica, thinned with warm water, and applied to clean wall surfaces, which must be warmed if they are not already dry; 1 cwt. will cover 120 to 150 sq. yd.

Among other processes which have been tried are - Solution of baryta followed by solution of ferro-silicic acid so as to fill the pores of the stone with an insoluble ferro-silicate of baryta; solution of baryta followed by solution of superphosphate of lime producing an insoluble lime phosphate and baryta phosphate. Soluble alumina oxalate applied to limestones produces insoluble lime and alumina oxalate. These 3 processes last alluded to all possess the advantage of producing by the changes they undergo within the structure of the stone an insoluble substance, without at the same time giving rise to the formation of any soluble salt likely to cause efflorescence, which necessarily attends the use of alkaline silicates.

During the erection of large buildings, the surface of the masonry built in the earlier stages of the work is smeared over with a sort of thin mortar, so as to preserve it from atmospheric influence, and to make it easier to clean down.