Soluble silica.- 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 crystallise 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, however, be dissolved by being digested 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 silicate of lime; 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 silicate of potash or silicate of soda.
The hardening of the surface is due to the decomposition of the silicate of potash. If the material operated upon be a limestone, carbonate of potash, silicio-carbonate of lime 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.1
When applied to sulphate of lime, crystallisation takes place which disintegrates the surface.
In order to correct the discoloration of stone sometimes produced by the application of preservative solutions, M. Kuhlinann proposed that the surfaces should be coloured.
Surfaces that are too light may be darkened by treatment with a durable silicate of manganese and potash.
Those that are too dark may be made lighter by adding sulphate of baryta to the siliceous solutions.
Bansome's Indurating Solutions consist of silicate of soda or potash, and chloride of calcium or barium.
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 from 1 to 3 parts of soft water until it is thin enough to he ahsorhed by the stone freely. The less water that is used the hetter, so long as the stone is thoroughly penetrated by the solution.
The solution 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 chloride of calcium 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." 1
The effect of using these two solutions in succession is that a double decomposition takes place, and insoluble silicate of lime 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 silicate of lime is white or colourless, "in the second dressing the prepared chloride of calcium may be tinted so as to produce a colour harmonising 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."
The following cautions are given in Messrs. Bateman's circular :-
"1. The stone must be clean and dry.
"2. The silicate should be applied till the stone is fully charged, but no excess must upon any account be allowed to remain upon the face.
"3. The calcium must not be applied until after the silicate is dry; a clear day or so should intervene when convenient.
"4. 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.
"5. 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 four gallons of each solution will be required for every hundred yards of surface, but this will depend upon the porosity of the material coated.
It has been used at St. George's Hall, Liverpool, for preserving the sculpture; at Trinity College, Dublin; Cardiff Town Hall; Greenock Custom House, and for other buildings both in this country and in India.
It is applicable not only to stone and brick surfaces, but also to those rendered with cement or lime plaster.
Szerelmey's Stone Liquid is stated by Professor Ansted to be "a combination of Kuhlmann's process with a temporary wash of some bituminous substance."
The wall being made perfectly dry and clean, the liquid is applied in two or three coats with a painter's 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.
1 Patentee's circular.
The stone liquid is transparent and colourless, but Szerelmey's stone paint is opaque and of different colours, and is applied like ordinary paint (seep. 412).
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 from 120 to 150 square yards.
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 phosphate of lime and phosphate of baryta.
Soluhle Oxalate of Alumina applied to limestones produces insoluble oxalate of lime and alumina.
"These three 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." 1
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.