Several methods have been proposed - a great number of different solutions and preparations have been tried; but none of them combine efficiency and cheapness to such an extent as to have come into very general use.
1 Descriptive Catalogue, Museum of Practical Geology, Jermyn Street.
It is unnecessary to give a description of these preparations in detail, but they naturally range themselves under two distinct classes which may be noticed.
The first of these classes consists of preparations containing dissolved organic substances; these fill the pores of the stone, and preserve it for a time, but they are themselves subject to decay, and therefore can afford only a temporary protection.
The preparations of the second class are solutions of substances which act either upon the constituents of the stone to which they are applied, or upon one another (when more than one is applied) so as to form insoluble compounds which fill the pores and harden the structure of the stone, at the same time making it also denser, more impervious, and abler to resist atmospheric influences.
Many processes are successful in the laboratory of the chemist; but none is likely to be of use in the practical execution of engineering or building works, which is not economically applicable on a large scale.
It has been recommended that stones should be placed in vacuum chambers so as to introduce solutions more readily - also that stones should be heated, or immersed in solutions. All these methods are impracticable in dealing with large blocks, on account of the expense and inconvenience attending the manipulation.
Any preservative solution, to be of practical value, must be capable of application to the surface to be protected by means of a brush.