(6) Linseed oil dissolved in benzine.
(7) Linseed varnish dissolved in 3 parts of benzine or petroleum ether.
(8) Solutions of stearine or paraffin wax in benzine.
(9) Collodion (free from acid). Zapon[96].
(10) Kessler’s fluate.
It may be added that, as a general rule, solutions for this purpose must be used as dilute as possible, for two immersions in a dilute solution are preferable to a single soaking in a concentrated one, which often scarcely penetrates into the pores.
As the preparation of solutions of shellac, gum-dammar, and of such substances as resin, stearine, and paraffin, necessitates heating, and as the solvents are very inflammable, it is advisable to make use of the solution of linseed varnish in benzine. This solution may be obtained at any time at any degree of concentration without the use of heat. Although it has the advantage that it hardens more rapidly than a simple solution of linseed oil it has also one disadvantage, for it gives a somewhat darker colour to light-coloured limestones. No more of the mixture of varnish and benzine should be prepared than is required for the impregnation, for this solution, on standing, throws down a gelatinous precipitate which is not re-dissolved even by heating. As this alteration is accelerated by the action of light, the mixture should always be kept in a dark place.
Collodion and zapon[96], on account of the expense, should only be used for small objects. After impregnation the objects should be covered with glass jars, cardboard boxes, etc., to prevent the precipitation of moisture upon them, as the result of the rapid evaporation of such volatile substances as benzine and ether upon exposure to the open air.
Rice water, tapioca water, or size (the latter of no greater strength than 2%) are only applicable to specimens which are kept in dry rooms, for in damp rooms they readily become sticky, and are liable to be attacked by moulds. Waterglass solution, probably because it is generally applied in too concentrated a form, instead of penetrating the object has a tendency to form a pellicle, which readily strips off. Even dilute solutions, however, are said to be unsuitable, from the liability to the efflorescence of alkali salts.
In the case of marble objects and antique statues of porous limestone, showing colours which are still bright on excavation, but which would soon fade, Rhousopulos[97] recommends impregnation with a very dilute solution (1 in 1000) of waterglass to preserve the colour. The solution should be as neutral as possible: in any case not alkaline. This is several times sprayed upon the object, which is allowed to completely dry between each spraying.