A material which is suitable for large objects to which the solution can only be applied upon the surface is Kessler’s fluate[98], which is soluble in water, and which hardens the limestone without completely closing the pores. It offers the additional advantage that it is applicable to thick limestone blocks, the dryness of which is not certain. The solutions numbered 1-9 must only be used when the limestone is dry throughout its mass. The fluate to be used in any particular instance must be decided from the nature of the case. Those most generally applicable are magnesium and zinc fluates and the so-called “double fluate.”
The stones from the Meten Chamber were hardened in the following manner: The limestone blocks were placed upright and the surface dusted by the air-current from a Dechend’s spray apparatus[99] which was then used to spray them repeatedly with a solution of “double fluate” of sp. gr. 1·16. Owing, however, to the injurious effect of the fine spray of the fluate upon the nose and lungs the stones were turned to a horizontal position, and a solution of fluate of sp. gr. 1·38 was applied by means of a large brush until the fluid was no longer absorbed. For the treatment of limestones on which there are remains of colours the use of a solution of shellac, gum-dammar, or collodion is recommended. Fluates should not be applied until their suitability for the particular purpose has been tested.
All specimens should be kept after impregnation in rooms which as far as possible are free from dust, for the dust which falls upon the surface will set in the varnish whilst it is hardening.
Impregnation without Previous Steeping. If a preliminary examination has shown that specimens of limestone will not bear steeping in water, recourse can be had to impregnation only. The treatment of such specimens must be thorough, for merely to paint the fluid upon the surface with a brush almost invariably proves a failure. Instead of penetrating the stone the impregnating medium forms a firm coating which is liable to be lifted, and in parts broken, by the crystallisation of salts, and thus allows the destructive processes to continue uninterrupted. Aqueous solutions, e.g. size, cannot of course be applied, and as it is necessary to make a preliminary trial of a fluate spray, it is generally found preferable to make use of the varnish-benzine mixture. In spite of this, salts may still make their appearance in the form of a crystalline powdery layer on the surface, which can be wiped off with a wet sponge; any moisture must however be removed with a soft dry linen cloth.
Removal of Incrustations and Dust. Incrustations of earth, lime, or gypsum should be washed off with water or removed by mechanical means, such as gentle rubbing with the finger. The solvent action of acids upon limestone precludes their use for this purpose. Any dust which adheres can be removed by rubbing with stale bread-crumb.
(b) Marble and Alabaster.
It is usually only necessary to clean marble with a soft brush and warm water, with the addition perhaps of some good neutral soap. In rare cases the presence of sulphates may perhaps cause some friability. The crystalline structure of marble renders steeping futile, and accordingly impregnation is resorted to. The use of Kessler’s fluates may be recommended. Adherent pitch or resin is best removed by a mixture of alcohol and ether. Alabaster seems to remain permanently sound and may be cleaned in the same way.
(c) Earthenware.
Steeping. The same line of treatment should be followed as in the case of limestones. A preliminary examination should always be made to test the power of resistance in water, which is always satisfactory if the clay has been sufficiently baked.
In the case of coloured terra-cotta care should be taken to ascertain whether the colours are likely to suffer during steeping. There is no danger of injury if the steeping is not too prolonged; in fact, the removal of the dust during the procedure often brings out the colours more clearly. If the Egyptian ostraca (clay fragments with black script) require to be washed they should be carefully watched in order to preserve the script, and therefore should be placed in the bath in such a way that the lettering is visible.