This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Keim's process claims attention as being the result of nearly 12 years' thoroughly scientific labour and research on the part of the inventor, and is based on the stereo-chrome process of Schlotthauer and Fuchs, differing however from that in such important particulars as to constitute, practically, an entirely new process of itself.
In 1S4S, Prof. Schlotthauer, of the Munich Academy, who had for some time been engaged in experiments with a view to discovering some permanent process for mural paintings, turned his attention to the substance known as water-glass (sodium silicate), the invention of the chemist Fuchs. The result was the adoption of the stereo-chrome process. In this process the surface to be painted on consisted of an ordinary mortar of lime and sand, impregnated with water-glass. Upon this surface the painting was executed in water-colour, and was then fixed by water-glass thrown against the surface in the form of a fine spray, the water-glass in this case forming the fixative for the painting. In practice, it toon became evident that a simple spraying of water-glass, applied to heterogeneous pigments, without reference to their peculiar properties as regards chemical composition, cohesive capability, etc, was not sufficient to ensure their permanence. Certain colours in particular, as ultramarine, umber, and black, were observed to be always the first to detach themselves in the form of powder, or by scaling off from the painting; thus pointing to the fact that their destruction was not owing to any accidental defect in the manner of their application, but to some radical unsuit-ability arising from the chemical conditions of the process.
In Keim's process regard is paid in the first instance to the ground upon which the painting is to be executed. A careful study of the best examples of the fresco paintings of former times, convinced him that the painting ground was a feature of supreme importance. The wall to be treated must contain no damp or decaying stones or bricks, and the latter must have been sufficiently baked, otherwise they will develop an efflorescence most injurious to the process. If the wall be already covered with stucco or mortar, this will serve as the first ground, provided it be in a thoroughly sound and dry condition, and it will then be sufficient to clean and level it before applying the second, or painting ground. If not, the stucco must be cleared off, the bricks laid bare, and the mortar between the bricks picked out to a depth of about 3/4 in.
This more thorough preparation is always preferable in a work of greater importance, or where special pains are advisable to secure durability, as, for instance, when undertaking the exterior decoration of a building. Upon this surface a thin squirting is cast, composed of the following mortar - coarse quartz sand, infusorial earth, and powdered marble, mixed in certain proportions. Of this mixture 4 parts are taken to 1 of quicklime, slaked with distilled water. Upon this squirting-cast, the object of which is to secure adhesion to the surface of the wall, follows mortar of ordinary consistency, composed of the same ingredients, to fill up all inequalities and produce a smooth surface, and upon this, again, the second or painting ground is applied.
The painting ground is composed of the finest white quartz sand, marble sand, artificially prepared, and free from dust, marble meal, and calcined fossil meal (infusorial earth). The sand composed of these materials, carefully mixed in the proper proportions, is mixed with quicklime slaked with distilled water, in the proportion of 8 parts sand to 1 of slaked lime. This mortar is applied to the wall as thin as possible, not exceeding 1/8 to 1/4 in. in depth.
For work executed on the exterior of buildings, Keim recommends the employment of pumice sand, in addition to the other ingredients of the mortar. When coated with a stucco of this composition, the wall presents so hard a surface as to admit of sparks being struck from it with a steel. It is absolutely essential that throughout the work, only distilled or filtered rain-water be employed. The reason for this is to obviate any possibility of the water containing lime, as that would affect the solution employed for fixing so as to impair the effect of the painting.
In this process Keim not only is careful to follow the best examples of antiquity in the manner in which the stucco is laid on the wall, but he has adopted the use of a mortar composed of carefully selected materials, in preference to that of an ordinary kind, such as was employed in the stereo-chrome process. The object of this is to attain a far higher degree of durability. The nature of the sand selected for this purpose is eminently calculated to ensure this. Marble sand, such as he employs (calcium carbonate in crystalline form), has been proved by experiment to add very greatly to the firmness of the mortar, containing many advantages above quartz sand, such as greater porosity for the absorption of the colours and fixing liquid, etc. Again, the infusorial earth mixed with it (a form of silica) has a double effect in consolidating the mass. First, it acts mechanically, cementing and binding together, with the lime, the coarser particles. Secondly, it forms, to some extent, with the lime, a calcium silicate, such as afterwards results from the addition of the water-glass. The presence of this silicate within the mortar adds, in a very high degree, to its hardness and power of resistance to chemical and mechanical influences.
When the mortar is perfectly dry, down to the stone or brick of the wall, it is treated to a solution of hydro-fluo-silicic acid, to remove the thin crust of crystallic lime carbonate which has formed on the surface, and thus to open the pores. It is then soaked with 2 applications of potash water-glass (potassium silicate) diluted with distilled water, and when dry, the ground will be found hard, but perfectly absorbent, and ready for painting.