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.
It is significant of the success which has attended Keim's thorough appreciation of the requirements of the pigments, that his labours in this direction have so perfectly adapted them to the chemical condition of the ground as to show that, to a very appreciable extent, fixation will be found to have already taken place before even the application of the solution employed for that purpose.
In 1878, a large mural painting was executed by this process on the exterior of the parish church at Eichelberg, near Regensburg. Before its completion, and therefore before any of the fixing solution had been applied to it, it was drenched by a heavy storm of rain. Contrary to anticipation, it was found that the painting, so far from being in any degree washed away, had held perfectly firm, and even in some places seemed to be as hard as if already fixed. Keim's explanation of this unexpected result, which he subsequently confirmed by experiments, was, that a chemical cohesion had already taken place by the action of the alkali set free in the mortar upon the silicates in the pigments.
Again, when it was determined to execute the mural paintings in the Franciscan Monastery, at Lechfeld, in 1879, it was desired to wash off a painting executed in this process a year previously, which had never been fixed. Neither water, nor even a tolerably strong solution of acetic acid, had the slightest effect upon it.
So far from approaching in any degree the difficulties or inconveniences possessed to a greater or smaller extent by fresco-painting, or any of its more modern substitutes, this process is even far pleasanter and easier to work in than oils or water-colours. Every variety of treatment is possible, and it adapts itself to any individual style of painting. It presents perfect facility for transparent glazing as a water-colour; and for painting in body colour it even surpasses the capabilities of oil colours in its power of opaque treatment.
The most delicate tints, when laid over darker tones, do not in the slightest degree darken over them, as they are apt to do in oils, but keep their full value perfectly. Retouching and correction can be effected with the greatest ease, and to an almost unlimited extent. The system admits also of great economy. To begin with, the pigments are by no means expensive, in spite of the labour expended on their preparation, and a very sparing use of them is sufficient to meet all possible requirements in painting, a far less amount requiring to be expended than in other processes. This is due mainly to their being ground so exceedingly fine, so that they need only be very thinly laid on; in fact, this consideration has always to be borne in mind, that the thinner the coat of painting is, the greater the degree of security that can be attained by the fixing. Moreover, there need be no waste of pigment at the end of the day's work, as in oils. The palettes employed for the process are constructed with small pans to hold the pigments. If any paint remains after the work is finished, it can either be replaced in the bottle, or it can be kept moist in the pan with distilled water for the next day's work.
Even if a considerable amount of the pigment should by inadvertence have been allowed to become dry, all that need be done is to grind it up again with a little distilled water, a task involving no labour. The process has the further recommendation of great cleanliness, distilled water being the only medium used in painting. The porous nature of the ground, and its peculiar texture, have had great fascination for those who have made practical acquaintance with the working of it.
The last stage in the process is the work of fixing. In the stereo-chrome process the fixing medium employed was potash silicate, thoroughly saturated with silica, in combination with sufficient sodic silicate to prevent it from opalescing. The chief defect of this lay in the fact that it was often apt to produce spots upon the painting. Keim has substituted potash silicate treated with caustic ammonia and caustic potash. The action of the carbonic acid in the atmosphere and in the water during the process, leads to the formation of carbonated alkali, which makes its way to the surface, and would form, when dry, a whitish film over the painting. To obviate this danger, as well as to expedite the process of converting the potash silicate with the basic oxides existing in the substance of the painting into silicate, the fixing solution is treated further with ammonia carbonate. The effect of this upon potash silicate is that silica is precipitated in a fine gelatinous form, and ammonia set free.
This latter volatilizes, and potash carbonate is formed, which is easily removed by washing alter the completion of the fixing.
Having regard to the value of heat in accelerating the action of chemical processes, the fixing solution is employed hot, with the advantage of obtaining a quicker and more perfect formation of silicate than was possible in the stereo-chrome process, where the solution was applied cold.
The effect of the fixing is not very difficult to understand. It has been already pointed out, in speaking of the pigments, that the result of their being treated with certain substances is to effect the formation of silicate, both in the constituent parts of the pigments themselves, as well as of those in combination with the painting-ground. The additional presence of the fixing solution intensifies this process to the greatest extent. The free alkali of the solution acts upon certain of the substances which have been added to the pigments - such as zinc oxide, alumina hydrate, and silica hydrate - at first by dissolving them. By the action of the carbonic acid in the atmosphere, these solutions are again decomposed by parting with the hydrates, which, through this process, are converted into silicates. The pure colours are enclosed in these silicates; whenever that is, the pigments themselves do not take part in the formation of silicate.
The hardening process of mortar has been described - in speaking of fresco-painting - to be due to the formation of a crust of lime carbonate upon the surface. The action of the fixing solution in Keim's process, when applied before and after the painting, is to form, in addition, a calcium silicate with the particles of lime, the presence of which within the mortar increases beyond comparison the hardness and durability of the whole; calcium silicate, no less than lime carbonate, being, as is well known, a constituent of some of the hardest marbles.
Briefly described, then, the effect of the fixative as it sinks into the ground, which has already absorbed the pigments, is to convert the painting into a veritable casting, uniting with colours and ground in one hard homogeneous mass of artificial stone, partaking of the nature of marble in its power of resistance to mechanical disturbance, partaking of the nature of glass in the impervious front it presents to the chemical action of the atmosphere.
The finished painting has proved itself absolutely impervious to all tests. It will admit of any acid, even in a concentrated form, being poured over it (save, of course, hydrofluoric acid). Caustic potash, also, has no effect upon it; indeed nothing can be employed with greater advantage than this for cleansing the painting when its condition requires that process. Soap and water may be applied with a hard brush, as vigorously as desired. The surface is so hard as to present a perfect resistance if scratched with the finger-nail. The hardness and durability of the finished painting have been subjected to very severe trials abroad. It has defied the elements in very had climates, having been exposed to the weather on the exterior of buildings for some years. In Munich a specimen of the process was subjected to incessant tests for 2 years, and, at the end, was as fresh and uninjured as at the beginning.