Manufacturer and builder 1, 1869
The art of stereochromy must be considered as involving an entirely new mode of creating durable pictures upon walls, inasmuch as a new binding material is applied, which differs from any employed in the usual modes of mural painting. By this binding material, which is the soluble glass of commerce, the colors become, as it were, silicified; and pictures executed in this way distinguish themselves by a certain freshness and a power of resisting atmospheric influences which ordinary frescoes do not possess.
In giving an account of this interesting mode of painting, we shall confine ourselves to a general outline of the method by which Kaulbach, the celebrated artist of Munich, and Echter, have executed, in the new museum at Berlin, four large pictures, which are generally acknowledged to exhibit a great advance in the art of mural-painting. The wall to be painted is first coated with a layer of ordinary lime-mortar, in order to equalize its unevenness. The sand employed, which may be either silicious or calcareous, must he of even grain and well washed beforehand. Lime must be sparingly employed, so as to render the cement rather poor than otherwise. In this and in all the subsequent operations, pure rain-water must be used. The plaster, thus prepared, must be well dried and exposed to the air for several days. so as to become entirely carbonated. Caustic lime would decompose the soluble glass. Fuchs, the inventor of stereochromy, recommends the moistening of the wall several times with a solution of carbonate of ammonia, so as to accelerate tie saturation of the lime. When dry, it is washed over several times with a moderately diluted solution of the socalled "double waterglass," allowing it to dry each time.
The ground being thus prepared, the upper layer may be soon after added. It consists, like the lower one, of a lime mortar, and is spread in the thickness of about one tenth of an inch. The sand employed must be well washed, and of a grain not exceeding a certain size. Very fine powder must be rejected; and for this reason it is best to pass it through a sieve. A rough grain is rather advantageous; Kaulbach says "it ought to feel like a rasp." For a picture to be viewed at a great distance, a coarser grain is required than for one designed for closer inspection.
When the ceating is perfectly dry, it is sometimes rubbed with a sharp sandstone, in order to remove the layer formed of carbonate of lime. It is better, however, to accomplish this by means of diluted phosphoric acid. The phosphate of lime thus formed []is the soluble glass, a solution of which, when the coats dry, is spread over the surface. The same is diluted with its equal bulk of water, and the operation is twice repeated. To much water-glass prevents the ground from taking the colors. The ground being thus prepared, the painting may be at once proceeded with; some delay, however, increases the capacity for aborption. The colors to be used must be ground with pure water, (we will speak of their preparation in a subsequent article,) and the wall has also to be frequently sprinkled with water, in order to displace the air from the pores, and to insure thus the adherence of the colors. Nothing further remains to be done than to fix the colors properly with a solution of the soluble glass referred to, which operation is accomplished by sprinkling the painting in the form of a fine shower or mist, then letting it dry, and repeating the operation until the colors adhere so firmly that they can not be any more rubbed off by the finger.
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