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.
If a plastered wall be new, and has not been whitewashed, it will do to size it with glue water; but if it has been kalsomined or whitewashed, which is often the case, no glue sizing should ever touch it. Any preparation of that kind is liable, sooner or later, to peel off and spoil the surface for any future finish. A safer way is to take oil and coat the whole surface before painting, which makes a fast union of any wash to the wall. On such a base oil paints will adhere perfectly. But the principal trouble in painting walls is found in the defective character of the plastering. If one is building a house, he can place the studding 12 in. from centre to centre, so that strong laths will not spring and break up the mortar at every pressure. The laths, too, should be spread 1/4 in. apart, and the mortar have 12 lb. of hair to the barrel of lime. This will make a wall that will stand like the walls of a house plastered 100 years ago. The reason why the plastering falls off from our modern houses is because the laths are laid so close that the immediate swelling cuts off the clinch, and the mortar is usually too sandy, and has but 6 lb. of hair.
On such a surface are laid 3 coats, when the clinch will fail to hold 1. Professional lathers or masons themselves ought to lay the laths and be sure of a large spread; then if the mortar is strong or rich, with plenty of hair, there can be no falling off. If the work is well done, the ceiling as well as the side walls may be painted to advantage. When any portion becomes soiled or smoked, it will then be an easy matter to wash it off. Rooms once thoroughly prepared in that way last for a lifetime, and always look substantial and neat. In case of cracks, make some putty of the same colour as the paint and fill up.
The following remarks are condensed from an interesting paper on mural painting by Rev. J. A. Rivington, read before the Society of Arts.
Fresco-painting, properly so-called, is the process of painting in water-colours upon wet mortar containing lime. In this process, the action of the carbonic acid in the atmosphere converts the lime of the mortar into carbonate of lime, and this latter it is which forms the preservative or fixing medium for the colours. The carbonic acid is driven out of the limestone or chalk originally by the process of burning, and the lime remains. When slaked, the lime is converted into a pulp of hydrate of lime. In this form it exists in the mortar, and greedily absorbs the water with which the colours are applied. This water, together with that already in the mortar, dissolves a portion of the hydrate of lime, and after a time this solution finds its way, through the supervening layer of colour, to the surface, where it absorbs carbonic acid gas from the atmosphere. By this means it becomes converted into carbonate of lime, and lies upon the surface of the painting in the form of a thin crystallic film, protecting and securing it to such a degree that it will admit of being washed, provided no great amount of friction he employed.
Experiment has shown that in fresco-painting the colour does not sink farther into the ground than in the case of any water-colour laid on a dry ground. On the contrary, the pigment becomes saturated with the solution of hydrate of lime which exudes from the mortar, and which can only become converted into a film of carbonate of lime on the surface; beneath this, the adherence of the pigment to the mortar is very slight, as may be easily proved when the crystallic film has been scraped off, or dissolved away by the application of an acid, or even removed, as is sometimes possible, by merely rubbing the surface with the moistened finger. After the removal of the protecting film of carbonate of lime by some such means, the pigment gives way readily when rubbed with the finger, and with even still greater readiness if moisture be also applied. A very striking illustration of this is afforded by the fate of the frescoes executed about 18 or 20 years ago on the exterior of the new Pinakothek in Munich. On the northern and eastern sides, the hail and rain have destroyed and washed away not only the protecting film of carbonate of lime, but also almost every vestige of colour.
The tendency to peel off in flakes, which paintings executed in fresco have often shown, admits likewise of a very simple explanation.
As a consequence of the greedy absolution by the mortar of the water contained in the pigments, the particles of the latter adhere mechanically to the surface of the mortar by capillary attraction, and that so closely as to permit of a second layer being very shortly after laid upon the first, without mixing with it in any way. Similarly, the second layer will admit of a third being superimposed. All 3 layers now become saturated with the solution of hydrate of lime, and are united by a real process of cohesion. This process is, however, only in the highest degree perfect where the superimposed layers have been applied before the hydrate of lime has completely penetrated the pigments. In those cases where it has so penetrated, and the crystallic film has already partly formed, the saturation cannot be so perfect; and where colours have been laid on after the film is fully developed, these can only adhere to the surface in a very imperfect degree. It follows that damp, or other causes, are sufficient to induce them to peel off very readily from the more firmly attached layers beneath.
The more or less inefficient modern substitutes for fresco are infinitely less deserving of respect. Most of them, if not all, such as wax colour, casein, as employed abroad, do not profess to be capable of resisting the influence of weather, when exposed to the open air. They are, therefore, only comparatively permanent, even when used for interior decoration, and may be dismissed without further mention.
Gambier Parry's process of "spirit fresco" appears to possess merits beyond such methods as are employed abroad, but, like them, it is not intended for exposure to the open air, and cannot enter into competition with Keim's process. It is, perhaps, unnecessary to remark that the only sure guarantee for the permanence of any painting must rest its claims on a thoroughly scientific observance of, and adherence to, the laws of chemistry. Unless the painting is executed under conditions which can be proved to comply with the demands of chemical laws, its permanence is a mere matter of haphazard experiment, and a perfectly open question, which even the test of time itself can hardly settle conclusively, since, without a thoroughly scientific basis, there is no real guarantee that the conditions will not vary. A substitute for fresco-painting has been adopted of late years in this country, for paintings on a small scale, by the employment of oil colours, with a matt medium to destroy the gloss peculiar to oil pigments, and to impart the dead surface so necessary to mural decorative paintings. Very little consideration is required to show that this method presents, perhaps, the least guarantee of any process, for the permanence of the painting.
In oil colours, it is the oil which, by filling the pores of the pigments, serves at once as a preservative and binding medium, while the varnish forms an additional protection against atmospheric influence. The various mediums used to destroy the characteristic effect of oil, effect this by expelling or neutralizing it. The volatile elements of the mediums then evaporate, leaving the pores open for the chemical action of carbonic acid gas, sulphuretted hydrogen, or any other deleterious agent in the atmosphere, to destroy the colour, while little or nothing remains to hind the substance of the pigments together. The comparatively rapid ruin of such paintings is the only possible result.