Some of the various processes of wall-painting lay more stress upon the preparation of the wall than do others. The most modern methods attach the greatest possible importance to this matter, sometimes, indeed, appearing to consider the permanence of the painting more important than its quality; while others rely upon interposing an impermeable layer between the wall and the painting, and thus neutralising any damage which might result from dampness or efflorescence ; and there can be no doubt that, other things being equal, it is better to have a perfectly constructed wall upon which to apply decoration, especially in a climate which has not the dryness of Egypt nor the brilliant sun of other southern lands. But even in Italy and Greece the greatest care was prescribed in the construction of such walls, and was often taken, with what excellent results recent discoveries at Cnossus have shown us, where wall-paintings executed some 3,600 years ago still showed bright and vivid colouring when disinterred from the earth which had shrouded them for many centuries.
The preparation of the wall according to the Greek method as given by Vitruvius was as follows. The first process was called trullisatio (for they knew nothing of damp courses), and consisted in coating the wall with trowelled plaster composed of rough sand, pounded brick and tile, and lime, in the proportion of one part lime, two parts river sand, and one part pounded brick or tile, which was left rough. The second process was called arenatum or sand mortar. Not less than three successive coats of this were laid over the rough plaster, each laid while the preceding coat was damp. The more substantial the mortar was, the more durable and solid the stucco turned out The proportions were: one part lime and two parts river sand. The third process was called marmoratum or marble mortar. This consisted of lime and marble dust of different degrees of fineness, laid on in three successive coats, using the coarsest first. It was so tempered that when in the right condition it did not adhere to the trowel in working, but left it clean. The first and second coats were allowed to dry before the third was put on. It consisted of one part lime and two parts marble dust It was sometimes done with less expenditure of time and material (for there were jerry builders even in those days), but it was found that if only one layer of arenatum and one of marmoratum were put on over the trullisatio, the colours did not show with their proper brilliancy, and the plaster was likely to crack and the whole decay after a time. When done well and thoroughly the surface of the stucco was so smooth that it might be polished, and it was sometimes used for mirrors. In the baths of Titus the stucco was one and a quarter inch thick, and was as hard as marble.
In modern times the Italians use three kinds of stucco, the first called pozzolano, which consists of two parts lime and one of pozzolana, or Roman cement; the second marmorato, made of two parts lime and one of marble dust; and the third arenato, made of two parts lime and one river sand. Signor Forni, however, calls the first plaster arricciato and plasters generally smalto. The rough plaster was always made of pozzolana, lime, and sand. The lime for the plastering made from limestone or marble. It should be slaked long before being used, so that parts not sufficiently calcined may have time to be dissolved, and be reduced to a proper consistency; for should the lime be fresh, or not thoroughly slaked, it , will "blow," that is, emit pustules and destroy the surface of the stucco.* The Greeks tempered their armatwm in a pit, the lime and sand being compounded with wooden rammers until thoroughly amalgamated, and the same method was adopted with the trullisatio.
* A story comes to my mind told to me by the late G. T. Robinson, a man whose knowledge was encyclopaedic and his experience immense. When a young man he was studying in the Campo Santo at Pisa, and made the acquaintance of an artist who was engaged in repairing the frescoes in that building, who promised him to tell him the secret of compounding the intonaco used by the mediaeval painters. Accordingly on the last day of his stay he led him across the piazza to his house and showed him in the undercroft, exposed to the air, but sheltered from wet, three casks containing slaked lime. Tapping the first, he said, " My grandfather: unfortunately very little of him left"; tapping the second, *' My father: I shall soon have to commence him " ; and tapping the third, " Myself: this is what I am preparing for my children." This shows how not only traditional knowledge and receipts are handed down from father to son in a land where tradition. is not yet dead, but the very materials with which the work is done, and reveals the tine secret of its permanence.
To ascertain when it is fit for use cut it with an axe; if it comes in contact with lumps, the lime is not well tempered; if the iron comes out dry and clean, it is perishing and weak, but when fat and well macerated it will adhere to the iron like glue. Eiver sand only must be used. Fit sand causes fermentation in conjunction with lime, sea sand is salt and the salt dissolves the plastering. Pozzolana is a red earth found at Puteoli and a strong cement. Roman cement is an imitation of it. The marble dust is chippings of marble sifted into three grades of coarseness, the first being a very fine poWder and the last almost like small pebbles.
The plaster is better if laid some years before, but if the wall which it is proposed to decorate is covered with old mortar the ingredients of which are unknown it should be entirely removed. The Germans use for their roughcast three parts of sand to one of lime. At Munich the lime was slaked in this manner. A pit was filled with clean burnt lime stones, which on being slaked were stirred continually till the substance was of an impalpable consistence. The surface having settled to a level, clean river sand was spread over it to the depth of a foot or more, so as to exclude the air, and lastly the whole was covered with earth. The German painters preferred it to remain thus for three years, and Cornelius prepared the lime for the Ludwig Kirche eight years before he painted there. The pits were simply dug in the earth, but Professor Hess recommended them to be lined with brick. At Genoa the lime is left in the pit for from eight to twelve months, according to its ascertained strength.