§ 181. The scraping of Marbles, which have been blackened or turned green by the air and dampness, has long been considered inexpedient, since, whatever precautions may be taken, the work which is to be restored is always scratched more or less, and it is impossible to practice it in the excavated parts without breaking the delicate sculptures, or causing sad incongruities between the designs in relief and those which are sculptured. It is therefore very desirable to find a wash that may be substituted for this destructive process. Several persons have described powders and waters, with which experiments have been made at the Luxembourg and other places, but none seem to be satisfactory. Alkalized water, prepared with potash, has also been proposed, as well as water dashed with hydrochloric acid.
In respect to soiled articles, which have not been tarnished by exposure to the open air; to restore their original color, it is sufficient to use the potash water, then to wash them in pure water, and finally, to finish them with the chlorureted water. Soap and water is often sufficient in such cases, it is spread on with a brush, and introduced into the sculptured parts by a somewhat stiff pencil.
I have heard a kind of varnish made of white wax highly extolled as a preservative; this is laid on by means of heat, and is afterwards rubbed with a cushion; it is said that this varnish was used by the ancients, and that the preservation of their chefs d' ouvre may be attributed to this; but proof has never been given of the efficacy of this method, which seems, like the rest, to be difficult of application.
The water and hydrochloric acid have been successfully tried in the Place de la Concorde, upon the statues and stone balustrades which ornament it, and the architect who superintended its use, considered it the most economical and expedient method known. The essential point in this operation is, to always use water which is perfectly clean, and in an abundant quantity. Showering by a garden pump, or a syringe with several holes, appears to me to be the best method of removing the chlorureted water from all the places in which it may rest despite the washing with the brush.
§ 182. Pozzolana may be classed among the number of natural cements which are often needed by the Marble-worker, but which are rarely used on account of the difficulty of procuring them. This is a natural cement, formed by volcanic scorias and lavas. It was much used by the Romans for aqueducts, reservoirs, and all works exposed to a constant moisture. Pozzolana, when mixed with the requisite proportions of good lime, sets in the water, and forms a mortar so adhesive and compactly united, that it can resist the action of the waves without suffering the least change.
There are several varieties of Pozzolana, namely: First, The gravelly and compact, and the basaltic pozzolana. The compact lava and basalt, reduced to small splinters or gravelly fragments, either by nature or by pulverization in the mills used by the Dutch for crushing a softer lava, known by the name of tras or Andernach stone, also furnishes an excellent pozzolana, which may be used either in or out of the water.
Second. The porous pozzolana, formed by spongy lavas, which are crumbly, and reduced to powder or small irregular grains. This is the common pozzolana which abounds in the suburbs of Bayes, Pozzuoli, Naples, Rome, and in many parts of the Vivarais, etc. The ferruginous origin of these colors of those volcanic earths have been produced; there are red, black, reddish, grey, brown, violet, and other colors. All of these, when mixed with lime, possess the property of acquiring a great solidity in water.
§ 183. This stone, so light, porous, and useful in almost all the arts, in Marble working most especially, is used for polishing, either in powder or in fragments; it removes the asperities, and prepares the material for receiving the last polish.
M. Daubenton was the first to observe and remark that the pumice stones were composed of particles of an almost perfect glass, and M. Dolomieu has also made many reliable observations respecting the origin and nature of this volcanic production; he has observed in his Voyages, that the island of Lipari is the immense warehouse which furnishes pumice stones to all Europe, and that several mountains of this island are entirely composed of it; he also says that he has found isolated fragments in a white, mealy powder, which was itself but a pulverable pumice.
The substances of these stones, particularly of the lighter ones, is in a state of frit, closely resembling a perfect glass; their texture is fibrous, their grain rough and dry, they look shining and silky, and are much lighter than either the porous, or cellular lavas. This distinguished traveler points out four species of pumice, which differ from each other in the closeness of the grain, the weight, the texture, and the arrangement of the pores.
"The pumice Stones," says he, "appear to have flowed in the same manner as the lavas, forming like them, broad currents, which have been discovered lying at different depths above each other, around the mountains of Lipari..The heavier pumice stones occupy the lower part of the currents or masses, the lighter stones being above them; the same is also true of the lavas, the lighter and more porous always occupying the upper part."