Next comes the polishing, which is principally performed with rollers of woolen cloth or list made to the size of about three inches diameter. As the sixth process, a rubber is charged with flour emery and a moderate degree of moisture; this rubber is worked uniformly over every part until the marble acquires a kind of greasy polish; seventhly, the work is completed with a similar roll of cloth charged with putty powder and water. Some prefer, as the polisher, an old cotton stocking not made into a rubber, and in some few of the more delicate works crocus is used intermediately between the emery and the putty powder. It is necessary to wash the marble after each operation, so that not a particle of the previous polishing material may remain, otherwise the work will be scratched.
The dull parts of sculpture are finished in four different manners, or rather the complete process of smoothing is discontinued at various stages so as to form four gradations, which may be described as follows:
The marble is sometimes left from the long and very slender statuary's chisel, the reverse end of which is formed with a sharp circular edge or ridge, just like a hollow centre, in order that the metal hammer, which is of soft iron, tin or zinc, may be slightly indented by the chisel, so as to avoid its glancing off; the chisel marks leave the surface somewhat rough and matted, intermediate between the granular and crystalline character.
For surfaces somewhat smoother, rasps are used to remove the ridges left by the chisel; the rasps leave a striated or lined effect suitable for draperies, and which is made more or less regular according to the uniformity of the strokes, or the reverse.
Files are employed for still smoother surfaces of the same character; and it is to be observed that the files and rasps are generally curved at the ends, to adapt them to the curvilinear forms of the sculpture.
For the smoothest of the dull or unpolished surfaces, the faint marks left by the file are rubbed out with Trent sand or silver sand and water, applied by means of a stick of deal cut to a point, and rubbed all over the work in little irregular circles, as a child would scribble on a slat and if the end of the stick is covered with two or threre thicknesses of cloth the marble receives a still rounder or Softer effect than from the naked stick, for which the cabbage wood or partridge wood is sometimes used, and the end of the stick is slightly bruised, so that the fibres of the wood may assume the character of the stiff brush, known by artists as a scrub.
Mr. Thomas Smith tells us that he has successfully copied the minute roughness or granulation of the skin, by a kind of etching which he was induced to try, by imagining that he could trace such a process to have been used in some of the most perfect of the ancient marbles that had not been exposed to the open air. The work having been smoothed with sand, as above, he takes a hard, stubby brush and therewith dots the marble with muriatic acid, and which quickly, yet partially, dissolves the surface. The strength of the acid, which must not be excessive, is tested upon a piece of waste marble; the brush is hastily dipped in the acid, applied to the work, quickly rinsed in water, and then used for removing the and from the marble. It is obvious the process calls for a certain admixture of dexterity and boldness, and sometimes requires several repetitions, the process occupying only a few minutes each time.
The bright parts of sculpture. Few of the works in sculpture are polished, and such as are, are required in the first instance to pass through the four stages already explained for producing the smooth but dull surface; after which, slender square pieces of the second gritstone and of snake-stone are used with water as a pencil, and then fine emery and putty powder on sticks of wood; but the work is exceedingly tedious, and requires very great care, that the artistical character of the work, and any keen edges that may be required are not lost in the polishing.