ished with the aid of the burnisher, which we will describe further on. It is the general usage, however, before introducing articles of silver into the gilding vat, to submit them to the operation known as "scratch-brushing."

CHAPTER IV.

SCRATCH-BRUSHING-VARIOUS KINDS OF SCRATCH-BRUSHES-
SCRATCH-BRUSH LATHE.

Scratch-brushing.

SCRATCH-BRUSHING is one of the indispensable and constantly recurring operations of the galvanoplastic art. It often precedes, and nearly always follows the operation of deposition, and therefore requires a special description. Scratch-brushing" consists in brightening the surface of an object, by the rapid and long-continued friction of the points of a collection of stiff and straight metallic wires.

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The instrument used for this purpose is called a "scratchbrush," and its shape varies with the articles to be operated upon.

The hand" scratch-brush" consists of a bundle of fine wires of hardened brass, which should be selected from a large coil in order that the single wires which compose the bundle may have less tendency to bend or curve when in place.*

To make a good hand scratch-brush, proceed as follows: select a coil of brass wire of the proper degree of fineness

Scratch-brushes of spun glass, with fibres of extreme fineness and elasticity, are also used for scouring highly delicate objects.

(Fig. 13), and bind it tightly with strong twine for about two-thirds of the intended length of the brush (6 or 8 inches) (Fig. 14). Then, with a chisel, cut the bundle of wire close to the cord at one end, and about 2 inches from it at the other end. Then dip the close cut end into a neutral solution of chloride of zinc, and plunge it into melted tin, which solders all the wires, and prevents their separation and injury to the hand of the operator. The tool thus made may be used as it is, but it is preferable to fix it, by means of another string, to a thin wooden handle which projects above the soldered end (Fig. 15).

Fig. 13.

Fig. 14.

Fig. 15.

Fig. 16.

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Scratch-brushes are also made by cutting a coil of wire for a length of from 12 to 16 inches, binding it in the middle, and doubling it so as to unite the two ends (Fig. 16). This process is less economical, and the wires have a tendency to become entangled.

Very small scratch-brushes are necessary for reaching sinuosities and depressions in the work, and other parts difficult of access.

An old scratch-brush, the wires of which have been bent in every direction, when fixed to a long handle, is useful for scouring the interior of certain pieces, such as Etruscan vases, coffee-urns, etc. (Fig. 17).

The varnishers on metals use, for rapid work, a kind of brush (Figs. 18 and 19) with long and stiff brass wires. Such brushes are only used for the preparation of articles of considerable size, such as clock dials, hearth furniture, and the like.

Scratch-brushing is scarcely ever done dry; the tool, as well as the pieces, must be constantly wet with a solution, which may set up a chemical action, but which most generally acts simply as a lubricant, and which, at the same time, carries away the impurities that the brush

detaches.

The scratch-brush affords an excellent test of the quality of the metallic deposits; when good they are only polished by the friction; when bad they scale off, from defective adhesion to the subjacent metallic surfaces.

A good adjunct for scratch-brushing is a shallow wooden tub for the solution employed, with a board laid across it, nearly level with the edges, which, however, project a little above. This board serves as a rest for the pieces (Fig. 20).

Fig. 20.

The liquid employed is water and vinegar, or sour wine (stale beer), or solutions of cream of tartar or alum when we desire to brighten a gold deposit which is too dark. But that most generally used is a decoction of liquorice root, of horse-chestnut, of marshmallow, of soapwort, or of the bark of Panama wood, all of which, being slightly mucilaginous, allow of a gentle scouring with the scratch-brush, with the production of an abundant froth.

The board on which the piece is rested while being scoured, should be above the surface of the liquid. It suffices to frequently wet the scratch-brush and the piece by dipping in the solution.

Every five or six days the old liquid is removed by careful decantation, so as not to carry away the sediment at the bottom, which always contains a little of the precious metals, and these, together with various residues of the workshop, are collected to be treated afterwards, as we shall explain further on.

With small objects and articles of jewelry, the operator holds the scratch-brush as he would a writing pen, and moves it over the article with a back-and-forward motion imparted by the wrist only, the forearm resting on the edge of the tub (Fig. 21).

Fig. 21.

For larger articles of bronze, on the contrary, the operator holds his extended fingers close to the lower part of the scratch-brush, so as to give the wires a certain support, and, with raised elbow, strikes the piece repeatedly, at the same time giving the tool a sliding motion. When a hollow is met which cannot be scoured longitudinally, a twisting motion is imparted to the tool. These various motions are better understood when seen than when described.

This mode of hand scratch-brushing is slow and tedious; besides, it is open to the inconvenience, on account of the irregular motion of the arm, of leaving on soft metals cross marks which have a bad appearance.