§ 130 Scratch-brushing.
This process is generally indispensable, and to its omission is to be traced most laboratory failures in electroplating. Scratch-brushes may be bought at those interesting shops where “watchmakers’ supplies” are sold. It will be well, therefore, to purchase a selection of scratch brushes, for they are made to suit particular kinds of work. They are all made of brass wire, and vary both in hardness and in the fineness of the wire. The simplest kind of scratch brush consists merely of a bundle of wires bound up tightly by another wire, and somewhat “frizzed” out at the ends (Fig. 90). A more useful kind is made just like a rotating brush, and has to be mounted on a lathe (Fig. 91).
images/Image136.gifFig. 90. Fig. 91.
The scratch brush is generally, if not always, applied wet; the lubricant generally recommended is stale beer, but this may be replaced by water containing a small quantity of glue, or any other form of gelatine in solution — a mere trace (say.1 per cent) is quite sufficient. Very fair results may be got by using either pure or soapy water. The rotating brushes require to be mounted on a lathe, and may be run at the same speed as would be employed for turning wooden objects of the same dimensions.
Since the brush has to be kept wet by allowing water or its equivalent to drip upon it, it is usual to make a tin trough over which the brush can revolve, and to further protect this by a tin hood to keep the liquid from being thrown all over the room. In many works the brush is arranged to lie partly in the liquid, and this does very well if the hood is effective.
There is a superstition that electro-deposits stick better to scratch-brushed surfaces than to surfaces which have not been so treated, and consequently it is usual to scratch-brush surfaces before electro-deposit. However this may be, there is no doubt that adherence and solidity are promoted by frequent scratch-brushing during the process of depositing metal, especially when the latter tends to come down in a spongy manner.
Gilt surfaces — if the gilding is at all heavy — are generally dull yellow, or even brown, when they come from the bath, and require the scratch brush to cause the gold to brighten, an office which it performs in a quite striking manner. The same remark applies to silvered surfaces, which generally leave the bath a dead white — at all events if the deposit is thick, and if ordinary solutions are employed. In either case the touch of the scratch brush is magical.