Metals are polished either by burnishing or buffing. The process of burnishing consists in rubbing down all the minute roughnesses by means of a highly polished steel or agate tool - none of the metal being removed.
The action of the burnisher appears to depend upon two circumstances; first, that the harder the material to be polished the greater lustre it will receive; the burnisher is, therefore, commonly made of hardened steel, which exceeds in hardness nearly every metallic body. And secondly, its action depends on the intimacy of the contact betwixt the burnisher and the work; and the pressure of the brightened burnisher being, in reality, from its rounded or elliptical section, exerted upon only one mathematical line or point of the work at a time, it acts with great pressure and in a manner distinctly analogous to the steel die used in making coin; in which latter case the dull but smooth blank becomes instantly the bright and lustrous coin, in virtue of the intimate contact produced in the coining press between the entire surface of the blank and that of the highly polished die.
It by no means follows, however, that the burnisher will produce highly finished surfaces, unless they have been previously rendered smooth, and proper for the application of this instrument, as a rough surface, having any file marks or scratches, will exhibit the original defects, notwithstanding that they may be glossed over with the burnisher which follows every irregularity; and excessive pressure, which might be expected to correct the evil as in coining, only fills the work with furrows, or produces an irregular indented surface, which by workmen is said to he full of utters.
Therefore, the greater the degree of excellence that is required in burnished works, the more carefully should they be smoothed before the application of the burnisher, and this tool should also be cleaned on a buff stick with crocus immediately before use; and it should in general be applied with the least degree of friction that will suffice. Cutlers mostly consider that burnishers for steel are best rubbed on a buff stick with the finest flour emeiy; for silver, however, they polish the burnisher with crocus as usual. Most of the metals, previously to their being burnished, are rubbed with oil to lessen the risk of tearing or scratching them, but for gold and silver the burnisher is commonly used dry, unless soap and water or skimmed milk are employed; and for brass furniture, beer or water, with or without a little vinegar, is preferred for lubricating the burnisher.
Buffing is performed by rubbing the metal with soft leather, which has been charged with very fine polishing powder. The rubbing is sometimes done by hand, but more frequently the buff is made into a wheel which revolves rapidly in a lathe and the work is held against it.
The polishing powder that is selected must be chosen with special reference to the metal that is to be buffed. Thus, fur steel and brass the best polishing powder is crocus or rouge, which may be purchased of any dealer in tools, or may be made by exposing very clean and pure crystals of sulphate of iron to heat, according to the directions given hereafter under the head of Polishing Powders. The hardest part of the rouge must be selected, and great care must be taken to have it clean and free from particles of dust and sand, which would inevitably scratch the article to be polished and render it necessary to again repeat all the previous processes of filing, grinding, etc.
Soft metals like gold and silver may be polished with comparatively soft powders, such an prepared chalk or putty powder (oxide of tin).
When metals are to be polished in the lathe the process is very simple. After being turned or filed smooth the article is still further polished by means of fine emery and oil, applied with a stick, and in the case of rods or cylinders, a sort of clamp is used so that great pressure can be brought to bear on the part to be polished. The work must be examined from time to time to see that all parts are brought up equally to the greatest smoothness and freedom from scratches, and as fast as this occurs polishing powder of finer and finer quality is used, until the required finish is attained.
In polishing metals or any other hard substances by abrasion, the great point is to bring the whole surface up equally. A single scratch will destroy the appearance of the finest work, and it cannot be removed except by going back to the stage to which it corresponds, and beginning again from that point. Thus, if in working with a smooth file we make a scratch as deep as the cut of a bastard file, it is of no use to try and remove this scratch with the smooth file, we must go back, and taking a bastard file make the surface as even as possible with it, and afterwards work forward through fine files and polishing powders.