Polish on metals

Abstract

The burnishing and rubbing of a metal surface to give it a smooth shiny finish is often used to give metal ware, jewellery and pieces of apparatus a better appearance. This is called polishing and is done by abrazing and then finishing by rubbing with a soft, fine powder. Abrazion usually removes thin layers off the surface by taking out many small pieces of the metal. The sharp grains of the abrasive cut numerous small pieces out of the surfaces and work the surface down roughly to the desired level. The higher degree of polishing works the surface mechanically without removing much of the metal itself. When lastly a soft, fine powder is used, made into a very wet paste, this polishing material remains uncontaminated after having been rubbed on the metal continuously. A well-polished surface has the same colour as the rough metal had originally but the glassy finish reflects so highly that no structure is seen with the eye. It is difficult to describe the surface of a polished metal but Professor Flinders Petrie aptly describes it as having a “wet lustre.” The microscope usually shows scratches left from the polishing which look like gently rounded furrows . Sir G. T. Beilby made a scientific study of polished surfaces with the use of a microscope and it is in supplement to his work that these experiments have been done. Beilby noticed in the final stages of polish that the surface appeared to flow and acted like a film to cover over scratches and pits and give a glassy finish. This film is called the Beilby layer and has the appearance of a super-cooled liquid. During intermediate stages the parallel edges of a scratch in the surface could be seen to be drawing together and at points had come together and bridged over the furrow. Afterwards the film, which covered the scratch completely and hid it, could be removed by etching and the scratch would appear as it looked originally. The mechanical working of the metal to make it flow, moves the surface very much in relation to the greater portion of the metal underneath and is a kneading and twisting of the metal on the surface. In the following experiments pieces of copper, silver, gold, and chromium of 1× 1·5×0·5 cm. size were used as different test metals. They were worked on different grades of emery paper and with polishing powders, and electron diffraction pictures were taken at different stages of polish to notice changes in the crystal structure.