Technical Briefs deals with technical concerns of interest to a wide audience, addressing topics as diverse as vanguard technology and ancient arts. It will also answer questions from readers and respond to suggestions, always trying to explain in simple terms the magic and wonder of working with metal. For this article, Tim offers understanding sandpaper better.
I’ve been working in metal for several years now, but I’m embarrassed to say I don’t really understand much about sandpaper. Maybe you can help me out.
If we are to be technically accurate, we should use the term “coated abrasives” instead of sandpaper. This refers to a class of products in which abrasive particles are adhered onto a backing. The topic is not difficult to understand but does involve a number of interrelated factors, so it can seem complex. Any coated abrasive can be described in terms of the hardness of the particle, the size, shape and distribution of the particle, the method of adhesion and the nature of the backing material.
Since as far back as we can guess, chunks of gritty rocks were rubbed against an article to wear down its rough spots. Ayr Stones are, of course, exactly this ancient technique, virtually unchanged. First records of sandpaper come from 13th-century China, where broken seashells were glued onto parchment. In Europe a few centuries later, glass was glued onto paper.
By simply rubbing one material against another it’s easy to tell which is harder. Early craftsmen would have experimented with stones of their region and established a hierarchy of abrasive materials. Some of these, like garnet, emery and sandstone are still in common use. Science, specifically magnification, has allowed us to better define characteristics of abrasive particles, but the idea of coated abrasives hasn’t been altered in over six centuries.
Besides hardness, another important factor is the size of the abrasive particle. To insure efficient cutting, the particles should be of uniform size. This is most easily established by sifting the grains through screens of varying mesh sizes. These sizes became convenient devices for classifying the size of the particles, so the number of the screen was applied to the particle size. Smaller numbers (screens having fewer holes per square inch) referred to large particles, and so on.
Anyone who has walked barefoot on both a pebble beach and a gravel road will know that the shape of a particle is as important as its size. Investigation has shown that minerals break differently when crushed, some having sharp splintery edges, others breaking into more rounded forms. These shapes dramatically affect the cutting action of the abrasive. It has also been discovered that the method of reducing large pieces of stone to small particles, either by crushing or pulverization, can result in grains of different shapes The ideal abrasive is one that splinters as it is worn, constantly presenting fresh, sharp cutting edges. Garnet is an example of such an abrasive.
Throughout most of history natural stones have been used to meet abrasive needs. One problem with this was the diversity found in materials gathered from different sites. Another was the increasing scarcity of some substances. Late in the 1800s, scientists discovered how to make silicon carbide and aluminum oxide, now the two most commonly used abrasives. Both rely on the high temperatures of huge electric furnaces to fuse ore into very tough particles. Subsequent research has seen the development of increased control in the size, hardness and other qualities of these products.
Backing materials in popular use are paper and cloth. Some applications require flexibility while others need rigid support. Water-soluble resins are usually used in flexible coated abrasives, while waterproof glues are better for applications where liquids are present. Many metalsmiths prefer waterproof (wet-or-dry) paper since it offers the versatility of double use and because its adhesive tends to be less sticky as the paper breaks down in use.
Grains of abrasive particles are laid onto their backing in either dense or scattered distribution. The latter, usually called open coat, is less likely to clog and so is preferred for “gummy” materials like pewter, lead or plastic. The denser structure, closed coat, leaves a finer finish on tough, clean-cutting materials.
As far as their use in metalsmithing goes, much can be said about the right and wrong ways of using abrasives. Careful reading of a good jewelrymaking textbook will prove worthwhile, but here I can at least summarize some of the important points: Finishing must proceed in an orderly sequence from coarse to fine papers; probably to include something in the 100s, the 200s, the 300s and the 400s, at least. Strokes should run in different directions with each stage of the process, and at each stage the metal must be brought to a uniform surface. Abrasives require muscle behind them, so devices that increase leverage and good grip (like a sanding stick) are important. Keep in mind the difference between hardness and size. Silicon carbide has a Moh’s hardness of over 9, making it a lot harder than even your largest, nastiest-looking file. This is especially relevant when working around a stone. Even a 600-grit paper, as harmless as it seems, will scratch all stones except diamonds.
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