Emery and the emery industry

In the growth and development of the machinery industry, the pressure of world competition soon led to a recognition of the accuracy of the proverb "time is money ". The value of the work performed by the hand of the industrious workman, formerly so highly appreciated, diminished progressively in proportion as ingenious inventors busied themselves with the replacing of limited human endurance by the tireless force of machinery. It is true that manual labour could not be entirely dispensed with, but the modern workman became more and more the servant of the machinery; and every industry that desired to keep abreast of the times had to fall into line in this respect. The fulfillment of these novel requirements led, in turn, to the establishment of entirely new industries, which have already acquired high rank. When the file and grind- stone, which formerly constituted the grinding tools of the machinery industry, proved no longer sufficient for its triumphal progress, urgent need arose for mechanical aids; and this need gave the first stimulus to the establishment of the industry of abrasives and grinding machines which, from small beginnings, developed in a few decades, under the pressure of circumstances, into a condition of prosperity that has rarely been equaled in the time. Even now there is no standing still; and, indeed, that will be impossible for this new industry so long as the older industries continue to struggle in the world market.

It is the object of the present technical study to bring both the engineer and the layman to a fuller comprehension of this new industry, the development and importance of which are still imperfectly known, even in trade circles.

One of the earliest acquirements of prehistoric man was the comprehension of the term "grinding". That this should be so was perfectly natural, because as soon as man began to understand the use and production of tools, and to recognize the difference between a sharp tool and a blunt one, he naturally sought for means by which the requisite sharpness could be imparted to his tools or weapons. Nothing could be more self-evident than to work the stone, intended to serve as a weapon or hatchet, with the aid of another stone. In this way he very soon learned, not merely the importance of the frictional motion stone upon stone, but also which kind of stone produced the proper effect in this treatment. He then gradually learned to appreciate quartz and felspar, and to use the wide-spread sandstone; and the applicability of emery for such purposes was recognized in the earliest ages.

Within historic times we have reliable knowledge of this, the old Greek historians having a good deal to say about the riches of the island of Naxos and its "smyris" (emery), which quickly became known as an excellent material for grinding, in competition with sandstone; but as it could only be used in the form of grains or powder its employment was merely a restricted one for a long time, sandstone grindstones being used both in the handcrafts and in the industries that subsequently developed from these latter.

It was not until the seventies of the last century, when a method was invented for binding the grains together with a suitable medium and compressing them into moulds under heavy pressure so as to form grind- stones, that the sphere of application of emery attained rapid extension.

In proportion as the demand for these grindstones and grinding wheels increased, so also was the method of manufacture improved as the result of experience Before long, moreover, other natural grinding agents became the object of investigation, and attempts were also made to overcome the irregularities present in the native raw materials by making artificial emery and other abrasive materials.

In this way the industry of emery and the abrasive materials in general has now become fairly extensive and of considerable importance to the entire machinery industry, though it is still for many and probably even the majority a terra incognita, the further exploration of which should certainly be profitable to manufacturers and others, if only from the standpoint of self-interest.


CHAPTER I - ABRASIVE MATERIALS

In order to facilitate achieving the purpose mentioned at the close of the Introduction, it will be necessary to consider the various abrasive materials more thoroughly, in order to gain from their properties the information requisite for their appropriate utilization in technical practice. The term "abrasive materials " is capable of wide extension ; and, indeed, the Report of the Geological Survey of the United States arranges under this heading the following substances : oilstones, scythe stones (whetstones), millstone quartz and millstones, grind-stones, flint, pumice-stone, kieselguhr and tripoli, crystalline quartz and felspar, granite, corundum and emery, carborundum, steel powder, and artificial corundum. This list, however, may be considerably shortened, when it is considered that only a comparatively small number of the foregoing materials are actually used for grinding.

Thus, only a relatively small proportion of the total production of sandstone is used for making whetstones and grindstones, the chief purpose of this stone being for building. The same applies also to oilstones and scythe-stones; whilst millstones are generally used for other purposes, namely, grinding corn. On the other hand pumice-stone must certainly be classed as an abrasive material, this being its sole use, in one form or another. Crystalline quartz and felspar, on the contrary, are imperfectly adapted for such purposes; whereas emery and corundum again must be regarded as abrasive materials par excellence, as also the granites that are not used for decorative purposes.

A more satisfactory classification of these materials is obtained by arranging them in accordance with their nature, a distinction being drawn from the outset between the two main groups: natural and artificial abrasive materials.

1. NATURAL ABRASIVE MATERIALS.

The natural abrasive materials are more numerous than the artificial ones, and may be divided, according to their nature, into three classes:

1. The purely siliceous materials, consisting of pure silica, free from any other compounds. This class necessarily includes the whole of the quartz family. The members of this family exhibit a degree of hardness corresponding to No. 7 of Moh's scale, and comprise:

(a) Ordinary quartz gangue, crushed and pulverized, used for polishing timber and stone.

(b) Quartz sands, which are used for the same purpose and occur chiefly in the eocene and oligocene beds of the older Tertiary formation, the so-called Bassin de Paris.

(c) Sandstone that can be reduced to fine dust by the blows of a hammer, thus furnishing an admirable abrasive material.

(d) Siliceous earths, which are abundantly found in calcareous strata.

(e) Millstone, originating in the decalcification zone of siliceous limestone deposits and used for grinding in corn mills. (In France, the fresh-water quartzites of the Paris basin are preferably used for this purpose.)

(f) Tripoli, a kind of kieselguhr, named after the town of Tripoli from whence it was first obtained.

2. The mixed siliceous abrasives, consisting of silicates in admixture with various bases. The chief member of this group is pumice-stone, a product resulting from the solidification of volcanic masses ; another being almadin, or ferrous-oxide granite, frequently met with in crystalline strata. These granite-bearing sands are still won, especially in Spain at Hoyazo near Cap de Gaeta, and also in Brittany. Granite paper is mainly used in the shoemaking industry.

3. The aluminous abrasives, the active constituent of which is pure alumina. The chief representative of the group is emery.


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