This article first appeared in Practical Engineering 1940 Vol1 No26. The information contained within the article is accurate as of 1940. The article highlights developments within Industry and Engineering at the time.
Asbestos, despite the fact that it was little known before the coming of the present century, is, in fact, an ancient material which has been rediscovered. The word itself is an ancient Greek one, meaning "inextinguishable," and it is believed that the people of the pre-Christian civilizations had the ability of weaving asbestos fabric, which they used as a funeral cloth in which to wrap the dead body prior to its cremation, thereby ensuring that the ashes of the body might eventually be gathered together unmixed with those of the surrounding fire.
The honour of first exploiting the commercial properties of asbestos goes to a British syndicate which, in about 1862, first mined this indestructible material in the Aosta Valley region of the Italian Alps. Simultaneously, the material was located in Canada—in Quebec—but all attempts to work these deposits failed until fresh sources were found in this Canadian region in 1877. In the following year about fifty tons of the material were commercially produced.
Sources of the Mineral
The success of this Canadian asbestos enterprise soon led to a rush on the part of prospectors to acquire asbestos-bearing land. Canada steadily increased its production of raw and refined asbestos until up to as recently as 1930, the Dominion constituted the world's chief source of asbestos. Asbestos was known even by the Romans to exist in the Ural Mountains of Russia, and after the 1914-18 war that country made every effort to exploit its supplies of the valuable mineral fibre—so much so that after 1930 the output of Russian asbestos exceeded that of the Canadian product.
Southern Rhodesia, the Union of South Africa, and the Island of Cyprus are also asbestos-producing regions. Despite this competition, however, the Canadian producers have been able to maintain an output of some 60 per cent of the world's annual supply of the commodity, a fact which clearly indicates the high quality of the asbestos which is mined in the rock-bearing regions of this vast Dominion.
Asbestos is a term which has hitherto been applied somewhat loosely to a number of fibrous minerals. Strictly speaking, however, asbestos, or at least the variety of it which serves the greatest engineering and industrial use, is a mineral termed "chrysotile," which is a mineral of the complex magnesium-silicon group containing combined water. This group of magnesium minerals produces the longest fibre type of asbestos, and it is interesting to note that the fibre of the mineral always runs across the direction of the mineral veins in the rock.
Arrangement of the Fibres
In any good quality asbestos-bearing land, the composition and purity of the asbestos remain extraordinarily constant, but strange to say, the degree of toughness or softness of the individual mineral fibres varies a great deal. Some types of asbestos have a brittle, wiry feel, akin to that of glass wool, whilst others are smooth, soapy and almost oily to the touch. The hard, wiry types of asbestos contain iron and sodium, and now this grade is seemingly unobtainable.
The United States produces a small amount of a type of asbestos material named Anthophyllite, but it is far inferior to the Canadian mineral for general structural, fire-resisting and engineering purposes, and it has the added disadvantage of being unsuitable for weaving.
Most of the asbestos material which makes its appearance in a hundred-and-one different guises in the engineering works and factories of our country is mined in the form of greenish-looking clumps of earthy mineral, having a "stringy" or fibrous nature and which can (given sufficient patience on the part of the operator) be pulled completely to pieces by the fingers and reduced to the form of a mass of single fibres. With the more valuable grades of material, this process of "hand splitting" is adopted, in many of the mines for the purpose of cleaning the asbestos and air stream or by being mechanically passed through screens or wire-mesh sieves.
Thus is produced the raw asbestos of commerce, a material which frequently takes the form of masses of clumped-up fibrous mineral which is packed (and sometimes lightly compressed) into sacks for the convenience of shipment. The manufacturer who receives such consignments usually completes the grading, the breaking-down and the general purification of the fibrous mineral in accordance with his own special requirements, which may, of course, be many and devious.
If, for example, the asbestos material is required for weaving-up into cloth, with or without admixture, it is given a most rigorous cleaning and grading, only the longest fibred material being selected-for this important use. If, on the other hand, the asbestos is required for compression into flat sheets from which the familiar packings, washers, gaskets and other articles are produced, the relatively more abundant shorter-fibre asbestos suffices, and its purification need not be carried out to the same extent.
The Uses of Asbestos
Asbestos is a ubiquitous material. In the general engineering and structural industries it appears in the form of numerous types of protective clothing, such as gloves, aprons, leggings and even entire suits. It is commonly woven into webbings and bandings for various industrial uses whilst, of course, in the automobile industry, its employment as a frictional material for car brake-linings is universal.
In the electrical industry, too, asbestos has many uses, for it is an excellent insulator as well as being an effective heat-protector. The rotor-bars of giant power station turbine-alternators are frequently wound with woven asbestos insulation, whilst, in steam-raising practice, the employment of the lower short-fibre and "powder" grades of the material for the lagging of pipe-lines and boilers is well known.
The earliest products of the asbestos industry were asbestos packings, which, at first, comprised merely the compressed material. Nowadays, however, there are two main types of asbestos packing—proofed and plaited. Plaited asbestos packings are made by plaiting the material by one of several different methods. Proofed packings, on the other hand, constitute ordinary asbestos cloth which has been impregnated with rubber to render it water-resisting.
The shorter-fibre and the average grades of the material, after being blasted out of the asbestos-bearing rock veins, are usually mechanically crushed, passed through vibrating machinery, or "shakers'" as they are called, in order to remove contaminating dirt and dust, and finally the loose fibres are graded. And, asbestos is employed to some extent as a material for the constructions of electrical "composition " insulators. The non-proofed sheet material is used for the lining of innumerable ovens and furnaces, an employment for which it is as long-lasting as it is effective, for should the asbestos lining become contaminated with deleterious material, fumes, or vapours, it is simply detached and burnt off by being placed in a hot furnace, after which the material is restored to its pristine condition.
In Building Construction
As a material for modern factory building and for other structural purposes, asbestos of reasonable quality is being used to an increasing extent. Asbestos, even of the lower grades, can be used as an “aggregate" or inert filler in Portland cement, its presence increasing the strength of the resulting concrete many times above normal. The same applies to the use of asbestos fibre in asphalt floorings and in “vertical" work carried out in this plastic bituminous material, an asphalt containing asbestos being up to half a dozen times as resistant to "flow" as one of normal composition.
Flat sheets of asbestos-cement composition, on account of their weather proof and heat-insulative nature, are much used in erecting light sheds and factories. They are also employed for roofing tiles, while recently it has been found possible to add to their attractiveness by the incorporation of suitable bright pigments or colouring agents in the concrete "mix."
Very recently, too, it has been found possible to spray short asbestos fibres, with or without admixture of other fibrous material, on to walls and other surfaces in order to provide sound-insulating partitions. This employment of asbestos has, of course, been temporarily stopped by the war.
Perhaps the one use of good quality woven asbestos with which we are all familiar is the now traditional fireproof theatre curtain. This article comprises a double asbestos cloth which has been metallised and suitably painted or coloured. In the stitching of the cloth, no ordinary thread is used, the whole of the stitching and banding being effected by means of coarse and fine asbestos "string" and thread.
Users of asbestos should note that the material, in most of its forms, although it is heat proof, is not waterproof. The material is absorbent, unless it be purposely "filled" with other substances to reduce this absorbency. However, as stated previously, any combustible or volatile substance which may be contaminating asbestos material may readily be got rid of merely by running a blowlamp over the contaminated area or, better still, by placing the whole of the asbestos material into a hot fire and withdrawing it after the extraneous matter has been burnt off.