Although metallurgy is concerned with the extraction and purification of metals, few of them are used in the pure state. Practically all of the metal produced finds its way into alloys before it comes to the con sumer's hands. Alloys, from the Latin, alligo, ato bind to,x' were defined by Birminguccio as ((amicable associations of metals with each Alloys may be formed by fusion, com pression or electro-deposition. By far the most of them are formed by fusion. By the alloying of metals new properties result which increase their suitability for commercial uses; for in stance, an alloy is produced with a coefficient e ' expansion of zero to make possible the man facture of clocks which will be accurate at a: temperatures. Wire is produced with the Sift coefficient of expansion as glass for use in ele tric light bulbs. Before the metallurgist gm us this alloy we used the rare metal platimc for this purpose. Alloy steels of unheard,. strength and anti-fatigue properties are pm duced for use in automobiles and aeroplanes Innumerable articles of common and span. use show that most metallic objects are alloy possessing special properties to meet speaa needs. Frequently the change of a few of a per cent of the amount of a constime• present in an alloy will cause the most startlin changes in the properties of the resultant meta: Of all the innumerable operations of metal lurgy there is hardly one which does not re quire the application of heat for its successiO promotion. Heat is usually supplied ITT the combustion of a fuel and must be applied many different ways and under various car! dons to suit the metallurgical process to to carried out. Therefore, knowing the metallce gical requirements we must know the charactr istics of the available fuels in order to deice mine which are to be used and the methods ce using them to produce desired results.
Fuels.—Although fuels are complex chew ical combinations, the main heat-producing ties ments of all are carbon and hydrogen. Who a fuel is once kindled the combustion will coz tinue as long as the heat evolved is suffither to keep the temperature above the igniticc point. Essential conditions for perfect tor bustion are: first, sufficient air brought into In timate contact with the fuel; second, suffice% time in which it may act; third, temperatue suitable for combustion must be maintained throughout. From these considerations it c evident that less excess air need be used Rig gaseous than solid fuels to get an intimate inee ture. Mechanical methods of producing ma tures are frequently employed. The form solid fuel affects combustion, porous fuels tit ing more air, caking fuels causing imperier combustion, etc. With perfect combustion. th products are carbon dioxide and water. atmosphere in the furnace resulting from its products of combustion may be either neuter oxidizing or reducing. A neutral atmosphere which results from perfect combustion is Ter difficult to maintain. Under ordinary cone tions we either supply an excess of air whist results in an oxidizing atmosphere, or we el: not furnish enough air for perfect combesnce and this results in a reducing atmosphere There are long and short flame fuels dependinc upon the amount of combustible gases evolved and the facilities existing for mixing air and gas. All these are important considemtions the use of fuel and design of cominictiee chambers. Fuels are rated by their caloric
values. There are three classes: solid, and gaseous. The solid are classified as nat ural (wood, peat, lignite, coal) and prepared (charcoal, coke, begasse, etc.). The Wel fuels are natural, as petroleum; and artificiaL is distilled oils, tars, etc. The gaseous fuels may be divided as natural gas, or artificial (mana factured gas, oil, water, coal, blast furnaces,, producer gas, etc., etc.). Each of these differ• Cut classes of (gels presents its own problem combustion and its characteristics must be known and considered in designing the fur nace in which it burns. A furnace which burns coke well, will not burn tar at all. In blast-furnace work the compressive strength of the fuel must be considered as that determines the weight of charge it can support.
Apparatus.— Having determined the con ditions and methods by which the heat is to be generated and applied to the metallurgical process, we must consider the requirements of a containing vessel which will stand the often times intense heats employed, have a certain amount of strength and at the same time exert beneficial, not harmful, chemical effects on the charge. The materials to which we turn to play this part in our sequence of processes are known as refractories. An ideal refractory not only should be a poor heat and electrical conductor, but must also resist the action of the high temperature causing it to melt away, resist the effects of sudden temperature changes, have mechanical strength and resist chemical changes. As no single material having all these qualities exists, metallurgical furnaces must be constructed in parts, i.e., a refractory inner lining, an intermediate insulating section and an outer wall for strength and protection. The principal constituents of refractory ma terials are aluminum silicates or clays, silica, alumina, lime, magnesium, chromite. These materials, though relatively infusible in them selves at the temperatures of metallurgical proc esses, must in order to be in shape to be used as furnace walls, be mixed with other ingre dients which increase their mechanical strength at the expense of refractoriness. The common forms in which refractories are used, are as follows: Fireclay, composed of hydro-silicate of alumina combined with fragments of other materials. It is plastic when wet, becomes brick-like when burned at red heat. It is used as clay for binder and in making fire bricks. Silica, mostly used as silica bricks, is made from quartzite, sandstone, ganister, etc., all consisting principally of silicon dioxide. Bauxite clay, principally an oxide of alumina is combined with a little silica and made into bricks. Lime pure is not used as a re fractory. Limestone mixed with fireclay is used in lining the test of a cupelling furnace. Cal cined magnesite. containing a high per cent of magnesium oxide, is used either in the granular or brick form. Dolomite, a magnesium calcium carbonate, is not much used in brick form but is usually, after calcining and crushing to small size, mixed with dehydrated tar and rammed in to make a furnace bottom. Chromite, a compound principally of the oxides of chro mium and iron, is used in the brick form and as a cement. Carbon bricks also find a use where a reducing atmosphere is required.