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ALUM.— Alumina.
high temperature, and to the joint presence of theseveral reagents, the whole of the sulphurous acidgas is converted into sulphuric acid, and in conse-quence masses of the burnt shale come down, inlarge part converted into soluble aluminium sulphate.The' method of lixiviating employed involves thetreatment of the hot crude sulphate with water intubs which run on an inclined tram, and work on theprinciple of the lixiviating vats for alkali. Thecrude sulphate being lixiviated while it is at a hightemperature, the liquor becomes boiling hot, and isdrawn off in a highly concentrated state; so thatvery little expense is incurred for fuel and evapor-ating pans.
Pure aluminium sulphate crystallizes, although withdifficulty, in thin pearly flakes, and takes up at thesame time 18 molecules of water. It is soluble inhalf its weight of cold water, but is scarcely at allsoluble in alcohol. When heated it parts with itswater of crystallization, and leaves a white porousmass, which is anhydrous aluminium sulphate(A1 2 (S0 4 ) 3 ). This modification dissolves very slowlyin water.
Uses. —Alum is largely employed in the arts,chiefly in consequence of the affinity which aluminahas for colouring and other vegetable matters; forgelatine, and a number of like bodies. The fibre ofcotton, wool, &c., has such an affinity for aluminaor a basic salt of aluminium, that when immersed ina solution of alum a basic salt is formed, which adheresto it so firmly that it cannot be removed by washing.
This basing with aluminium salts enables the clothto unite with larger quantities of the colouring matter,and retain it with greater persistency. Some pre-cautions are to be observed in its use in the handsof the cloth printer, which will not here be enteredinto, as they form a part of the subject to be treatedof under Calico-Printing.
Alum is used in preparing white leather, techni-cally “ tawing; ” to clarify water, by the combinationswhich it forms with the foreign matters present, andalso in the preparation of bookbinder’s paste. A largoquantity of alum is taken medicinally, as a topicaland internal remedy in various diseases. Alum mixedwith gypsum (calcium sulphate) forms the chiefingredient with which the outer chambers of Milner’sfire-resisting safes are lined, so as to protect theinterior from being injured. This it does from twocauses: firstly, the large quantity of water it contains,which helps to moisten the inner chamber when thebox is heated, and so prevents the contents frombeing consumed; and secondly, from the non-con-ducting power of the mixture after the water isexpelled. The great success which has attended theapplication is the best testimony to its efficiency.
ALUMINA.— Aluminium, Oxide. Alumine , French;alumium oxyd, alaunenle, thonerde, German. Formula,Al 3 Og.—This body is assumed to be a sesquioxide,from its being isomorplious with ferric oxide(Fe 2 0 3 ). _
The utility of alum in calico-printing rests solelyupon its being a soluble modification of this earth.Alumina is extensively used, united with silica, in
the manufacture of all kinds of earthen and porce-lain ware; in the manufacture of crucibles, mortars,and cements. It is particularly employed, almostpure, for the manufacture of glass pots, the refractorypower of the earth being so great as to permit thefusion of the frit, itself remaining unaltered either bythe intense heat or by the molten substances.
Alumina occurs in the earth’s crust, but rarely ina pure state. When so found it constitutes themineral corundum, varieties of which, distinguishedby their colour, are the sapphire, the ruby, orientaltopaz, amethyst, &c.
Alumina constitutes a large proportion of all theslaty and shaly rocks. It is the main ingredient alsoof pipe-clay and argillaceous soils, which increase intenacity in proportion to the quantity of aluminathey contain.
Though it exists so largely in the soil, it contri-butes but little in a direct manner to the nourish-ment of plants, as the ash they leave contains, ingeneral, a very small quantity of alumina.
Emery, much used for polishing and grinding, isan opaque variety of corundum. The Bessemerwheel for grinding steel without loss of temper iscomposed of emery bound together by Ransome’ssilicate of lime. It thus forms a cutting stone cap-able of grinding chisels from old files, at a cost ofless than 5 per cent, of the stone in comparison withthe metal.
Preparation .—Alumina may be precipitated by analkali from any of its solutions as a bulky, white,amorphous powder, which, when collected, wellwashed, and dried, is pure. It is a white, tasteless,earthy substance, adheres to the tongue, has a speci-fic gravity of 2'00, and is insoluble in water, but dis-solves easily in caustic potassa and soda, and in mostacids, at least when newly thrown down. Whenheated to redness, however, it becomes hard anddense, as in burned clay and firebricks, gives outsparks when struck with steel, and can hardly boscratched with a file. It is then almost insoluble,even in the strongest acids. Rose states that thespecific gravity of alumina increases after long igni-tion. After exposure for some time in a porcelainfurnace alumina has the spec. grav. 3'999, which isvery nearly that of native corundum.
Pure alumina is totally infusible at all moderatetemperatures; but exposed to that of the oxyhydro-gen blowpipe it fuses into transparent globules,which can be made to resemble the ruby by additionof a small quantity of potassium chromate. Ebel-men succeeded in making minute artificial rubies byheating together in a porcelain furnace a mixture of1 part of alumina with 3 to 4 parts of borax. Thealumina dissolves in the borax, and as the latter isevaporated by the intense heat, crystallizes out ascorundum. The addition of potassium chromate, asin the previous case, gives to these crystals the colourof the ruby.
Crystallized alumina is insoluble in all acids, butcan be rendered soluble by fusion with potassium orsodium hydrates.
Alumina forms three compounds with water:—