261 



ALUMINA. 



ALUMINIUM. 



162 



refrigeratory process. It takes from eight to fourteen days for the 

 liquor to cool and deposit its crystals ; the mother liquor remaining in 

 the centre ifl then run off through a hole bored in the side of the 

 crystalline mass, which is then broken up into irregular square blocks 

 and sent into the market. 



By the use of a purer raw material than the alum shale above men- 

 tioned, the processes may be much simplified, so far at least as regards 

 the production of the sulphate of alumina. Thus, in some manu- 

 factories, pipe-clay or china-clay containing little else than silicate of 

 alumina, contaminated with only a small quantity of oxide of iron, is 

 gently roasted in a reverberatory furnace, and then decomposed by hot 

 sulphuric acid which combines with the alumina, expelling the silicic 

 acid chiefly in the insoluble form. The mass thus obtained is now 

 lixiviated with water, and there is thus obtained a concentrated solution 

 of sulphate of alumina, which is subsequently converted either into 

 potash or ammonia alum in the manner already described. 



A large quantity of alum is now made by Spence's process, which is 

 peculiarly fitted for coal districts, inasmuch as the whole of the materials 

 employed are derived from coal and the coal strata. Thus the iron 

 pyrites so frequently met with amongst coal in lenticular masses or 

 distinct strata, furnishes sulphuric acid [SULPHURIC ACID] which, when 

 heated in large shallow leaden pans with roasted coal shale (a material 

 very similar in composition to alum shale), yields sulphate of alumina, 

 containing an excess of sulphuric acid. A quantity of the ammoniacal 

 liquor of gasworks [GAS] is now boiled in a close vessel, and the vapours 

 of carbonate of ammonia and sulphide of ammonium arising therefrom 

 are conducted into the hot acid solution of sulphate of alumina, where 

 they are converted into sulphate of ammonia, which immediately 

 forms ammonia alum with the sulphate of alumina present, whilst 

 carbonic acid and sulphuretted hydrogen gases escape. On cooling, the 

 solution deposits a copious crop of alum crystals, which are purified by 

 washing and recrystallisation, in the manner already described. 



Properties. Alum presents the appearance of colourless, tolerably 

 transparent octohedral crystals, which possess a sweetish astringent 

 taste, and are soluble in about 18 parts of cold water and less than 

 their own weight of boiling water. These crystals contain 24 equi- 

 valents of water, which they lose on exposure to a heat of about 400, 

 and are converted into a light, porous, white, insoluble mass termed 

 /mi-lit alum. At a bright red heat ammonia alum is totally decom- 

 posed ; ammonia and sulphuric acid are volatilised, whilst pure and 

 anhydrous alumina is left behind. For the chemical relations of alum, 

 see ALF.MS. 



I'ten. Alum is largely employed in dyeing and calico-printing, and 

 in the manufacture of paper ; it is also used in the production of pig- 

 ments called laka, and is sometimes mixed with the flour used for 

 bread. A minute quantity of alum thus employed renders the bread, 

 especially if made from the inferior kinds of flour, lighter, whiter, and 

 probably more digestible ; it also enables the bread to retain a larger 

 amount of water after baking. It is easy to perceive that these pro- 

 perties may, on the one hand, be made fraudulently subservient to the 

 tion of bread apparently of the best quality from inferior flour ; 

 and on the other, may be legitimately used as means whereby such 

 Hour, when it must be employed as food, may be converted into a 

 more palateable and wholesome bread. The amount of alum (about 

 the weight of the flour) required to produce this effect, is too 

 minute to exercise any deleterious effect upon the consumer of such 

 bread. 



In all these applications it is immaterial whether potash alum or 

 ammonia alum be employed; weight for weight, however, ammonia 

 alum is rather more valuable than potash alum, since it contains 11'90 

 per cent, of the active ingredient, alumina, whilst potash alum contains 

 mly 10-82 per cent. 



About 20,000 tons of alum are now annually manufactured in the 

 United Kingdom. 



ALU'MINA (Al, 3 ). The oxide of aluminium ; it constitutes a 

 large proportion of all clays, which in a great measure owe to it their 

 plastic property. The name of alumina is derived from alumen, the Latin 

 for alum, the salt from which it is generally obtained in a pure state, by 

 in, :ui* which will presently be mentioned. Alumina is widely diffused in 

 nature ; the adamantine spar or corundum, the ruby, and sapphire, are 

 alumina nearly pure and crystallised, and are among the hardest sub- 

 stances in nature. The diaspore is a crystallised mineral, which con- 

 sists almosts entirely of alumina and water ; and in North America 

 another hydrate of alumina has been found, and called gibbsitc. 

 KiM'Ty, likewise remarkable for its hardness, and hence much used for 

 grinding and polishing, is also alumina coloured with the oxides of iron 

 and manganese. 



The following in the process recommended by Berzelius (' Traite" de 

 I 'himie/ ii. 369) for procuring pure alumina : Dissolve and crystallise 

 alum repeatedly, to deprive it of the peroxide of iron which it usually 

 contains ; when thus rendered pure, a portion of the alum dissolved in 

 water, and added to a solution of potash in excess, gives a precipitate 

 at first, which is afterwards completely redissolved. To a boiling 

 solution of the purified alum, add one of carbonate of potash, as long 

 as precipitation takes place; then a slight excess of the carbonate 

 being used, digest with a gentle heat to decompose the subsulphate of 

 alumina formed. Wash this carefully on a filter, and redissolve it in 

 hydrochloric acid ; precipitate the clear solution with ammonia or car- 



bonate of ammonia, and wash the precipitate, which, when dried with 

 a gentle heat, is hydrate of alumina, and when heated to redness 

 becomes pure alumina, by losing its water. 



If intended for the purpose of solution in acids, it is better to keep 

 the alumina in the state of hydrate ; for when once rendered anhydrous, 

 acids act upon it slowly and with great difficulty. Pure anhydrous 

 alumina may also be obtained by heating ammonia alum to redness. 



Hydrate of alumina, when recently precipitated, presents the ap- 

 pearance of a white, bulky, semi-transparent, gelatinous substance ; on 

 drying it contracts greatly and forms a white powder, which adheres 

 to the tongue strongly. Hydrate of alumina readily combines with 

 acids, except carbonic acid, forming salts which are generally soluble 

 and uncrystellisable. They all possess an acid reaction. If the acid 

 be volatile, a portion of it is generally expelled on boiling a solution of 

 the salt, and a basic compound is precipitated. With acetate of alu- 

 mina this takes place at ordinary temperatures ; as, for instance, in 

 the ageing of calico, which has been mordanted with acetate of 

 alumina, acetic acid passes off during the ageing process, and a basic 

 and insoluble acetate becomes fixed in the fibre, forming what is 

 termed the mordant. Hydrate of alumina, as usually prepared, is 

 insoluble in water ; but Walter Cram has described a curious modifi- 

 cation of it, which dissolves in large quantity in water containing a 

 minute amount of acetic acid. Hydrate of alumina is readily soluble 

 in solutions of the fixed alkalies, but insoluble in ammonia. If its 

 solution in caustic potash or soda be exposed to the air, carbonic acid 

 is gradually absorbed, and terhydrate of alumina (A1 2 3 , 3 HO) depo- 

 sited in small but well-defined crystals. 



When strongly heated, hydrate of alumina becomes suddenly in- 

 candescent, contracts greatly, and loses its water of hydration. It is 

 then nearly insoluble in acids, but may be fused before the oxyhydrogen 

 blowpipe, yielding an exceedingly hard vitreous mass, resembling 

 corundum. 



Alumina is an important substance, whether regarded as the ore of 

 the metal aluminium, as a constituent of soils, or with respect to its 

 extensive employment in the operations of the dyer, calico-printer, and 

 colour-maker. It is also a necessary ingredient in all kinds of porcelain, 

 earthenware, bricks, and tiles. 



ALUMINA, SULPHATE OF. (Al,0 3 , 3S0 3 + 18aq.) A salt formed 

 by dissolving hydrate of alumina in dilute sulphuric acid, and then 

 evaporating the solution. It crystallises in colourless flexible scales, 

 containing eighteen atoms of water. They are soluble in twice their 

 weight of cold water. Sulphate of alumina has an astringent and acid 

 taste ; it is permanent in the air, and is nearly insoluble in alcohol. 

 It is used in dyeing and calico-printing, in the place of alum, for the 

 production of mordants ; and, as a general rule, it may be employed 

 for all the purposes to which alum is applicable. For these applications 

 sulphate of alumina is manufactured on a large scale by treating the 

 white previously calcined clays of Devonshire and Dorsetshire with 

 then- own weight of dilute sulphuric acid, specific gravity 1 '200 at the 

 temperature of boiling water ; and then, after separating the insoluble 

 silica, precipitating the small amount of peroxide of iron, which is 

 dissolved as sulphate, with ferrocyanide of potassium. The Prussian 

 blue thus formed is allowed to subside, and the supernatant liquor is 

 concentrated by evaporation, until it solidifies on cooling into a white 

 mass, which is nearly pure sulphate of alumina, containing from 13 to 

 1 4 per cent, of alumina. The commercial salt thus manufactured is 

 sometimes called patent alum. 



A less pure, but almost equally efficacious, sulphate of alumina is 

 made by a still more simple process, and is sold under the name of 

 alitminoia cake [ALUMINOUS CAKE]. 



ALUMINIUM, the metal contained in alumina. Aluminium was 

 first obtained in a state approaching to purity by W'ohler, in 1827, by 

 decomposing chloride of aluminium by means of potassium. Its pre- 

 paration has since been simplified and rendered practicable on the large 

 scale by Bunsen and by Deville. The former chemist prepares it by 

 the electrolytic decomposition of the double chloride of aluminium and 

 sodium (A1 2 C1 3 + NaCl). The melted salt is maintained at a tempera- 

 ture of about 400 ; and as the metal separates at the negative pole, 

 which consists of a plate of gas-carbon, it sinks to the bottom of the 

 crucible containing the melted salt, and is thus protected from 

 oxidation. On subsequently raising the temperature, the particles of 

 aluminium coalesce into a large globule. 



M. Deville, to whom we are indebted for a process capable of pre- 

 paring the metal on a manufacturing scale, obtains aluminium by 

 decomposing its chloride with sodium. The chloride of aluminium he 

 prepares as follows : anhydrous alumina is mixed with charcoal, made 

 into a paste with oil or tar, and ignited in a covered crucible. When 

 cold, the mass, which now consists of alumina and carbon, is broken 

 into fragments and placed in an earthenware retort, with the short 

 neck of which a glass receiver is connected. The retort and its con- 

 tents being heated to dull redness, a current of dry chlorine gas is 

 introduced through the tubulare, and solid chloride of aluminium 

 immediately begins to condense in the receiver. The following is the 

 reaction : 



3C 



A1 2 C1 3 + SCO. 



Alumina. Carbon. Chlorine. Chloride of 

 aluminium. 



Carbonic 

 oxide. 



