ALUDEL. The recipient of vapourizing sub • stances subjected to the operation of heat. They are generally of earthenware, and of various forms and size. Sometimes tubes are employed as aludels, and sometimes vessels of large capacity, according to the nature of the substance which is to be condensed in them. The process of condensing the product is much facilitated by keeping the aludel constantly cool by wet cloths or a stream of water. ALUMINA. A primitive earth existing in great abundance in clays, earths, ochres, rocks, &c. Soils containing much of this substance are called argil laceous. It is found in a state of areat purity in many previous gems, as the sapphire, topaz, emerald, garnet, beryl, &c. When pure, it is of a white colour, pulverulent, and soft to the touch. It adheres to the tongue, but is tasteless. It is insoluble in water, but is readily dissolved by acids, and also by caustic, potash, or soda. If moistened with water, a very ductile and tenacious paste is formed, which, by heating, becomes exceedingly hard, and is, therefore, ex tensively employed in the manufacture of porcelain, earthenware, and all kinds of pottery. Bricks, tiles, crucibles, and stone ware, contain large quantities of alumina. It is infusible, per se, in the strongest heat of a furnace, but small quantities may be fused by the oxy-hydrogen blow-pipe. If mixed, however, with certain proportions of lime and silica, it fuses readily. Pure alumina may be obtained easily from the triple salt, containing ammonia instead of potash, by heating it intensely. The acid and the alkali are dissipated by the heat, and the pure earth remains. It is usually procured by adding a small quantity of solu tion of bicarbonate of potash to a solution of common alum, which precipitates the iron contained generally in that salt. The filtered liquid is then to be added to the liquid ammonia, which combines with part of the sulphuric acid of the alum, and precipitates the earth in a spongy mass. This must be washed fre quently in distilled water, and then heated to dryness. According to the expe riments of Sir H. Davy, it appears, that, like the other earths, alumina is a metallic oxide. By passing potassium in vapour over alumina heated to whiteness, a great part of the vapourized metal was converted into the alkali potash :—a decisive proof that alumina contains oxygen. By treating the chloride of aluminum with potassium, a grey powder is obtained, which, by burnishing, acquires metallic lustre. This powder burns with much splendour if heated to redness, and is converted to alumina by absorption of oxygen. The metallic base is called aluminum; and it is found that 100 parts combine with 8 of oxygen. Alumina has the unusual property of contracting by heat, and in pretty exact proportion with the intensity applied. On this property the ingenious Mr. Wedgewood constructed his pyrometer for estimating very high temperatures. See PYROMETER. This useful earth has a powerful affinity for colouring matter, and also for, greasy substances. Fuller's earth and pipe-clay owe their useful properties to the large quantities of alumina they contain. ALUM, a triple salt of great importance in the arts, is composed of sulphuric acid, alumina, and potash. It is sometimes found native, but only in small quantities, and is artificially manufactured, chiefly from the mineral called alum slate. This is found in great abundance in the north-east district of Yorkshire, more particularly between Whitby and Stockton. In the reign of Queen Elizabeth, Sir Thomas Chaloner established a manufactory of alum at Gie borough, in Yorkshire, and engaged several expert workmen, acquainted with the mode of manufacture, from the dominions of his Holiness the Pope, who fulminated bulls and anathemas against him and them in vain. The alum
slate is also found in abundance at Beckel, in Normandy. There are other aluminous minerals from which alum may be obtained; but none appear so well adapted, nor so plentiful, as the alum-elate, or aluminous schist. The mineral from which alum is manufactured, is broken into small pieces, and placed on a bed of fuel, until it is about 4 feet high. The fuel is then ignited, and as the calcination proceeds, more of the mineral is added from time to time, until an enormous pile, sometimes equal in area to 200 square feet at the base, is formed The combustion proceeds so rapidly, that it is necessary to prevent access of air, by plastering the crevices with small schist (alum-slate,) made into a lute with water. It requires about 130 tons of the mineral to produce one ton of alum. When the calcination is effected, the residue is digested in pits containing large quantities of water. The liquid is withdrawn by pumps, and added to fresh calcined ore. This is repeated until its spec. gray. becomes about 1.15. This saturated liquor is sometimes placed in pits, to deposit sulphate of lime, and other earthy matter, which may contaminate it, and sometimes boiled to pro duce the same effect. The purified liquid is then concentrated at a boiling heat, in large sloping leaden pans. This is then removed to a settling cistern, and a solution of muriate of potash, or the impure alkali of the soap-maker, is added to it. The quantity necessary is ascertained by experiment with a small portion in a basin. The alkaline solution reduces the spec. gray. from 1.4 or 1.5 to 1.35. The quantity of alkali required is, therefore, easily ascertained by an hydrometer. If the spec. gray. of the liquid be more than 1.35, it does not yield crystals, but a solid magma resembling grease, and it becomes necessary to reduce it and re-crystallize. Urine is sometimes added for this purpose; it is crystallized in the usual manner by slow evaporation. These crystals are purified by washing and boiling in a leaden vessel. The saturated solution is then poured into casks, and in about a fortnight the casks are unhooped and taken to pieces, the alum remaining in a solid mass. This last process is called ruching. It is calculated that 22 tons of muriate of potash, or 31 tons of the black ashes of the soap boiler, or 73 of kelp, are necessary to make 100 tons of alum. Ammonia may be employed instead of potash, but it is more expensive. The impure sulphate of soda formed in the manufacture of aqua fortis, may be economically employed in the manufacture of alum, as it contains two ottt of the three ingredients necessary to its formation. Th celebrated Chaptal manufactured alum artificially in France to a great extent. A chamber of very large dimensions was constructed of masonry, and floored with brick ; the bricks being cemented with a composition of pitch, wax, and The roof was of timber, but the planks were closely grooved into each other, no nails being employed. Lastly, the whole of the ulterior was covered with a thick coating of the above-mentioned cement, applied as hot as possible. The purest and whitest clay is then, after calcination, strewn on the floor, and sulphur burned in the chamber. By this process sulphuric acid is formed, which in a few days saturates the alumina of the clay, converting it into sulphate of alumina. This is known by the efflorescence that takes place. The salt is then removed and exposed to the air, that the acid may penetrate more effectually the alumina. It is then lixiviated, treated with potash, and crystallized as before described. One of the most ancient manufactories of alum was at Roche, in Lyria, whence is derived the term Roche-alum.