When it has been boiled down until it stands just above the point at which cryatals are depoaited, it is run off into coolers in which crystals of sulphate of iron separate out. The mother liquor from these is run off' into another cistern. When pure potash alum is required, a saturated solution of chloride or sulphate of potaah is run into the cistern ; but for ammonia alum, impure aulphate of ammonia (usually in the form of gas-liquor) is employed; the ordinary alums of commerce are, however, generally mixtures of the two. The correct quantity of these solutions has been added when the addition ceases to produce a cloud or milkinesa in the cistern. To produce 100 parts of alum from the sulphate of alumina liquor, the theoretical quantities are:— Chloride of potash .. „ parts.
Sulphate of potash .. „ 18.4 „ Sulphate of ammouia „ „ 13.9 „ The exact proportions required may be determined by teating a small quantity of the aluminous liquor before introducing the alkaline solution, but in practice the above indication is a sufficient guide. The mixture is next allowed to become perfectly cold, when the mother liquor is pumped or siphoned off, and the residue, consisting of alum in fine crystals, or " flour," is well drained, and washed several times with a little cold water. The alum flour is theu placed in a leaden boiler, and dissolved in boiling water. While still boiling, or baying just ceased, the liquor is run into large casks or tubs. Theae tuba are built in pieces ; at the bottom is a large flagstone, and the pieces, each of which is lined with lead, are built round it, and kept in their places by strong iron hoops screwed together. The diameter of the tubs is greater at the bottom thau at the top, and they are about 6 ft. high. When the hot solution is drawn off into these tuba, they are covered with wooden covers. In about four days, the sides of the tubs may be taken down, as there will then be a sufficient thickness of alum to hold the mother liquor. It now stands in thia condition for fourteen days more, and a hole is made near the bottom of the block through which the mother liquor is drained out. This block is afterwards broken up and packed into casks for the market. The bottom part, which is not so pure as the rest, is usually redissolved.
3. From Coal Shales (Spence's process).—By this process, which was patented by Peter Spence, of Mancheater, in 1845, about two-thirds of the alum produced in this country is manufactured. The shales used, which are black owing to tbe presence of from 5 to 10 per cent. of carbonaceous matter, are found underlying the coal seams in South Lancashire. They are calcined in the following way : A number of air-channels, one or two feet apart, are constructed by laying two parallel lines of bricks, each line about 4 in. distant from the other, and then laying bricks across the top of these so as to form a channel of about 4 in. section, the bricks being laid loosely in order to permit the air to pass freely between them. Upon these channels, one workman throws a layer of burning coal, while another covers the coal with the more bituminous shales, broken up small. When com bustion commences, more shale is laid on gradually, care being taken not to put out the fire, and at the same time keeping down the heat to low redness, the object being to render the alumina of the shale aoluble in sulphuric acid. If the temperature be raised too high, the clay will vitrify, and the alumina become insoluble. Calcination usually occupies about ten days, and when complete, the shale is of a pale red colour. It is then placed in long tanks or pans, made of sheets of cast iron, screwed together, lined with lead, and about 40 ft. long, 10 ft, wide, and 3 ft. deep. Before
being charged with shale, the bottoms of these pans are covered with tiles, about 9 in. square, in order to prevent the shale from coming in contact with the lead, because the heat would dry the shale and burn the lead. The charge of each pan is about 20 tons. The shale is then digested with about 10 tons of sulphuric acid, of sp. gr. 1.25. During four or five days the contents of the pan are kept at a temperature of 105° (220° F.), partly by means of a fire underneath the pans, and partly by the introduction of ammonia in the form of gas-liquor, which is boiled down in boilers. Steam is also driven in, in order to maintain the tempetature. From time to time the liquor is tested to see if it be of the proper strength. A small quantity is put into a aquare, shallow leaden dish, and according to the time it takes to crystallize, it is known whether the liquor is ready to be drawn off into the coolers. These coolers are large, rectangular leaden vessels, about 29 ft. long, 17 wide, flud 1 ft. 9 in. deep. While the liquor is in tbe 000ler, it is constantly agitated by means of a long wooden arm, which is worked by steam ; this prevents the formation of large crystals. On an average, the liquor remains iu these coolers about fourteen hours, at the end of which time there is a bed of small crystals deposited, several inches in thickness. This deposit is greenish in colour, owing to the presence of sulphate of iron. The crystals are then thrown into a large, square box, lined with lead; in this they are washed well with mother liquor and then allowed to drain, the operation taking about two hours. When thoroughly washed, the crystals are thrown upon an iron grating, the bars of which are about in. apart ; this is done in order to break the lumps and wash out the mother liquor. The crystals are then ready to be dissolved, which is effected by means of steam in a strong cylindrical vessel, 2 to 3 ft. high, and 2 ft. in dianaeter. It has two divisions ; one part is open to allow the crystals to be thrown in, and the other part has a division and is closed. This diviaion is to prevent large crystals from passing through undissolved. At the bottom of the open part of the cylinder is a coil of lead pipo, perforated with small holes, through which a current of steam (about 20 lb. pressure) is driven. This, passing through the alum, dissolves it as fast as oue man cln throw it in. At the top of the cylinder is a pipe, which communicates with a wooden tank, lined with lead, called the dissolving box. Thie is 14 ft. leieg, 8 ft. broad, and about 3 ft. deep. It ia to receive the solution of alum before it is drawn off into the crystallizing tubs or reaching pans; the solution remains in this tank about 3 hours, and is covered over with boards, the joints of which are loosely packed with cotton waste. When this tank is nearly full, and under full pressure of steana, about four quarts of size is poured in through a pipe, in order to cause the mechanical impuritiea t,o settle out. When the solution is ready to be tapped off into the tuba, those which are at the furthest end of the shed— say 60 ft. from the tank—are filled a few minutes before thoae at the nearer end. The appearance of the alum after crystallization depends largely upon the temperature at which the boxes have been filled. When it ia too hot, the liquor forms large crystals outside, but when t,he temperature is about right, the exterior of the block of alum ahould resemble a loaf of auger. The processes of crystallizing and removing the crystala are precis* the same as described above.