The manufacture of borax from the Tuscan boracic acid is performed in the following way ;— About 23 cwt. of c trbonate of soda crystals are placed in s large copper, or wooden vessel lined with lead, containing such a quantity of water as will raise the total weight to 2 tons. Heat is then applied, by the introduction of steam from a perforated coil lying at the bottom of the vessel, and the mass is then kept at the boiling point till all the soda is dissolved. The cryetala of boracic acid are then added, in instalments of 10 to 12 lb., till there is sufficient to complete the decom position of the sodic carbonate—usually about 1 ton. Carbonic acid is disengaged with lively effervescence, and borate of soda (borax) remains in solution, the liquor exhibiting a specific gravity of 1.66 (33° Tw.) at the conclusion of the operation. The heating is then suspended, and the vessel is tightly closed, and left to stand for ten to twelve hours, at the end of which time the clear solution is run into shallow crystallizing pans. In cool weather, the crystallization usually occupies three or four days. The mother-liquor is siphoned off, and is put back into the copper to assist in the solution of more carbonate of soda. The resulting crystals of borax are redissolved in boiling water, to which is added about 1 cwt. of carbonate of soda for each 2i tons of borax. The solution is heated to the boiling point and is run into lead-lined inverted cones. These are covered with matting, in order to conserve the heat as lung as possible, to produce larg,e aud well-defined crystals. At 30° (86° F.) the mother-liquor is run off, and the crystals, after standing a few hours to prevent their becoming friable, are carefully detached by means of a chisel.
To avoid the tedious nature of the above operation, it has been proposed to calcine the native acid, with perfectly dry carbonate of soda, in a furnace at a red heat ; the result is a hard, spongy borax of excellent quality. But as the impurities inherent in the 'acid can only be eliminated by crystallizing the borax, the improvement on the older plan is not apparent.
In making octahedral borax, the liquor must be run off to crystallize at 75° (167° F.), when the sp. gr. is 1.3 ; the mother-liquor is removed when its temperature is 55° (131° F.). The product is harder than ordinary borax, and contains only half as much water. It is preferred for soldering purposes.
The refining of tineal is performed by placing the crude mineral in pans, covering it with cold water to a height of 2 or 3 in. above the surface, and leaving it to stand for some hours. Receutly slaked lime is then added, in the proportion of 1 part to 400 parts of tincal ; the mixture is stirred, and left to stand for twelve hours ; it is then again strongly agitated, and the muddy supernatant liquor is decanted. This liquid is not thrown away, but is preserved to wash the impure borax, tho solid matters in suspension being first separated by settlement and decantation. The washing is continued with the same liquor, clarified by subsidence as often as applied, till it is no longer rendered turbid. In this way, a great portion of the fatty rnatter is washe,d away as an
insoluble soap of lime. The salt thua purified is dissolved in 2/ parts of boiling water, to which has been added a solution of chloride of lime, containing 2 parts of that salt to 100 parts of tincal. A precipitate is thereby produced, consisting chiefly of insoluble aoap of lime. The liquor is separated from the precipitate by filtration, and is evaporated down to a density of 1.14 or 1.16 ; it is then run off into crystallizing vessels, and is cooled very gradually, in order to produce large crystals.
Borate of lime in a raw state has been used with fair success, as a subatitute for borax, in porce lain manufacture : but it is liable to spoil the colour. When treated by the following process, however, it is found to be quite equal to the sedic salt. The mineral is levigated in a manner similar to that adopted in c6lour works, and a small jet of steam is admitted into the dolly tub to promote purification. The sand and coarser impurities are separated in the dolly tub, while the finely divided borate of lime flews on through spouts into tanks ; there it subsides, leaving the soluble impurities in solution in the supernatant liquor, which is siphoned off. The borate of lime is dried in dishes in stoves, or in reverberatory furnaces, with a gentle surface heat, and is occasionally stirred about. For coarse ware and inferior glass, it auffices to wash off the sand, &e., adhering to the crude nodules of mineral. In using this substance as a "fret" for potters, the consumption must be regulated so as to have the same proportion of boracic acid as would be represented by a specific quantity of borax.
The extraction of boracic acid from borates of lime has been already described. There only remains to be mentioned a process sometimes employed for converting the borate of lime directly into borate of soda. This is effected in two ways, known respectively as " wet " and " dry '' ; the firat is peculiar to France, as the second is to England. The former closely resembles the manufacture of borax from Tuscan boracie acid, already detailed. The finely-powdered mineral is heated with water in a similar way, and a quantity of carbonate of soda is added, sufficient te neutralize the previously determined beracie acid present. Under the influence of the heat, a double decomposi tion takes place, and when cornpleted, the copious precipitate of carbonate of lime is allowed to subside, while the clear solution of borate of soda is decanted, concentrated, and crystallized. According to the second plan, a mixture of powdered berate of lime and carbonate of soda, with the addition of a little sand or other siliceous substance, is heated, in a reverberatory furnace, to a state of incipient fusion, and is then left to cool, after which the semi-fused mass is powdered, and the botax is extracted by lixiviation.