The _raw soda_ thus obtained in the black ash furnace is subjected to lixiviation by water in iron tanks in which the impurities or tank waste (see below) are deposited. The crude soda liquor so obtained is then further treated and converted into calcined soda, crystal soda, or caustic soda. In the production of calcined soda the crude soda liquor is first purified (‘oxidised’ and ‘carbonised’) by blowing through air and carbonic acid gas, pressed through a filter press, and crystallised by evaporation in pans and calcined, i.e. deprived of water by heat. [Illustration: FIG. 5.—Revolving Black Ash Furnace—Elevation (_after Lueger_) A Firing hearth; B Furnace; C Dust box.] _Crystal soda_ is obtained from well-purified tank liquor by crystallising in cast-iron vessels. Caustic soda is obtained by introducing lime suspended in iron cages into the soda liquor in iron caustic pots, heating with steam, and agitating by blowing in air. The resulting clear solution is drawn off and evaporated in cast-iron pans. As already mentioned, the _tank waste_ in the Leblanc process, which remains behind—in amount about equal to the soda produced after lixiviation of the raw soda with water—constitutes a great nuisance. It forms mountains round the factories, and as it consists principally of calcium sulphide and calcium carbonate, it easily weathers under the influence of air and rain, forming soluble sulphur compounds and developing sulphuretted hydrogen gas—an intolerable source of annoyance to the district. At the same time all the sulphur introduced into the industry as sulphuric acid is lost in the tank waste. This loss of valuable material and the nuisance created led to attempts—partially successful—to recover the sulphur. The best results are obtained by the Chance-Claus method, in which the firebrick ‘Claus-kiln’ containing ferric oxide (previously heated to dull redness) is used. In this process calcium sulphide is acted on by carbonic acid with evolution of gas so rich in sulphuretted hydrogen that it can be burnt to sulphur dioxide and used in the lead chambers for making sulphuric acid. Sulphur also as such is obtained by the method. These sulphur-recovery processes which have hardly been tried on the Continent—only the United Alkali Company in England employs the Chance-Claus on a large scale—were, as has been said, not in a position to prevent the downfall of the Leblanc soda industry. Before describing briefly the Solvay method a word is needed as to other processes for manufacture of sulphate and hydrochloric acid. _Hargreaves’ process_ produces sodium sulphate (without previous conversion of sulphur dioxide into sulphuric acid) directly by the passage of gases from the pyrites burners, air and steam, through salt blocks placed in vertical cast-iron retorts, a number of which are connected in series. A fan draws the gases through the system and leads the hydrochloric acid fumes to the condenser.