File:Reverberatory furnace for evaporating soda solution.png

English: The surface-evaporating furnace, shown in fig. 613, is a very admirable invention for economizing vessels, time, and fuel. The grate, A, and fire-place, are separated from the evaporating laboratory d, by a double fire bridge b, c, having an interstitial space in the middle, to arrest the communication of a melting or igniting heat toward the lead-lined cistern D. This cistern may be 8, 10, or 20 feet long, according to the magnitude of the sodawork, and 4 feet or more wide. Its depth

should be about 4 feet. It consists of sheet lead, of about 6 pounds weight to the square foot, and it is lined with one layer of bricks, set in Koman or hydraulic cement, both along the bottom and up the sides and ends. The lead comes up to the top of c, and the liquor, or lye, may be filled in to nearly that height. Things being thus arranged, a fire is kindled upon the grate A ; the flame and hot air sweep along the surface of the liquor, raise its temperature there rapidly to the boiling point, and carry off the watery parts in vapor up the chimney e, which should be 15 or 20 feet high, to command a good draught. But, indeed, it will be most economical to build one high, capacious chimney stalk, as is now done at Glasgow, Manchester, and Newcastle, and to lead the flues of the several furnaces above described into it. In this evaporating furnace the heavier and stronger lye goes to the bottom, as well as the impurities, where they remain undisturbed. Whenever the liquor has attained to the density of 1-3, or thereby, it is pumped up into evaporating cast-iron pans, of a flattened, somewhat hemispherical shape, and evaporated to dryness while being diligently stirred with an iron rake and iron scrajier.

This alkali gets partially carbonated by the above surface-evaporating furnace, and is an excellent article.