1G8
ALUM. —Crystallization.
bottom arc found lined with alum, now nearly white,to the thickness of 8 or 9 inches. The small eup-likeclusters of second alum crystals, which form on thesurface of the coolers at this stage of the process, andsuccessively fall to the bottom, assume the beautifulappearance exhibited in Fig. 8. The mother lie,
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mm
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which should have a specific gravity of 26°, is re-moved, and used for washing up the first alum asalready described; and the crystals being broken up,are rinsed with pure water and drained, previous tothe finishing process.
Fig. 7.
Crystallization .—The powder or alum flour, con-sisting of minute or imperfect crystals, prepared asjust mentioned, is very seldom sent into the market,the consumers preferring to use the salt in theform of large crystals, the alum being less liable tobe adulterated with any other impurities when in
a well-defined crystalline state than when it isamorpl ous.
Crystallization of the alum flour is effected bydissolving it in as little boiling water as possible, andwhen no more of the substance is taken up, runningthe saturated solution into crystallizing vessels, whichare denominated “roching casks,” or “growingvessels.” Steam is better adapted than water forthe purpose of dissolving the flour. A largo woodenvessel, lined with sheet-load, is provided for thispurpose, and in the interior the powder is sustainedoccasionally on perforated shelves, so that the steamwhich issues from the pipe that enters at the bottommay have more extensive contact with it, and thusdissolve it more readily. As the steam penetratesthrough the several diaphragms bearing the powder,part is dissolved ; that which is liquefied in the upperones is concentrated by the hot steam as it falls downto the bottom of the vessel, and being hot, it is atonce run into the vessels for crystallization.
Fig. 9 is a drawing representing the roching-pan asit is usually found in the alum factories of England and
Fig. 8.
Scotland ; it works on the above principle, and there-fore renders a subsequent evaporation unnecessary.It is a large eovered leaden cistern, into which steamis forced, and the alum is shovelled in to meet it; asolution takes place rapidly, without any addition ofwater. The temperature of this solution havingreached 96° Fahr. (35° - 5 C.), the heat is raised ashigh as possible, generally to 224° Fahr. (10G°-G C.);and the steam being shut off, the pan is closelycovered up, and allowed to remain at rest for fouror five hours, or till the temperature has fallen toabout 200° Fahr. (93° - 3 C.) The water which hasbeen used for washing the crystals, is taken again forwashing the first alum, as above.
At Valmunstcr this operation is carried on invessels constructed as follows:—The wooden troughsare lined with lead in the ordinary way, but areclosed by a lid in which are two openings; one ofthe openings rqpeives the steam-pipe, and the othera wide leaden funnel. The sides of the funnel areperforated with small holes, 300 of which are insertedin every square foot of surface. In this funnel thealum flour is placed, and the steam injected through