564 BOOK XII. 



are poured into the caldron the same number of amphorae of the solution as of 

 congii of the lye of which I have already spoken, and into the same caldron 

 is thrown as much of the already made saltpetre as the solution and lye will 

 dissolve. As soon as the mixture effervesces and forms scum, it is trans- 

 ferred to a vat, into which on a cloth has been thrown washed sand obtained 

 from a river. Soon afterward the plug is drawn out of the hole at the 

 bottom, and the mixture, having percolated through the sand, escapes into 

 a tub. It is then reduced by boiling in one or another of the caldrons, until 

 the greater part of the solution has evaporated ; but as soon as it is well 

 boiled and forms scum, a little lye is poured into it. Then it is transferred to 

 another vat in which there are small rods, to which it adheres and congeals in 

 two days if there is but little of it, or if there is much in three days, or 

 at the most in four days ; if it does not condense, it is poured back into the 

 caldron and re-boiled down to half ; then it is transferred to the vat to cool. 

 The process must be repeated as often as is necessary. 



Others refine saltpetre by another method, for with it they fill a pot 

 made of copper, and, covering it with a copper lid, set it over live coals, where 

 it is heated until it melts. They do not cement down the lid, but it has 

 a handle, and can be lifted for them to see whether or not the melting has taken 

 place. When it has melted, powdered sulphur is sprinkled in, and if the pot 

 set on the fire does not light it, the sulphur kindles, whereby the thick, greasy 

 matter floating on the saltpetre burns up, and when it is consumed the salt- 

 petre is pure. Soon afterward the pot is removed from the fire, and later, when 

 cold, the purest saltpetre is taken out, which has the appearance of white 

 marble, the earthy residue then remains at the bottom. The earths from 

 which the solution was made, together with branches of oak or similar trees, 

 are exposed under the open sky and sprinkled with water containing saltpetre. 

 After remaining thus for five or six years, they are again ready to be made 

 into a solution. 



Pure saltpetre which has rested many years in the earth, and that which 

 exudes from the stone walls of wine cellars and dark places, is mixed with the 

 first solution and evaporated by boiling. 



Thus far I have described the methods of making nitrum, which are not 

 less varied or multifarious than those for making salt. Now I propose to 

 describe the methods of making alum, 10 which are likewise neither all alike, 

 nor simple, because it is made from boiling aluminous water until it con- 

 denses to alum, or else from boiling a solution of alum which is obtained 

 from a kind of earth, or from rocks, or from pyrites, or other minerals. 



10 There are three methods of manufacturing alum described by Agricola, the first 

 and third apparently from shales, and the second from alum rock or " alunite." The reasons 

 for assuming that the first process was from shales, are the reference to the " aluminous 

 earth " as ore (venae) coming from " veins," and also the mixture of vitriol. In this process 

 the free sulphuric acid formed by the oxidation of pyrites reacts upon the argillaceous material 

 to form aluminium sulphate. The decomposed ore is then placed in tanks and lixiviated. 

 The solution would contain aluminium sulphate, vitriol, and other impurities. By the 

 addition of urine, the aluminium sulphate would be converted into ammonia alum. Agricola 

 is, of course, mistaken as to the effect of the addition, being under the belief that it separated 

 the vitriol from the alum ; in fact, this belief was general until the latter part of the i8th 

 Century, when Lavoisier determined that alum must have an alkali base. Nor is it clear 



