ASSAV/A'G AXD SALT/A'G THE ORE. 49 



rod, which is pushed down to the bottom in order to have mud from the bottom, 

 the centre, and the superficies of the cake. The mud thus collected is put together, 

 well mixed up, and ready to effect the assay. The assay gives to the cake a rich- 

 ness lower than the one obtained when assaying the ground ore ; which difference 

 is caused both by the silver remaining in the arrastre and by the wear of the latter, 

 which, increasing the bulk of the cake, abates its relative richness. The quantity 

 of gold obtained in this assay will give an exact idea of the amount to be collected 

 from the arrastres, consequently, of the degree of perfection attained in such an 

 interesting operation. It is to be borne in mind that the assay of the cake, which 

 serves as a basis to estimate the ore, as well as the results of the reduction, is made 

 both by the miner who sells the ore and by the reducer who purchases it. Hence 

 the results of two different assays show the true value of the ore, with a proper 

 allowance for reduction, provided the operation be accurate. It is also to be 

 borne in mind that it is untrue, as some misinformed foreign writers have asserted, 

 that our assays show a richness thirty per cent lower than the right one ; for, since 

 the exportation of ores is allowed, many a quantity of them has been sold to 

 Europe, the assays made there having given a result very similar to ours. 



The quantity of salt to be put in a cake varies according to the richness of the 

 ore, and it is impossible to state a constant proportion. We have only noticed 

 that the fluctuation takes place in the same direction ; that is to sav, the richer the 

 mud is, the larger the quantity of salt required; so that, for instance, when the 

 cjuantity of silver in a tnotiton is from four to six marks, 125 pounds of salt are re- 

 quired, which is equivalent to 3.90 per cent of the mud. If the richness be from 

 six to- ten marks, 150 pounds of salt will be added, say 4.68 per cent. On adding 

 the salts to the cake, it must be evenly distributed over the surface in order that 

 its action may be uniform on the whole mass. 



Chlorure of sodium constitutes the active principle of salt, and is one of the agents 

 indispensable for amalgamation. The common salt used in the reduction of ores 

 contains other substances, such as chlorures of calcium and magnesium, sulphates of 

 the same bases and of soda, carbonates of soda and lime, some earth, and other acci- 

 dental substances. All those impurities being very variable, even in salts coming 

 from the same mine, and having great influence upon the result and duration of ore- 

 reduction, the salt must be assayed previous to being added to the cake in order to 

 ascertain the quantity of chlorure of sodium it contains. Besides, it must be borne 

 in mind that to employ poor salt, or to use an insufficient quantity of it, will prove 

 very detrimental. 



The cake being salted, with the above precautions, the first repaso is practised. 

 Repaso is the name people give to the operation of stirring the mud mixed with the 

 substances required for reducing ores, and is performed by mules, which are caused 

 to continually walk over the cake during eight hours. Twenty-four mules gener- 

 ally are employed for a cake containing a hundred vionfones (about 147 net tons). 



On the day following the salting of the cake ; the magistral and quicksilver are 

 added, an operation called incorforo (incorporation). Both the above substances 

 are to be perfectly spread over the surface of the cake; which operation offers no 

 difficulty at all, as the magistral is powdered, and the quicksilver is strained through 

 a linen, pressed by hand, and takes the form of small balls, coming out like drops 

 of rain. After having incorporated the cake, another stirring is immediately effected, 

 repeating said operation every other day, from six in the morning to two in the after- 



