Zii/r DL'silverizatlou. 93 



added. Pure zinc is uo longer used for all these additions. 

 The second, third and fourth scums of a previous operation, 

 which are not very rich in silver, are used for the first and some- 

 times for the second addition, thus greatly reducing the amount 

 of zinc required for the operation. When the lead is very poor 

 in silver, the first addition is used several times, in order to make 

 it as rich as possible. The object of dividing the additions is to 

 arrive, as quickly as may be, at the highest percentage of silver, 

 and to get an alloy so rich that there will be little liability to 

 oxidation in the subsequent liquation, thus shortening and cheap- 

 ening the process. The amount to be added in the first charge 

 will depend on the amount of copper in the lead. If it contains 

 but a small amount of copper and some gold, 100 lbs. are added, 

 at Cheltenham. If there is much copper, more zinc must be 

 added to bring out the copper, as most of the copper comes off 

 with the first Grasses. If gold is present in large proportion, 

 the quantity of zinc must be increased, since all the gold comes 

 off with the first scums. If no gold is present, two-thirds 

 of the charge of zinc necessary for the whole operation may be 

 added in the first charge. It is then stirred from one-half to 

 three-quarters of an hour with a flat spatula, which is 17 inches 

 in diameter, attached to a piece of gas-tubing 6 feet long. The 

 temperature during this time is kept above the melting-point 

 of zinc. The tool is made to work from the sides toward the 

 center, with a downward motion at the same time. When the 

 zinc is thoroughly incorporated, the fire is drawn, and the ket- 

 tle allowed to cool until the zinc alloy, which contains the silver, 

 rises and floats on the top of the melted lead. This time de- 

 pends on the heat of the metal, and on the season of the year. 

 In summer, it is four hours ; in winter, only two. The skim- 

 mings are taken off in perforated ladles, and put into one of the 

 smaller kettles. These first skimmings are carefully separated 

 from the rest, if the lead contains either much gold or much 

 copper, or both. 



At Cheltenham, the skimmings from the first addition of zinc 

 are charged into a small kettle between the two large ones. 

 At the Germania works, kettles Nos. 1 and 2 are skimmed into 

 Nos. 3 and 4. If the skimmings come from the first addition 

 of zinc, they are partially liquated in Nos. 3 and 4, and trans- 



