Zi)ic Desilverizdtion. 105 



into the retort to assist the discharge of the fumes. When no 

 more escape, the furnace is tipped down and the contents of the 

 retort discharged into a lined receiver, and there left until cool 

 enough to he cast into pigs. They generally contain from 2,000 

 to 3,000 oz. of silver, and not more than from 0.5 to 0.8 per cent 

 of zinc. The retort is now carefully scraped with an iron scraper, 

 to remove any slag or other material adhering to the sides. The 

 amount removed in this way is not large; but it is necessary to 

 keep the retort clean, for if the material was allowed to accu- 

 mulate, it might he difficult to remove it, and there would be a 

 risk of breaking the retorts in doing so. The material so collect- 

 ed, amounting usually to a few pounds, is reduced with the cu- 

 pellation litharges. The unburned charcoal is put back into the 

 retort. When the retort is cleaned, it is turned up partially, 

 and fine charcoal dust, or a piece of wood, thrown in, to make 

 a reducing atmosphere, and prevent the formation of litharge 

 from the oxidation of the very small quantity of lead attached 

 to the sides of the retort. This precaution is very neces- 

 sary, for if the litharge was allowed to form, it would soon de- 

 stroy the retort. The furnace is now turned up and is ready 

 for a fresh charge. 



The workmen are obliged to be careful in all these furnaces, that 

 in introducing the coke they do not push too hard on the retort, 

 which is quite soft. The fire must be kept at a constant tempe- 

 rature of white heat throughout the operation, which lasts from 8 

 to 10 hours according to the percentage of zinc in the alloy. But 

 when the lead contains antimony, it lasts a much longer time. 



The only precaution required during the operation, is to keep 

 the temj)erature high enough to prevent the formation of a crust 

 on the surface of the charge. To prevent this, and to know 

 what is going on in the interior of the retort, without removing 

 the condenser, it is probed from time to time to break the crust, 

 for if it should form, an explosion would be likely to take place. 

 The men can always tell the condition of the heat by looking into 

 the coke-charging hole. 



It is very necessary that the current of gas should always be out 

 of the retort. The retort should last from 1 to 20 operations on 

 an average, and it is generally broken before it is worn out ; but 

 when much antimony is present in the lead, they last a much 



