PAINTS AND VARNISHES 



375 



the buckles, which gradually become coated with the lead acetate 

 and turn white. The fermenting tan-bark is, meanwhile, generating 

 carbonic acid gas. As the acetic acid fumes turn the metal lead 

 to acetate, the carbonic gas turns the lead acetate to basic lead 

 carbonate. This step completes the process of corrosion, which 

 takes altogether from 90 to 130 days. 



(/) The stacks are then torn down ("drawn" or " stripped"). 

 The acetic acid having evaporated, the disks are found to have 



FIG. 186. 



become so brittle that they may be crumbled with the fingers and 

 to have changed from a bluish gray to a white color. 



(g) Next the corroded disks are subjected to several grinding, sift- 

 ing, and refining processes, the object being to separate and remove 

 the metal lead, so that only the pulverized white lead will remain. 



(//) When the last step in refining has been finished, the purified 

 white lead is thoroughly mixed with linseed oil. The mixture is 

 then given a final grinding, from which it emerges in the doughlike 

 chunks which form the white lead of commerce, 



