612 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



in the imperfectly corroded buckles. This white material is hy- 

 drated carbonate of lead, or white lead. The pots are emptied, one 

 by one, into a large container equipped with an exhaust fan, so that 

 the dust created in the emptying is carried off and does not contami- 

 nate the air about the workmen. 



From the stack the corroded buckles are conveyed to a separating 

 machine. Here they are broken up by tumbling about in a revolv- 

 ing screen called a " buckle beater," and all uncorroded portions of 

 lead are separated from the white lead. 



The white lead is then mixed with water and ground again and 

 again between heavy rapidly revolving millstones until it is fine 

 enough to float on water through a long, oblong box called a " drag 

 box." The coarse particles of white lead sink to the bottom of the 

 drag box and are mechanically returned to the millstones for re- 

 grinding. 



The white lead that is fine enough to float through the drag box 

 enters a series of four huge tubs in which the water is agitated con- 

 stantly by revolving rakes. These tubs are known as " rake tubs." 

 In the rake tubs, the white lead is washed free from traces of acid 

 and other impurities; here also any coarse particles that accidentally 

 passed the drag box settle to the bottom. 



After floating through the series of rake tubs, the white lead (still 

 floating in water) flows on to a screen of silk bolting cloth contain- 

 ing about 27,000 microscopic holes to the square inch. Only white 

 lead which is fine enough to pass through these holes is used in paint, 

 that which remains on the screen being rejected and reground. The 

 white lead passing through the screen is pumped into large settling 

 tubs where it settles to the bottom as wet pulp, the surplus water 

 being drawn off from the top. 



The wet pulp is then pumped upon a series of drying pans where 

 the moisture in it is evaporated by means of steam surrounding the 

 pans. (This operation requires periodical stirring of the pulp by 

 hand and the series of pans is so arranged and connected with a 

 revolving exhaust fan that when any door in front of a pan is opened 

 a strong draft of air is drawn in and carries away any dust particles 

 raised in the act of stirring.) From the drying pans the dried white 

 lead is conveyed to mixers, known as "chasers," where it is mixed 

 with linseed oil, then to stone mills, where it receives its final grind- 

 ing, after which it is packed in appropriate containers. Eight 

 pounds of linseed oil to ninety -two pounds of white lead is the usual 

 mixture. 



The other commercial methods of manufacturing white lead are 

 known as quick processes because they lessen the time required to 

 corrode the lead. The two most successful processes are the Carter 

 and the Matheson, the principle underlying both being the same as 



