558 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



The columns and pads were set in place in 1825. Wlien un- 

 covered the lead was observed to be thinly coated with a white crust 

 which was shown by X-ray diffraction analysis to be hydrocerassite 

 [Pba (003)2(011)2], corresponding to artificial white lead, commonly 

 used as a paint pigment. Wlien exposed to open air it will probably 

 further alter to cerussite as it often does in nature. 



In the Freer Gallery collection of ancient Chinese bronze cere- 

 monial vessels those with high lead content frequently have cerussite 

 as a mineral alteration product among the corrosion constituents. On 

 some of the vessels it covers large areas with rather ugly grayish 

 crusts interspersed with patches of reddish lead oxide (litharge) 

 and malachite. It is not uncommon to find cerussite as a distinct layer 

 underlying malachite. In some occurrences, cerussite is found as 

 well-formed crystals with glistening crystal faces; in others, it occurs 

 as roimded excrescences much like malachite, and in one early bronze 

 vessel cerussite took the form of the original bronze surface much as 

 tin oxide does when it replaces copper pseudomorphically. Cerussite 

 often reveals itself on ancient bronze by its pinkish yellow fluorescence 

 in long wavelength ultraviolet light. 



LEAD OXIDES 



Next to the carbonates the several oxides of lead seem to be the 

 most commonly occurring lead minerals. Caley (1955) identified 

 massicot or yellow lead monoxide, PbO, on lead objects excavated at 

 the Agora in x\thens. He also found dark brown patches of lead 

 dioxide or 'plaUneTite^ Pb02, in a thin layer next to the lead metal. 

 On the lead pads from the U.S. Capitol, already mentioned, patches 

 of a hard salmon pink encrustation gave the X-ray diffraction pat- 

 tern of litharge^ another form of lead monoxide. The third common 

 oxide of lead called minium (red lead) is found also in nature, but 

 its occurrence on a lead artifact apparently has not been recorded. 



LEAD SULFIDE 



A. G. Daubree (1875), reported the occurrence of galena^ PbS, on 

 a specimen of lead metal found in a thermal spring at Bourbonne- 

 les-Bains in France. Strangely, no other occurrence of this naturally 

 abundant mineral of lead has been noted. 



LEAD CHLORIDES 



The occurrence of the rather rare white lead mineral cotunnite^ 

 PbCla, was observed by A. Lacroix (1910) as an alteration product 

 on lead plates of a sunken Eoman ship found off Mahdia, Tunis, in 

 1907. Cotunnite was first found at Vesuvius as a product of sublima- 

 tion. Lacroix also reported another white lead mineral, fhosgenite 



