CORROSION OF IVIETAL ANTIQUITIES — GETTENS 563 



SILVER CHLORIDES 



Silver objects retrieved from desert soils are often encrusted with 

 a gray-brown or dull lavender crust of cerargyrite or silver chloride, 

 AgCl. Silver coins recovered from salt water are sometimes super- 

 ficially altered to this mineral. There are instances, however, where 

 silver chloride penetrates deeply into the metal structure. Many of 

 the silver ewers and bowls now displayed in the University of Penn- 

 sylvania Museum, recovered by the late Sir Leonard "Wooley from Ur 

 of the Chaldees, were originally heavily encrusted with cerargyrite. 

 After the crusts were removed by Kenneth A. Graham (1929) by 

 means of electrolysis and formic acid, much of the original appearance 

 of these precious antiquities was restored. A Persian silver plate 

 dating from Sasanian times recently acquired by the Freer Gallery 

 of Art has a continuous dull, purplish-gray color caused by alteration 

 of the surface metal to cerargyrite. The color of the surface resembles 

 the mauve color of the silver halides of an undeveloped photographic 

 film exposed to full light. The appropriateness of the common name, 

 horn silver, is apparent when one attempts to remove the deposit from 

 the object with blunt tools. Cerargyrite and paratacamite associated 

 with cuprite are sometimes found on base silver objects. 



We still do not find listed in Dana's System of Mineralogy a com- 

 pound mineral of copper and silver chlorides, although there is one of 

 silver iodide, Agl, called miersite in which copper substitutes for silver. 

 A new mineral of copper and silver chlorides may first be identified 

 on some silver artifact. This is one more reason to stay the hand of 

 those who may want to put unsightly freshly excavated silver objects 

 through the cleaning bath before they can be properly examined. 



IRON 



Rust on iron objects is so common and is held in such disfavor 

 that it is got rid of quickly if the object is of any interest. Since the 

 beginning of this centuiy quantities of iron artifacts have been stripped 

 of corrosion crusts by electrochemical and electrolytic procedures. 

 This has no doubt m.ade possible the recovery of many important ob- 

 jects, but it is not likely that much attention was given to the nature 

 of the iron alteration products. 



IRON OXIDES 



In the process known as rusting, iron is converted to its hydrous 

 oxide goethite^ FeO(OH), named for the German poet Goethe. In 

 the older mineralogical literature it is called limonite. The mineral 

 is dull yellow or yellow-brown in color. Daubree (1875) noted that 

 iron chains of Roman origin found in the thermal springs at Bour- 



