548 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 63 



corrosion product. A corrosion product that caimot be identified with 

 a known mineral may be analyzed and described in chemical termi- 

 nology until a corresponding earth mmeral has been discovered. Some 

 mineralogists insist that a true mineral can only be formed by geologic 

 processes w^orking on geologic material, yet there are no differences 

 either physical or chemical between compounds created by purely 

 natural processes and those formed on an artifact where man acci- 

 dentally assisted in bringing the artifact and the proper environment 

 together. Austin F. Rogers (1903), former professor of mineralogy 

 at Stanford University, early in the century described the corrosion 

 crusts on ancient Chinese coins and he named the mineral species he 

 found among them such as malachite, cuprite, and others. F. W. 

 Clarke (1924), in his classic work on geochemistry makes several 

 allusions to minerals in artifacts; in Dana's System of Mineralogy 

 (Palache, Berman, and Frondel, 19M, 1951), there is reference to a 

 number of occurrences of minerals on man-made objects. Admittedly, 

 this is an ambiguous area that has not been finally resolved by 

 mineralogists. 



The mineral crusts on ancient objects are important for a number of 

 reasons. Under certain circumstances the mineral alteration product 

 formed on a metal adds interest and even beauty to an object and 

 increases its value in the eyes of collectors. The artistic term "patina" 

 is applied by them to the colorful, thin, but continuous corrosion films 

 that form on the surface of copper and its alloys giving evidence of 

 age and long use. H. J. Plenderleith says (1938) ". . . patina is a 

 form of incrustation which is stable under normal conditions of tem- 

 perature and humidity, is protective in proportion as it is hard and 

 non-porous, and has often an aesthetic appeal in accordance with 

 its hue or the play of colours of the minerals of which it is com- 

 posed." Green and red patina on ancient bronze is so admired that it 

 is sometimes produced artificially on recently made objects to simu- 

 late appearance of age and authenticity. Even old leaden objects 

 with thin crusts of cerussite and litharge are more interesting than 

 lead in the raw. Unfortunately, however, most corrosion products 

 on metals are ugly and disfiguring. Rust on iron and tarnish on 

 silver have little appeal. Even copper and bronze is unattractive 

 when covered with chloride-bearing corrosion crusts. Thick corro- 

 sion can obscure fine details of decoration and modeling and can com- 

 pletely hide gilding, inlays, and inscriptions. Sometimes corrosion 

 crusts act as cementing agents for clay and earthy accretions. The 

 term patina does not properly apply to corrosion crusts and earthy 

 overlayers that conceal and disfigure. 



Because ugly corrosion crusts so often detract from the appearance 

 of metal artifacts, there is frequently an urge to clean them away 



