Bob, thanks, great article, certainly the author is "da man" for that topic. The only question I'd have is that in reading between the lines, he's only concerned with excavated objects, or excavated objects on display inside museums. I'd humbly submit that the cannons I'm talking about are all non-excavated and are in a similar environment, cycling between wet and dry, and usually exposed to sunlight.
However, much of his article probably directly applicable to our problem. The things that grabbed my attention include:
Moisture is bad, oxygen is bad, even light is bad. The "dirty white" outlines I see, often in the grooves left by engraving, are tin or oxides of it left after the copper has reacted. The fact that light plays a role backs up my observations that the cannons in shady areas at Ft. McNair were in better shape than those in the direct sunlight.
I also observed that the two 1759 French 4-pounders that had been painted black did not show any evidence of bronze disease. Why? The complete coat of black paint deprives the cannon of all of the "bad actors" needed to support the chemical reaction, namely moisture, oxygen, and light.
The reason none of the many bronze cannons on display at Watervliet Arsenal seem to have active bronze disease is that all but one are kept inside.
Here's my summary of the article you linked, which I will post on one other discussion I have running on this topic:
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Excerpts from article that may be important to conservation of US trophy cannon:
BRONZE DISEASE: A REVIEW OF SOME CHEMICAL PROBLEMS AND THE ROLE OF RELATIVE HUMIDITY
by DAVID A. SCOTT
published in: Journal of American Institute for Conservation, JAIC 1990, Volume 29, Number 2, Article 7 (pp. 193 to 206)
ABSTRACT—A general review of some of the theories proposed to account for the process of “bronze disease” is presented from both the historical and chemical points of view. The corrosion product of most serious concern, cuprous chloride, and its inter-relationship with some of the other important corrosion products of copper alloys, such as the copper trihydroxychlorides, is reviewed. The critical RH for the transformation of cuprous chloride is discussed and suggestions are made concerning both the storage conditions for bronzes and the variety of conditions under which cuprous chloride can occur in excavated bronze.
A. This article summarizes the salient information published to date on the subject of cuprous chloride and bronze disease.
B. Bronze disease may be defined as the process of interaction of chloride-containing species within the bronze patina with moisture and air, often accompanied by corrosion of the copper alloy itself, a process which has been more or less understood for the last 100 years. The products of the reaction are light green, powdery, voluminous basic chlorides of copper, which disrupt the surface and may disfigure the object. Several corrosion processes of copper are also enhanced by visible light. Cuprous chloride, for example, is a light-sensitive material and must be kept in the dark, preferably in a vacuum desiccator to prevent any chemical change.
C. Berthelot's essential conclusion—that the recurrence is due to a cyclical reaction involving both oxygen and moisture—is indeed correct. More is known about the process today, but we still do not know all the details of the corrosion chemistry involved.
D. It is clear, however, that there is no reason per se to reduce the RH of stored bronzes that are not showing signs of active corrosion to levels below 39%. Storage at an RH between 42% and 46% should provide adequate conditions for most objects. The humidity should not be allowed to rise above 55% because the reactions of cuprous chloride become very rapid as the RH rises and will not necessarily stop as soon as the RH is lowered again.
(author) DAVID A. SCOTT, B.Sc., B.A., Ph.D., C.Chem. MRSC, FIIC, has been head of Museum Services of the Scientific Program at the Getty Conservation Institute since 1987. He has been a lecturer in conservation at the Institute of Archaeology, University of London, Department of Archaeological Conservation and Materials Science, and, since 1984, an editor of Studies in Conservation. He was named a fellow of the International Institute for Conservation in 1989. His principal research interests are the analysis and technical study of ancient metallic objects and their corrosion products, the conservation of metallic artifacts, the study of Chumash Indian rock art and the archaeometallurgy of ancient South America, particularly Colombia and Ecuador. Address: The J. Paul Getty Museum, P.O. Box 2112, Santa Monica, Calif. 90406.
complete article URL:
http://aic.stanford.edu/jaic/articles/jaic29-02-007_1.html
