4 BRONZE DISEASE AND RELATIVE HUMIDITY

SOME OF the aspects of water layers formed on copper surfaces exposed to the atmosphere have been reviewed by Graedel (1987). Several monolayers of water can be adsorbed onto the surface of pure copper at moderate or high humidities. At an RH of 60% and a temperature of 20�C, for example, the number of water monolayers on a metal surface is estimated to be 15, while at 90% RH the number of layers is about 27. Current views in surface chemistry suggest that when the number of monolayers rises above three, the layer possesses the chemical properties of bulk water.

The kinds of surface interaction phenomena discussed here are quite different than those found on a pure metal, and they involve corrosion products of copper and tin as well as impurities that may exist within the alloy. Structural defects in crystalline phases and grain boundary effects may well play a role, too. Clearly, the potential exists for some reactions to continue independent of the critical RH for the transformation of cuprous chloride and for variations in stability to be apparent with different objects.

What is the best model for the RH values for storage that we currently have? It has been proposed as a result of empirical observations in museum collections that unstable bronze artifacts must be stored at an RH of less than 39% if the reactions of cuprous chloride are to be stifled. The situation is complex for a variety of reasons. First, the critical RH value for cuprous chloride in air in isolation from a metallic substrate is higher than 46% RH. An experiment was conducted by the author for two years in which compressed tablets of cuprous chloride (which becomes waxy when consolidated by compression in an IR press), powdered cuprous chloride, and copper powder mixtures were kept in a humidity cabinet over a saturated salt solution providing a humidity that only fluctuated between 42% and 46% RH during the period of the experiment. No observable change in either the pure cuprous chloride or the powder mixture occurred. Samples of the copper powder were mounted in resin for microscopic examination and polished for metallographic study. No change could be observed at the interface between the cuprous chloride particles and the copper substrate, showing that if any reaction had occurred it was quite negligible: the copper survived more or less intact.

When the same experiment was conducted at an RH of 70%, reaction was rapid; within a day the compressed cuprous chloride tablet exfoliated and burst as it changed to one of the copper trihydroxychlorides. Samples of polished copper sessile beads covered with crystals of cuprous chloride to which droplets of water are added periodically develop a waxy crust of cuprous chloride adjacent to the metal surface, with a covering principally composed of paratacamite. If the waxy layer that adheres to the copper is removed with a scalpel, clear indications are found that the copper surface has been attacked. The surface is dull and etched by the reaction with cuprous chloride under these conditions of wetting and drying. In relation to copper, cuprous chloride has a relative molar volume (RMV) of about 3.36, while the copper trihydroxychlorides have RMVs of about 3.99. A considerable force for expansion exists as a result of this transformation; the relative molar volume increase is even more marked compared with cuprite, which has an RMV of 1.67.

An unknown factor in coming to a conclusion concerning critical RHs for objects is the potential role that could be played by the existence of chloro-complexes of copper within the corrosion crust. Their effect, coupled with the uncertainty as to the presence of adsorbed water or internally trapped water due to microcapillarity, will be to produce continued activity until the object has either reacted with the available water or has dried out before the available cuprous chloride is exhausted.

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.

Although the reactions reviewed here suggest a cyclical process in the absence of further contamination the process will stop or will slow to low rates of reaction when the cuprous chloride has been transformed. The sound metal remnants comprising part of the object (if any metallic component was extant upon excavation) can suffer attack during the process, but attack of the remaining metal itself will not be appreciable, especially if the RH is kept below 46%; the primary problem is the cuprous chloride that is transforming, together with the potential effects of the alteration of cuprite within the patina.