| Abstract: | SUMMARIES. To inhibit the corrosion of lead objects and lead in high-lead bronzes, particularly in the presence of organic emanations from wood, paint, etc., and/or high humidity, is a difficult conservation problem. A method to prevent corrosion of lead in copper objects was therefore developed using benzotriazole (BTA). The most appropriate treatment method for the conservation of the objects was to immerse them in neutral BTA solution, for which a 0.5% (w/v) concentration of BTA was enough. For bigger objects, three or four coats of 1.0% solution applied by brushing were equally effective. The method was used successfully for the conservation of some ancient copper coins and bronze sculptures containing high amounts of lead; it can also be used to inhibit corrosion of lead objects.
CONCLUSIONS. Preferential corrosion of lead segregated as globules on the surface of high-lead copper objects may give rise to unsightly pitting. Stabilization of such objects may be difficult in museums due to the continued corrosion of lead in the presence of organic emanations from wood, paint, etc., and/or high humidity. Alcoholic or aqueous solutions of BTA have been widely used with success for the conservation of copper objects. Neutral BTA solution reacts with lead also, and the use of BTA solution to inhibit corrosion of lead in copper objects containing a high percentage of lead appears most appropriate. Although BTA solution neutralized by solid calcium carbonate reacted with lead even in very low concentrations (0.1%), 0.5% to 1.0% BTA solution was suitable for the conservation of objects. Best results were obtained by immersion of the objects in the BTA solution, for which a 0.5% concentration of BTA was enough; however, three to four coats of 1.0% solution applied on the objects by brushing were equally effective. The method developed can also be used to inhibit corrosion of lead objects, such as lead coins, lead came used in stained glass windows, etc. Emanations from oakwood sawdust and high humidity did not affect lead coupons treated by immersion in neutral 0.5%. BTA solution. However, a preservative coating on the objects may also be given after treatment for added advantage. It was found that neutral BTA solution reacts with lead to form a crystalline compound, which is not soluble in common solvents and is thermally stable up to 250°C. XPS studies showed that lead-BTA complex is formed as a film on top of the lead plates immersed in neutral BTA solution. The effect of neutral BTA solution in inhibiting corrosion of lead metal is due to the formation of a crystalline and polymeric lead-BTA complex on its surface. |
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