White efflorescences on lead objects were identified as a basic lead carbonate. It is supposed that this product is formed by the reaction of formaldehyde from impregnated wood. The formaldehyde oxidizes to formic acid and reacts with the lead to yield lead formate. This then transforms to lead carbonate by the action of carbon dioxide from the air. To preserve the corroded lead objects they were exposed to hydrogen sulfide, which transforms the lead carbonate to lead sulfide.