The presence of sulfuric acid in alum-conserved wood – Origin and consequences

Abstract

Abstract Dealing with the consequences of historical conservation treatments is an issue that conservators and conservation scientists will meet with increasing frequency as conservation materials naturally age and interact with materials the objects are made from. One example is the use of alum salts to treat waterlogged archaeological wooden objects during the 100-year period between 1850–1950. This now-obsolete method was widely used in Scandinavia, but also worldwide. Today many objects treated with alum are damaged and are actively deteriorating. The wood is highly acidic (pH 1-2.5), it is brittle with little remaining structural integrity. It is thought that, in addition to decay processes possibly initiated by absorbed metal ions, the presence of sulfuric acid, generated during treatment, plays a central role in the active deterioration process observed. To understand the alum treatment better, it was applied to recent wood, freshly excavated archaeological wood and pure cellulose paper. It was found that the samples became more acidic after treatment. The same material types were also treated with solutions of sulfuric acid, pH 0-3, to compare the effects of acid of known concentrations. The present contribution discusses the immediate effects of high acidity on these samples, due to the recent treatment by alum salts and by sulfuric acid. The newly treated samples were compared with alum-treated wood from the Oseberg find 100 years ago. It also includes preliminary investigations on the behaviour of alum salt during heating, in the solid state as well as in solution.