Temperature- and humidity-induced changes in alum-treated wood: a qualitative X-ray diffraction study

Abstract

Alum-treated wooden artefacts suffer from extreme deterioration, and the stability of these objects and the salts they contain to variations in climate conditions is an important issue. Responses of potassium alum (KAl(SO4)2·12H2O), ammonium alum (NH4Al(SO4)2·12H2O), potassium bisulfate (KHSO4) and alum-treated wood to changing temperature and relative humidity (RH) were therefore investigated at the crystalline level using powder X-ray diffraction (XRD). The XRD analysis showed changes in the crystal structures of alum salts that suggest some degree of dehydration starting at 40 °C at 15% RH. Rehydration of such dehydrated alum in Oseberg wood samples was still ongoing after 2 years. However, alum salts on their own and in alum-treated wood generally appeared to be stable to most of the changing climate conditions. Conversely, the minor potassium bisulfate component found in some samples was very sensitive to all changes in RH and temperature, and may even cause changes to the alum component under some conditions. This may be related to the efflorescence of K9H7(SO4)8·H2O crystals seen on some fragments after long-term exposure to high RH. These findings can help to refine RH and temperature limits for alum-treated objects, and also demonstrate the utility of temperature- and RH-controlled XRD for qualitative monitoring of climate-induced changes in such salt-impregnated materials.