Abstract
From the mid-1800s to the late 1960s, conservation by alum salts (KAl(SO4)2·12H2O—potassium aluminium sulphate), using various recipes, was a common method to prevent shrinkage and to strengthen waterlogged archaeological wooden objects. This method was mainly used in Scandinavia. The alum method appears to have also been applied to highly degraded archaeological waterlogged wood in other countries, for example in the U.S and Germany. Today, many of the archaeological wooden objects treated with alum show extreme deterioration and very low pH, which are attributed to the effects of the alum-treatment. This study investigated the extent of the current levels of chemical degradation in wooden objects conserved with alum salts at different points in time (1880s, 1930s and 1905–13) in order to understand their current condition and whether extent of degradation was in any way related to time of treatment, in an attempt to understand the rate of degradation. It was also an opportunity to compare the chemical state of preservation of alum-treated wood from different collections, as only the Oseberg collection has been intensively studied in this way up until now. Samples from historical wooden objects from the following collections were investigated and compared: the Dejbjerg collection (National Museum of Denmark in Copenhagen); the Oseberg collection (Museum of Cultural History, Oslo, Norway); the Glimmingehus collection (Swedish History Museum, Sweden). Analyses of lignocellulosic polymers and of inorganic compounds were undertaken to evaluate the chemical preservation of the wooden objects. The investigations were performed using a multi-analytical approach which consisted of: pH measurements, analytical pyrolysis (Py-GC/MS), X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDS). It was possible to link the extent of degradation with time, on a general level but we found a great variability in the state of preservation of the wood also within the same collection. It is clear, however that alum-treated wood is more degraded than archaeological wood not treated with alum.
Highlights
From the mid-1800s until the 1950s, and in some cases up to the 1960s, aluminium potassium sulphate was routinely used for the conservation of waterlogged wooden objects of an archaeological and historical value.In the 1960s it was eventually replaced with polyethylene glycol (PEG), as PEG-treated wood was less brittle
The lowest pH values were observed in the Norwegian (N-187 and N-250F) and Swedish (S-383) samples, all alum-treated samples gave lower pH values than that measured for sound maple
If the object is coated with linseed oil, surface pH readings likely do not dissolve the acidic species in the wood, but rather only alum salts concentrated on the surface
Summary
From the mid-1800s until the 1950s, and in some cases up to the 1960s, aluminium potassium sulphate (alum) was routinely used for the conservation of waterlogged wooden objects of an archaeological and historical value.In the 1960s it was eventually replaced with polyethylene glycol (PEG), as PEG-treated wood was less brittle. From the mid-1800s until the 1950s, and in some cases up to the 1960s, aluminium potassium sulphate (alum) was routinely used for the conservation of waterlogged wooden objects of an archaeological and historical value. The alum conservation method was simultaneously developed in Denmark and Germany in the 1850s and in 1861 both methods were published [1, 2]. It was one of the first methods used for the conservation of highly deteriorated waterlogged wood artefacts. The alum treatment method consisted of immersing waterlogged wooden fragments into a concentrated solution of alum at 90 °C from 2 to 24 h or more [3].
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