Air humidity

Do archaeological wood artefacts require certain climatic conditions to remain stable? To guarantee long-term preservation of treated wood the humidity of the surrounding air must be regulated.

It is crucial to monitor and control relative air humidity to conserve and stabilize archaeological wood. This parameter plays an important role in the conservation treatment and the storage of wooden artefacts. For instance, the efficiency of the consolidation agents, used to support the internal structure of wood, depends on the relative humidity (RH) of the surrounding air. Some consolidants used in conservation treatments are water-soluble compounds with hygroscopic properties that can attract water from the surrounding air into the wooden structure when relative humidity increases. If humidity of the surrounding air is unstable, constant adsorption and desorption of moisture can occur, causing dramatic physical and chemical changes inside the wood. In general, fluctuating climatic conditions can soften consolidants supporting the wood, stimulate microorganism’s growth, cause shrinking and cracking of the wood and cause the appearance of salt outbreaks on the wood surface. This is mainly due to the fact that moisture promotes ion migration inside the wood. For this reason, relative humidity in museums and store rooms must be stabilized after impregnation of wood with water-soluble consolidants.

Photo from Mikkel Christensen, Museum of Cultural History/UiO
Treated wooden wagon in a storage case with controlled relative humidity, exhibited at the Viking Ship Museum, Norway. Photo: Museum of Cultural History, UiO/Mikkel Christensen.

 

Consolidation agents

In certain conservation treatments, consolidants must penetrate into the cell wall of degraded wood in order to replace the adsorbed water, preserving its central part and preventing shrinkage. This method requires compounds with low molecular mass since they are more water-soluble; in the ArCo project, the most relevant consolidation agents used for this purpose are alum and low molecular mass PEG. However, these characteristics can make compounds like PEG more hygroscopic and consequently the wooden structure more sensitive to climate fluctuations. Thus, it becomes important to evaluate the behaviour of alum and PEG towards moisture, since wooden artefacts treated with these compounds are frequently found in Scandinavian museums.

 

Salt precipitates

Archaeological wood can contain metallic salts that resulted from certain conservation treatments or migration of metal ions into the wood, either from metal corrosion or from soil/water during burial. These unstable salts are deposited within the wood and contain sulphur compounds that, in the presence of oxygen and moisture can produce iron sulphates and sulphuric acid.  The main problem arises when fluctuating relative air humidity hydrates consolidants like PEG and causes these salts and acids to dissolve and migrate to the surface of wood, forming acidic precipitates that can degrade cellulose and lignin. Although many salts precipitate on the surface of wood when RH fluctuates, some hydrated sulphur salts migrate and re-crystalize inside the wood, conferring lack of stability to the wooden structure and causing cracks due to an increase in volume. This process is enhanced by the presence of iron ions in wood since they can trigger sulphur oxidation reactions and PEG degradation, increasing the amount of sulphate inside the wood and the hygroscopicity.

 

Climate chamber

Photo from National Museum of Denmark
Climate chamber containing archaeological wood samples from the ArCo project. Photo: Natmus.

To understand the behaviour of treated wooden artefacts towards fluctuating relative air humidity, archaeological wood can be placed into a climate chamber and be subjected to changing RH along time; alternate cycles of temperature and RH are established to reproduce museum’s climatic conditions. This provides information about the most resistant consolidation agents and the climate conditions at which wooden artefacts remain stable. The formation of salt precipitates and migration of ions, which are related to the moisture content in wood, can be monitored through simple observation together with conductivity measurements. Moisture sorption isotherms can be recorded to analyse the hygroscopicity of wooden artefacts, i.e. its water content at a certain relative humidity at constant temperature. In general, the behaviour of archaeological wood in controlled ageing chambers gives an insight into its climate demands during storage, providing valuable information for museums.

By Maria Laura Amberger
Published Oct. 6, 2015 3:41 PM - Last modified Oct. 29, 2015 1:49 PM