Abstract
Controlled air drying has been applied, with varying degrees of success, to several large and high profile conservation projects around the world. While the technique is popular, due to its low cost and accessibility, surprisingly little information exists on how best to apply it. Typically, waterlogged material is held in a high humidity environment, which is lowered incrementally down to a level at which it will be stored or displayed. However, the high natural variability of waterlogged archaeological woods often leads to inconsistent or unreliable results. The very high levels of dimensional change encountered also makes it essential to pre-treat material with bulking and/or consolidating agents such as polyethylene glycol (PEG). If controlled air drying is to be applied as a more reliable and consistent conservation technique, more information is required on the drying behaviour of waterlogged archaeological woods and how this is affected by different drying conditions.This study looked at the drying behaviour of oak, poplar and pine timber samples from the Tudor warship Mary Rose, as well as more heavily degraded oak from the Flag Fen wet terrestrial archaeological site. Initially, the physical and microstructural characteristics of the entire sample group were investigated. Degree of degradation was assessed using moisture content data and its relationship with volumetric dimensional change measured after oven drying samples. Light and scanning electron microscopy were used to investigate microstructural characteristics and the level and type of microbiological decay present.
High performance climatic test chambers were then used to examine the behaviour of untreated timber samples under a range of both constant drying conditions and complete drying schedules using stepped incremental changes. The rate and nature of moisture removal was investigated under each set of conditions, as well as the development and final amount of dimensional change for each sample type. Similar tests were then carried out for samples pre-treated with a wide range of polyethylene glycol solutions of different molecular weight (PEG 200, 600, 1500 and 4000) and concentration (10%, 30%, 50% and 70% w/v). Results were used to suggest the most favourable combination of air drying conditions and PEG pre-treatment for each timber type investigated and ultimately propose a potential air drying schedule for the PEG treated hull of the Mary Rose.
Degradation patterns differed for each of the timber types investigated, though all showed both highly degraded and well preserved regions. Microscope studies showed this to be due to the presence or absence of past microbiological attack. Relationships between moisture content and oven dry dimensional change were reasonably good for all species where samples were taken from microbiologically degraded regions. Correlations were highest between moisture content and volumetric dimensional change for poplar and pine samples, though for oak samples, longitudinal dimensional changes gave the strongest relationship.
Drying experiments revealed that untreated Mary Rose oak samples were best stabilised using short drying schedules, initiated at lower relative humidity and lower temperature, and utilising few stepped increments. One of the most favourable combinations of conditions was drying directly to 55% RH at 20°C, the conditions typically used to store dry archaeological wood. No advantage was gained from using the longer drying schedules, initiated at high humidity, that are typical of those employed in the seasoning of green timber. Mary Rose poplar and pine samples were also best dried at lower temperatures, but otherwise showed no clear trends according to drying conditions. Again, drying directly to 55% RH at 20°C, produced results comparable with all other schedules at this temperature. Even for oak samples however, differences in dimensional change between drying schedules was very small and even the best results were completely inadequate in terms of a stabilising conservation treatment. The use of consolidating and/or bulking agents in conjunction with this method is clearly essential.
Trials using different PEG treatments showed that concentrations of at least 30% w/v are required to stabilise very well preserved material (Mary Rose pine), though 50% w/v minimum concentrations were necessary for Mary Rose oak and poplar and for Flag Fen oak. PEG 200 and 600 performed best for Mary Rose oak and pine, whereas PEG 4000 gave far better anti-shrink efficiency values for highly degraded Mary Rose poplar and Flag Fen oak samples. Given that most timbers used as source material for samples, showed both well preserved and highly degraded regions, combinations of 50% PEG 200 or 600, followed by 50% PEG 4000 were recommended.
| Date of Award | 2005 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Rod Eaton (Supervisor) & Moss Steve (Supervisor) |