Reducing Decay During Environmental Change:
The Use Of High And Low Technology Moisture Monitoring Methods In Preserving The Timbers Of The Frigate UnicornDuring Dry-Docking.
Derek C. R. Sinclair, G. Colin Stove*, A. Odell*, Paul Durrant and John W. Palfreyman
Scottish Institute for Wood Technology
University of Abertay Dundee
Bell Street, Dundee, DD1 1HG
Int.- 44 - 1382 308930 / 308657 / 308675
The importance of effective moisture control for the long-term conservation of historic wooden ships is discussed in the context of the Frigate Unicorn- one of the oldest surviving intact ships. Two methods for ensuring effective deep moisture monitoring are detailed. Method 1 uses high technology sub-surface precision radar scanning systems especially adapted for the analysis of moisture content in timber structures. Method 2 employs traditional technology in the construction of a system of artificial moisture sinks. A long-term monitoring strategy using the two methods is discussed.
The Frigate Unicornwas built in 1824 at the Royal Dockyard at Chatham as part of the Royal Navy's post-Napoleonic War replacement programme. The vessel was placed 'in ordinary' after completion and has remained so since. The protective presence of a roof probably accounts for the vessel containing approximately 90 % of it's original timbers. It is presently preserved afloat under the care of the UnicornPreservation Society in Victoria Dock, Dundee, Scotland.
The original long-term conservation strategy for the Unicornenvisaged its restoration to a fully masted and rigged floating frigate. However, once the extent of original timbers remaining on the vessel was determined and it was realised that the ship's 'ordinary' condition was its original condition, the roof itself being an important historical artefact, this strategy was radically altered. The present strategy entails total conservation of the entire un-masted vessel in a permanently covered and enclosed dry-dock. This policy will ensure that the timbers of the Unicornreceive long-term protection from the environmental conditions which encourage decay processes.
However, removal of the vessel from it's present situation afloat will incur dangers to the structure in the short-term. Specifically, as the structure 'dries-out' the moisture content of many of the lower hull timbers are likely to achieve levels which are highly favourable to the proliferation of decay fungi. As the drying process may take several years this 'short-term' decay problem could result in very significant damage to timbers which at the moment are largely sound.
In their role as timber preservation consultants to the UnicornPreservation Society the Scottish Institute for Wood Technology (SIWT) is examining techniques and procedures designed to identify that moist region in the drying timbers, between the dry surface and waterlogged interior, where suitable oxygen and moisture conditions for decay processes are found. This paper reports the findings of a pilot project using radar technology which successfully identified specific timber layers, throughout the 1.4 metres depth of the oak keelson and keel of the Unicorn, on the basis of their moisture status. The paper discusses this high-tech non-destructive method and a complementary low-tech minimal destructive method for deep moisture measurement in large dimension timber artefacts. It is proposed that both methods will be used after dry-docking the Unicornto allow accurate identification of 'high decay risk' areas thereby facilitating timely intervention to pre-empt or minimise decay by use of preservative chemicals of low mammalian toxicity or improved drying procedures.
The Dry-Docking Of The UnicornAnd The Importance Of A Moisture Monitoring Strategy To Minimise Decay Of Drying Timbers.
The decision to permanently dry-dock the Unicornor any other historic vessel is one which will create heated debate between conservators the world over. It can be argued that a ship which is not afloat ceases to be a ship. Alternatively, a ship which is afloat but will never sail due to its historical importance or inherent structural weakness, may also not be regarded as a ship in the truest sense of the word. These arguments will no doubt continue as long as historic ships are conserved. However, the long-term maintenance of these structures, which were originally designed and constructed to have a maximum useful lifespan of perhaps 20 years at sea, is an exceptional undertaking and exceptional measures are required to achieve it unless continuous repair of the structure is regarded as an acceptable strategy.
Regardless of the subtleties of these arguments, permanent covered dry-docking is a conservation strategy which has much to recommend it to workers primarily concerned with long-term preservation of the timbers of conserved and/or replicated historically important ships. The strategy removes the ship from the influence of seawater and freshwater ingress, that provide the moist timber conditions which encourage the activities of decay causing fungi and which represent the greatest biological threat to the long-term preservation of any timber vessel. The influence of moisture on the decay process is profound. Maintaining the moisture content of wood below 20 % (w/w) renders it virtually immune to microbial degradation (Scheffer, 1973). Similarly, microbial degradation normally occurs only very slowly in timbers which are completely waterlogged, due to poor oxygen availability. Timbers with moisture contents which fall into the lower end of this range especially (approximately 30-50 %), are at great risk of microbial degradation as these conditions provide an ideal balance of moisture and oxygen availability for decay causing fungi to flourish. In a floating timber ship these conditions will usually be found at the 'wind and waterline' ie. between timbers of high moisture content in the lower hull and the dryer upper works.
A strategy for monitoring moisture in important structural timbers of the frigate Unicornis dictated by changes in environmental conditions within the timbers as a consequence of the dry-docking strategy. The major environmental changes will be to the moisture status and ambient temperature of the ship's timbers; the former being reduced and the latter most probably being increased. This will encourage an increase in the activity of wood-destroying fungi as timber conditions become more conducive to fungal growth (waterlogging down and temperature up) until environmental stresses result in proliferation of fungal fruiting bodies (waterlogging down and temperature up) followed by inactivation of the fungi (waterlogging down and temperature up). Therefore, dry-docking may place the lower hull timbers of a vessel under short-term threat of accelerated decay until the moisture contents of the timbers are reduced to levels of 10 - 15 %. These moisture contents are consistent with timbers found in enclosed dry locations and will not support active colonisation and growth of decay causing fungi. Though the moisture contents of the majority of the lower hull timbers of the Unicornhave never been measured, moisture contents of wood cores taken from the keelson indicated levels ranging from 20.10 to 170.77 % of timber dry weight (Palfreyman et al, 1994, White et al, 1996). It is therefore seems probable that many of the timbers in the hold area, at present, are either waterlogged or at moisture contents which are compatible with decay.
Though the oak timbers of the Unicornare very durable and resistant to fungal attack, populations of decay fungi are present in the keelson (Palfreyman et al, 1994, White et al, 1996). In addition, various drillings of this timber element using the Sibert detection drill decay (unpublished observations, SIWT) and testing using sonic technology (unpublished observations, Albright, M) have shown that these fungi have caused significant deterioration. Deterioration could be slowed by the application of diffusible preservative treatments such as concentrated boron solutions which can slow deterioration and can give 'long-term' protection to oak timbers in-situ with minimal effects on dimensional stability (McCutcheon et al, 1992, 1993). However, diffusible preservatives are leachable by nature and it is likely that 'long-term' in timber preservation terms falls short of 'long-term' in historic ship preservation terms. A strategy of continued application of presently 'environmentally friendly' preservatives to replace those lost by leaching is seen by some as an acceptable long-term option. That may be the case, but as the environmental debate continues a cautious approach to such a strategy must be taken.
A possible solution for the Unicornis dry-docking under permanent cover with associated environmental control to ensure drying of the timbers. During and after the docking process studies designed to identify those remaining largely sound timbers deemed to be 'at risk' of decay, by virtue of their dimensions, painted exteriors and/or position in the vessel allowing them to retain higher moisture contents over extended periods will be necessary. These studies in conjunction with isolation and identification of decay causing basidiomycete fungi and minimal invasive decay detection measurements, will provide early indications of particular areas which are under threat of decay and which may require targeted diffusible preservative applications or local changes to forced ventilation schedules employed notably in the hold area, after dry-docking.
As many of the timbers within the susceptible hold area will have moisture contents which are not measurable by standard conductivity methods (suitable only for timbers with moisture contents up to 100 %) other non-standard methods of moisture measurement must be employed. These methods must not involve significant damage to the fabric of the vessel and must be able to provide indications of deep moisture content to levels greater than 100 %. These constraints have led to the development of sub-surface precision radar as a non-destructive tool for deep moisture measurement.
The Use Of Sub-Surface Precision Radar As A Method For Deep Non-Destructive Moisture Measurement In Timbers Of The Unicorn: A Pilot Study.
All radar (radio detection and ranging) is based on the propagation of electromagnetic radiation and its detection. The principal of operation of subsurface precision radar (SPR) is based on the reflection of radio waves from the boundaries between zones of different dielectric constant within the material under study. The dielectric constant is defined in very simple terms as the ratio of epsilon/epsilon0, where epsilon is a measure of the permittivity or the ease with which a charge can cross a material and epsilon0 is a measure of the degree to which a given charge is repelled by an identical charge across a distance in a vacuum. Therefore, providing the radar signal can penetrate a given material the layers of different dielectric constant can be detected and if the properties of each layer are known the extent of each layer can be established.
The use of radar technology in archaeological investigations, shallow geologic surveys and the detection of underground features and defects is well known. However, the use of this technology for other applications, such as the investigation of in-situ construction materials, has been limited in the past by the size and portability of equipment, particularly the antennae, and by the requirement for highly specialised analysis of the signals received from the substrate under examination. These limitations have been studied by the authors of this paper and many of the results and details of the equipment developed and methods for its use are restricted due to patent applications. However, the primary objectives of the authors, to produce novel compact radar antennae linked to user friendly computer software has been realised and initial results, presented here, indicate the success of the approach.
An early prototype of the system using some standard equipment was employed in a short pilot study in June 1996 to examine moisture within the oak keelson and keel timbers of the Unicorn. The point in the hold area chosen for the test was to the rear of the mainmast. At this point the depth of timber is approximately 1.4 m. In order to calibrate the reflected signals to the dielectric constant of water, the transmitting antenna was positioned immediately above a 0.045 m deep layer of deionised water. This provided a computed dielectric constant of 81.06 equivalent to 100 %. Electromagnetic impulses were directed into the timbers with the transmitting and receiving antennae in a fixed 'side by side' scanning position. This was followed by a WARR (Wide Angle Reflection and Refraction image) scan where the transmitting antenna remained fixed and the receiving antenna was gradually moved away from the transmitter to receive reflected and refracted impulses at specific measured datum points taped on to the keelson. Precise measurement of datum points is crucial to any radar survey in order to optimise triangulation of range distance and ensure accuracy of scanning measurements. The initial 'side by side' scan was carried out to identify layers within the material under study and the WARR scan was carried out to provide additional data on the same timber section recorded from different distances and angles to support or challenge the initial findings. Analysis of the data from both scans indicated excellent agreement and these findings are presented in table 1.
Table 1. Identified layers, layer thicknesses and dielectric constants within the oak keelson and keel of the frigate Unicorn.
Layer Depth (m)
-0.045 - 0.000
0.000 - 0.312
0.312 - 0.383
0.383 - 0.537
0.537 - 0.727
0.727 - 0.800
0.800 - 1.059
1.059 - 1.217
1.217 - 1.331
NB* - Layer 1 represents the deionised water solution used to calibrate the equipment.
The overall depth figure of 1.331 m recorded for the keel and keelson by the radar survey (table 1) accorded very well with measurements on naval architects drawings of the vessel. The scans also precisely identified the depth at which the keelson and keel meet and indicated 8 distinct layers of differing moisture status within the timbers (table 1). Computed moisture values are not presented, as the radar calibration procedure used for this pilot survey did not employ the preparation of a range of oak timbers at a range of moisture contents and salinity values. However, the data do indicate the ability of the system to detect changes in moisture content throughout large dimension timber structures and the development of systems capable of measuring actual moisture content is progressing.
A Strategy For Monitoring The Moisture Status Of Timbers During The Initial Drying Phase Of Dry-Docking.
As indicated earlier, assessments of moisture loss from the timbers below the 'waterline' of the dry-docked Unicornwill be hampered by timber moisture contents being initially too high (> 100 % w/w) for standard conductivity measurements. In addition, the timber depths at which moisture must be recorded (approximately 0.6 - 1.4 m), without destructive interference with the artefact, are not consistent with the length of standard conductivity probes. However, based on the authors studies of moisture detection using subsurface precision radar it will be possible to carry out such measurements using this technology.
The use of subsurface precision radar for deep moisture measurement.
Long term moisture monitoring strategy of the Unicornusing radar technology will involve two phases. Phase 1 will employ six fixed scanning locations established in strategic locations in the hold area of the vessel (eg. the keelson). Each scanning location will support a surface mounted holding device to precisely locate one of two portable scanning platforms equipped with fully calibrated compact antennae. One of these platforms will be designed to suit the physical profile of the inside of the hull and the other will be suited for traversing the keelson. These scanning locations will be used to carry out a limited moisture survey of the vessel in order to establish appropriate locations for more widespread application of the technology in phase 2. These data will be supplemented by data recorded in locations in other parts of the vessel using completely portable scanning antennae. Phase 2 will involve the establishment of further fixed scanning locations as required and the design and construction of a number of custom built scanning platforms to incorporate fully developed antennae. At this stage a full moisture survey of the vessel will be carried out and this will be repeated to completely map the changing moisture status of timbers throughout the vessel at regular intervals. To monitor the effects of the drying regimes associated with dry-docking in detail, the first full moisture survey of the vessel will be completed immediately prior to removing the Unicornfrom the water.
To provide additional in-situ calibration of the radar antennae it is proposed that a monitoring method based on the deep insertion of artificial moisture sinks is used simultaneously.
Artificial moisture sinks for deep moisture calibration of subsurface precision radar antennae.
A number of small diameter boreholes (approximately 0.5 cm) will be drilled to various depths (eg. 25, 50 and 75 cm) into important structural members in the hold at several of the fixed radar scanning locations. Each borehole will receive a rigid wire insert of similar length to the borehole. Prior to insertion, the borehole tip of the wire will be coiled to hold a cylindrical plug of wood composite material (5 x 0.3 cm (L x D)) After insertion, the top of each borehole will be sealed with silicon sealant and it's position marked. If the borehole fills with water, leading to complete waterlogging of the plug this will indicate that the surrounding oak timbers at this depth are also waterlogged and subject to only very slow fungal decay. However, in the absence of water ingress this procedure is intended to allow the inserted plug to equilibrate with the moisture content of the surrounding oak timber at the base of each borehole. The plugs will be removed, and fresh plugs inserted immediately before radar scans are carried out. Each removed plug will be weighed and dried at 105 oC to constant weight to determine moisture content as a percentage of dry weight.
To ensure that plug moisture contents are representative of those in surrounding oak timbers, and therefore appropriate for calibration of the radar antennae, a pre-trial involving the preparation of oak timbers 'wetted' to a range of moisture contents will be undertaken. These timbers will be bored to receive plugs of various composites for equilibration and comparative gravimetric moisture analysis. The results of this test will indicate what composite material is most suitable for use, what timescale for equilibration will be required and what correction factor, if any, is required to account for the density difference between oak and the composite of choice.
The simultaneous use of these two quite different methods for deep moisture measurement will provide a sufficient amount of accurate moisture data from the dry-docked vessel to identify those specific timbers and areas where moisture contents are approaching or have achieved levels highly conducive to the growth of decay causing fungi. Such information will be invaluable in allowing timely preparation of chemical preservation protocols and/or amended drying regimes to be directed at susceptible timbers and areas.
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