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A response to the questions on PEG posted on the Conservation DistList by Dr Hamilton on the 13th August 2000. (Conservation DistList Instance: 14:13 Monday, August 14, 2000)
Date: 13 Aug 2000
From: Donny Hamilton <dlhamilton@tamu.edu>
Subject: Stability of waterlogged wood treated with PEGOver the past five years, the conservation staff of the Nautical Archaeology Program and the Conservation Research Laboratory at Texas A&M University has been conducting research on the conservation of waterlogged wood with polyethylene glycol (PEG) and it has resulted in a number of questions that I want to throw out to the Conservation DistList.
- Can anyone cite an instance where a ship's hull (or major structural timber) in North America has been satisfactorily conserved with PEG?
- Can anyone cite an instance where a ship's hull in Europe has been satisfactorily conserved with PEG?
- What have been the major problems of the waterlogged hull conservation projects.
Satisfactorily conserved means there was no major warping/shrinkage, and there are few, if any problems of the PEG puddling and dripping as it migrates out of the cells.
- Once conserved, how crucial to the conservation treatment is a well controlled environment.
This leads to another set of questions:
- What are the significant problems that have come up over the years for waterlogged wood (large or small) that have been conserved with PEG--especially those artifacts that have been treated for 10 to 20 years.
- Is PEG an effective treatment for waterlogged wood that has long term stability, and remains reversible or is it used because it is effective in the short run and is relatively cheap when compared to alternative treatments.
- To what degree does the PEG (a polymer) cross link with the cellulose, sugars and starches in the cell wall and is thus impossible to remove, thus not really reversible.
Additional perspectives are being sought.
Donny L. Hamilton
Director, Conservation Research Laboratory
Head, Nautical Archaeology Program
College Station, Texas 77843
979-845-6355
Fax: 979-845-6399
Dr Hamilton's questions are very challenging and they demand answers. The questions imply criticism of the use of PEG which is in my view, the only technique which has withstood the test of time, consistently performed well in research studies and has shown incredible versatility in the treatment of waterlogged wood. Most of the answers to these questions are documented in the literature.
There are some aspects of the questions which are not totally clear - for instance the first two questions which ask whether satisfactory treatments for hulls have been obtained in North America or Europe. This is narrow in scope. Hulls have been treated throughout the world by experts so why not also include Asia, or Australia, Africa or the Middle East?
Dr. Hamilton's questions
1. Can anyone cite an instance where a ship's hull (or major structural timber) in North America has been satisfactorily conserved with PEG?
Parks Canada in Ottawa has treated many large timbers with a variety of PEG treatments, and one complete Hull which was done as a unit. The complete hull of a "War of 1812" British Gun Boat known as the "Mallorytown wreck" treated and was treated in 1967 by total immersion in a PEG solution which was retroactively determined to have been 20% by weight in PEG 1000 and 12.5% by weight in PEG 1450. The Mallorytown wreck, meets Hamilton's criteria for satisfaction, and has for many years been on exhibition to the public.
2. Can anyone cite an instance where a ship's hull in Europe has been satisfactorily conserved with PEG?
In Europe it is much easier to come up with examples of PEG treated shipwrecks - and that is simply because that funding has been in place and supporting research has been sustained. In North America research efforts have been much more limited and sporadic.
The Wasa is the largest hull structure which has been stabilized. There was a large controversy in Sweden surrounding the Wasa's treatment because PEG penetration proved so difficult. I do not know whether the result can be considered as "satisfactory" because like most of the conserved wrecks objective assessments have not been published.
Smaller and more fragmentary than the Wasa are the Viking ships at Roskilde. These were treated with PEG many years ago by Brorson Christensen for the National Museum of Denmark. The results look beautiful, but without knowing the objectives of those who treated the wrecks it would be presumptuous to decide whether the results are satisfactory.
Perhaps the most interesting development in recent years in the treatment of hulls has been with Per Hoffmann's two-step PEG method. The Bremen Cog, a mediaeval merchantman was treated as a unit with this technique and is now after many years of impregnation, on display in the Schiffahrtsmuseum in Bremerhaven. No final evaluation has been published.
Whether any of the above projects are "satisfactory" is an interesting question. Satisfactory to whom one might ask? Although one can establish criteria (see Hamilton's below), judgments tend to be subjective. Since all treatment imposes a certain degree of alteration to wood, satisfaction is really determined by the extent to which such changes can be tolerated. One interesting point is that the more perfectly preserved and the more complex are the remains the greater is the challenge to conservation and the more difficult it is to achieve a satisfactory result because the criteria become more stringent. Recent intact wrecks are in that sense much more difficult to conserve to a satisfactory level than older more fragmentary remains.
Hamilton's definition was:
"Satisfactorily conserved means there was no major warping/shrinkage, and there are few, if any problems of the PEG puddling and dripping as it migrates out of the cells"
In summary satisfactory conservation depends very much on the features of the wreck that are deemed important to preserve and, as mentioned above, the degree to which any change can be tolerated. Such features could be tool marks on the surface, fragments of paint or tar. It could be that non-wooden structural elements (fastenings of various kinds) should retain structural strength.
At minimum overall dimensional stability should be achieved, superficial collapse should be minimal or non-existent, colour should be natural, and the timbers should be biologically stable and not subject to fungal deterioration or insect attack. Losses or degradation of impregnant should not occur and chemical change of mineral components of the wood should not occur.
3. What have been the major problems of the waterlogged hull conservation projects?
The major problems of waterlogged hull conservation projects have been:
There have been a number of documented problems in the literature that illustrate these points:
Lack of planning and funding: the Cairo in the Southern USA was allowed to languish in New Orleans for many years before rescue and conservation of what was left by the US Parks service, H. Thomas Mc Grath, Jr., "The Eventual Preservation and Stabilization of the USS Cairo", The International Journal of Nautical Archaeology and Underwater Exploration (1981) 10, 2 pp. 79-94.
Not enough research : The Machault in Canada, though ultimately successfully conserved with PEG, initially encountered problems when an untested sand drying process was tried out. This was in the early days of conservation in Canada, V. Jenssen and Lorne Murdock, "Review of the Conservation of the Machault Ships timbers (1973-1981)", Proceedings of the ICOM Committee for Conservation Working Group on Waterlogged Wood Conference, Ottawa, 1981, D.W. Grattan ed., J. C. McCawley ed. special discussions. (Ottawa: ICOM Waterlogged Wood Working Group, 1981)
Lack of planning and funding: The Alvin Clark was raised from Lake Michigan by amateurs in 1967 intact and still floated, but through absence of funding became "A tragic disaster, it is now tired, broken and decrepit" Peter H. Spectre in Woodenboat magazine May/June 1983 No. 52.
Not enough research : in the late 1970s in Marseilles a Roman hull was treated with an insufficiently tested atmospheric pressure freeze-drying procedure. This resulted in extensive surface collapse of the wood, which might have been avoided had there been a pre-treatment with PEG. Daniel Drocourt and Myriame Morel-Deledalle, "Marseille, Lyophilisation a pression Atmospherique D'une Epave de Bateau Romain", Proceedings of the Second ICOM Waterlogged Wood Working Group Conference, Grenoble 1984, (Grenoble: ICOM Waterlogged Wood Working Group, 1984) pp. 219-239.
One might add that, so far, the technical problems posed by PEG have been addressed by careful research. The outstanding research efforts of the National Museum of Denmark, the Wasa project in Sweden, the Batavia project in Australia, the CTBGE in Grenoble, the Bremen Cog project in Bremerhaven and the Mary Rose project in the U.K. are positive examples as to what can be achieved.
For example, difficulties with penetration have now largely been dealt with through Per Hoffman's two step procedure.
Per Hoffmann, "The conservation of the Bremen Cog: between the steps" York:, Proceedings of the 6th ICOM Working Group on wet organic archaeological materials conference, 9-13 September 1996, pp.: 527-543.
4. Once conserved, how crucial to the conservation treatment is a well controlled environment?
There is data in the literature on the impact of environment on waterlogged wood see Grattan's study of 1981 and Nishiura and Imazu's later work for example
D.W. Grattan, "A Practical Comparative Study of Treatments for Waterlogged Wood, Part II, The Effect of Humidity on Treated Wood", (Ottawa: ICOM Waterlogged Wood Working Group, 1981), pp. 243-252.
Taduteru Nishiura and Setsuo Imazu, "Experimental study of the dimensional change of highly degraded waterloged wood according to ambient humidity after preservation treatement"Bremerhaven: ICOM Waterlogged Wood Working Group, 1990) pp. 107-118.
The conclusion was that for PEG treated wood, If the relative humidity is below about 60% and the impregnation of PEG has been accomplished so that excess PEG is not present - treated wood is remarkably insensitive to variations in relative humidity.
5. What are the significant problems that have come up over the years for waterlogged wood (large or small) that have been conserved with PEG--especially those artifacts that have been treated for 10 to 20 years.
The largest problem with PEG treated wood has been that sometimes archaeological wood contains unstable sulphides which can oxidise to sulphates and eventually to sulphuric acid. This leads to destruction of the timbers. The same problem is well known for fossils composed of pyrites. Wood containing sulphides has been treated with ammonia successfully. This seems to achieve deacidification and stabilization, however, some of the wood associated with the Roskilde ships has been affected. It is not known whether PEG exacerbates the problem, and the scope of the problem globally, has not been established .
6. Is PEG an effective treatment for waterlogged wood that has long term stability, and remains reversible or is it used because it is effective in the short run and is relatively cheap when compared to alternative treatments?
This question is confusing so I have interpreted it as (1) what is the long term stability of PEG treated wood? and (2) what is the relative cost of PEG treatment.
We at CCI studied the long term stability PEG treated objects, and it seems that the objects are faring well in the long term. In addition there is an international study which has been running since 1987 with a large number of wood samples stored at the University of Tromsø in Norway. The samples are regularly and objectively evaluated every few years.
See: R Saeterhaug "ICOM's international comparative wood conservation project: an overview" Konserveringsetik. Nordisk Konservatorforbunds XI. Kongres, Reykjavik 20.-24. Juni 1988 pp. 166-173 - AATA Number: 26-676
David Grattan, "International Comparative Study of the Treatment of Waterlogged Wood", Newsletter of the Wetland Archaeological Research Project, No. 4 (April 1988) pp. 11-14.
Furthermore, there is accumulating evidence that PEG ages more slowly when impregnated in wood than it does as a pure substance. In our research at CCI we never found evidence of PEG degradation in timbers treated up to 30 years ago. Our assumption is that the lignin in wood acts as an anti-oxidant. Lignin has been shown to be an effective anti-oxidant.
The thrust of the questions suggest that the authors are principally concerned with moisture absorption and excessive hygroscopicity -leading to wood that tend to drip in humid conditions. CCI research showed that this is not a problem unless the relative humidity exceeds 80% - although as mentioned above we prefer a more conservative upper limit of relative humidity of 60%. In addition research has showed the importance of balancing the concentration of impregnation solutions with the concentration of PEG.
Malcolm Bilz, David W. Grattan, J. Clifford McCawley, Leslie Macmillen, Lesley Dean, and Clifford Cook "A Study of the Thermal breakdown of PEG" Proceedings of the Fifth ICOM Wet Organic Archaeological Materials Working Group Conference, Maine (1993) pp. 167-198.
J.A. Schmidt, C.S. Rye and Norayr Gurnagul, "Lignin inhibits the autoxidative degradation of cellulose" Polymer degradation and Stability 49 (1995) p.291.
Clifford Cook and David W. Grattan, "A Method of Calculating the Concentration of PEG for Freeze-Drying Waterlogged Wood", Proceedings of the Fourth ICOM Wet Organic Archaeological Materials Working Group Conference, ed. Per Hoffmann, Bremerhaven 1990, (Bremerhaven: ICOM Wet Organic Archaeological Working Group, 1987) pp 239-252.
Relative Cost is an almost impossible question to answer - it would perhaps make sense if one could compare different treatments with similar end results. This is not possible , because there are rarely treatments with which comparisons can be made. The reality is that there are trade offs. Slow drying might be cheaper than PEG as it does not involve the cost of purchasing impregnant - but we know that in most instances the result will be significantly worse. Costs are based on materials, equipment, labour and basic services such as buildings, heating lighting etc. hence the cost of the impregnant may only be a fraction of the total cost.
7. To what degree does the PEG (a polymer) cross link with the cellulose, sugars and starches in the cell wall and is thus impossible to remove, thus not really reversible?
At CCI we conducted research to measure the long term solubility of PEG. We tested this in the PEG from the 1967 Mallorytown treatment by Parks (mentioned above) , and the Ozette site wood, and found that as far as we can determine complete solubility , complete extractability. There is no evidence for cross linking with PEG - chemically it is unlikely. It is known that PEG does not cross-link as it ages. Recent debate has centered around whether the mechanism of degradation takes place by loss of monomer units by end-of-chain unzipping, or by random chain scission. At present evidence favours the unzipping mechanism.
Malcolm Bilz, David W. Grattan, J. Clifford McCawley, Leslie Macmillen, Lesley Dean, and Clifford Cook "A Study of the Thermal breakdown of PEG" Proceedings of the Fifth ICOM Wet Organic Archaeological Materials Working Group Conference, Maine (1993) pp. 167-198.
Vincent Cooke, Deborah Cooke and David W. Grattan, "Reversing old PEG treatments of objects from the Ozette site" Proceedings of the Fifth ICOM Wet Organic Archaeological Materials Working Group Conference, Maine (1993) pp. 97-110.