Stability of waterlogged wood treated with PEG

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A response to the questions on PEG posted in Conservation DistList
Instance: 14:13 Monday, August 14, 2000 by Dr Hamilton on the 13th
August 2000.

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. A complete hull of a "War of 1812" British Gun
Boat known as the "Mallorytown wreck" 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, 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 The Wasa is the largest  hull structure which has been
stabilized. There was a large controversy in Sweden surrounding the
treatment because PEG penetration proved difficult

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
wonderful.

Perhaps the most interesting development  in recent years in the
treatment of hulls has been 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.

Whether any of the above projects are "satisfactory" is an
interesting question. Satisfactory to whom one might ask? Although
one can establish criteria, judgements 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.

>    3.  What have been the major problems of the waterlogged hull
>        conservation projects.

The major problems of waterlogged hull conservation projects have
principally been non-technical in nature:

    1.  Lack of commitment to conservation in the project and
        archaeological plans

    2.  Obtaining sufficient money as well as recognition for
        conservation.

    3.  A lack of good supporting scientific research

    4.  Lack of knowledge of the state of the art of conservation.

There have been a number of documented problems in the literature
that illustrate these points:

The Cairo in the Southern USA was allowed to languish in New Orleans
for many years before rescue and conservation by the US Parks
service. See 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.

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". See Peter H.
Spectre in Woodenboat magazine May/June 1983 No. 52.

In the late 1970s in Marseilles a Roman hull was treated with an
atmospheric pressure freeze-drying procedure. There was 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.

>    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 see  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., and also Taduteru Nishiura and Setsuo Imazu,
"Experimental study of the dimensional change of highly degraded
waterlogged wood according to ambient humidity after preservation
treatment" Bremerhaven: ICOM Waterlogged Wood Working Group, 1990)
pp. 107-118.

The conclusion was that, If the relative humidity is below about 60%
and the impregnation of PEG has been accomplished so that excess PEG
is not present--PEG treated wood is remarkably insensitive to
variations in relative humidity. (Grade of PEG is a factor)

>    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. In Australia, 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 Tromso 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 and
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. Research has shown  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. See 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. And 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. The
reality is that there are trade- offs in the equation of cost versus
result.  Slow drying might be cheaper than PEG as it does not
involve the cost of purchasing impregnant--but 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. It seemed that all
the PEG dissolved out of the wood.  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.  See 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. And: 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.

David Grattan,  Canadian Conservation Institute.


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                  Conservation DistList Instance 14:16
                 Distributed: Sunday, September 3, 2000
                       Message Id: cdl-14-16-006
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