Marker for PEG

Karin Abelskamp <k.abelskamp [at] archeologie__nl> writes

>Currently, ArchaeoSpecialists is carrying out experiments in
>impregnating wood with PEG. To determine the extent of impregnation,
>we would like to "mark" the PEG (4000) solution to make the PEG
>easily recognized within the object after treatment (after
>cross-section). I can imagine that this has been done before. Does
>anyone have experience with the use of markers for this purpose?
>What type did you use and why?

Much of the research that has been done concerning the use of PEG to
give dimensional stability to (archaeological) wood has been
published in the postprints from the Wet Organic Archaeological
Materials (WOAM) conferences.  Three papers involving a staining
technique to determine the extent of PEG impregnation of wood cell
walls developed by Gregory Young at CCI have been published in these
postprints for conferences in the years 1981, 1987 and 1998.  The
name of the conference has changed over the years but following are
the three references, which are the same ones to which Cliff Cook
referred on a previous instance of the Cons DistList:

    1.
    Bilz, M.. T. Grant and G.S. Young, (1998), "Treating Waterlogged
    Basketry: a Study of Polyethylene Glycol Penetration into the
    Inner Bark of Western Red Cedar" in Proceedings of the 7th
    ICOM-CC  Working Group on Wet Organic Archaeological Materials
    Conference, Grenoble, 1999, pp. 249-253.

    2.
    Young, G.S. and Ritchie Sims, (1987), "Microscopical
    Determination of Polyethylene Glycol in Treated Wood--The Effect
    of Distribution on Dimensional Stabilization" in Conservation of
    Wet Wood and Metal: Proceedings of the ICOM Conservation Working
    Groups on Wet Organic Archaeological Materials and Metals,
    Freemantle, Western Australian Museum, 1989, pp. 109-140.

    3.
    Young, G.S. and I.N.M. Wainwright, (1981), "Polyethylene Glycol
    Treatments for Waterlogged Wood at the Cell Level" in
    Proceedings of the ICOM Waterlogged Wood Working Group
    Conference, Ottawa, ICOM, 1982, pp. 107-116.

Before describing the staining technique, I should point out two
things related to your original question.  First, the staining
technique is applied to thin cross-sections of wood following PEG
treatment, not to the PEG itself prior to treatment.  Second, PEG
4000 (referred to as PEG 3350 in North America) is too large of a
molecule to penetrate the wood cell walls (see reference #2 page 115
PEG 3350).  Therefore, you should find that the PEG 4000 bulks the
cell lumen but does not penetrate the cell walls.  A lower molecular
weight PEG, such as PEG 400, is normally used to penetrate the cell
walls to give dimensional stability to the wood.

Note that the stain preparation used in reference #3 was improved by
the time of the later references.  The following description is of
the latest technique.

A saturated solution of cobalt thiocyanate in dry ethyl ether was
made. When staining was to be done, a portion of this stock solution
was brought up to saturation with phenol (about 85% w/v).  The
cobalt thiocyanate gave a blue colour to the solutions.  Thin
transverse sections were cut from PEG treated wood that had been
freeze-dried as is normal following a standard PEG treatment.  These
sections were immersed in the cobalt thiocyanate / dry ethyl ether /
phenol solution.  After one and a half hours, the sections were
dipped in ether (to remove excess stain), then in xylene (to be
compatible with the mounting medium) and then were mounted on
microscope slides with Permount (Fisher Scientific mounting medium).
The prepared slides were left overnight with a small weight (40 g)
on the cover slips.  Photomicrography was done the next day.

Under the microscope, the normal colour of PEG treated cell walls is
normally white to pale yellow in the absence of the stain.  Thin
sections that have been immersed in the cobalt thiocyanate stain but
have not been PEG treated also are white to pale yellow since the
stain is held by complexing only with the PEG.  Only the thin
sections that both have been stained with cobalt thiocyanate and
have PEG retained in them show regions of blue.

A better way to show the extent to which the PEG has penetrated the
cells is to view the thin sections in fluorescence mode rather than
brightfield mode on the microscope.  Not only does the cobalt
thiocyanate complex with the PEG but it also quenches the
fluorescence of the lignin when the PEG stain complex resides in the
cell wall.  An incident illumination at about 400nm gives emitted
radiation at about 550nm (yellow/green) from the fluorescing lignin.

If PEG is present, the cobalt thiocyanate held by the PEG quenches
the lignin fluorescence to give a dark green/black appearance in
those areas.  Low molecular weight PEG (such as PEG 400) can
penetrate the cell walls via the cell lumen in intact wood samples.
With wood treated with low molecular weight PEG, you should see
progressively more quenching of the lignin fluorescence in the cell
walls, spreading from the inside edge of S3 (if there is one), then
throughout the thicker S2 layer as you extend the time that the wood
is immersed in the PEG solution.  With wood treated with only high
molecular weight PEG (such as the PEG 4000 that you mention) which
cannot penetrate the cell walls, the fluorescence should remain in
the cell walls regardless of the length of time that the wood is
immersed in the PEG solution.  However, in the brightfield mode, you
should be able to see the high molecular weight PEG bulking the
lumen after drying.

Malcolm Bilz
Conservation Scientist
Canadian Conservation Institute
1030 Innes Road
Ottawa, Ontario K1A 0M5
Canada
613-998-3721
Fax: 613-998-4721


                                  ***
                  Conservation DistList Instance 20:41
                 Distributed: Sunday, February 25, 2007
                       Message Id: cdl-20-41-005
                                  ***