Offgassing from plywood shelving

In reply to Claudius Schettini's problem of a science library with
new veneered plywood shelving, Sean Harrison suggests that a clear
latex sealant will trap gases inside.  Although there are references
in the conservation literature to the use of clear acrylic varnish
as a vapour barrier, I think recent literature indicates that no
transparent coating is going to provide a really good vapour
barrier.  The clear acrylic varnish might slow down emissions but
won't stop them.  Would anyone else like to comment on this?

In "Sealing of MDF to prevent corrosive emissions", The Conservator,
no. 22. United Kingdom Institute for Conservation of Historic and
Artistic Works, London, 1998, p.49-56, David Thickett describes the
disappointing results of liquid coatings tested as vapour barriers.
With one exception, liquid coatings on an MDF substrate did not
significantly decrease the corrosion of lead coupons in accelerated
corrosion tests.  Mr. Thickett did find that many of the liquid
coatings significantly reduced formaldehyde emissions, so perhaps
the clear acrylic varnish would reduce the problem of formaldehyde
off-gasing from the plywood.  But the liquid coatings in Mr.
Thickett's study did next to nothing to keep in organic acid
vapours.  And this is something that should be
remembered--formaldehyde is not the only harmful component coming
out of the plywood.

I'm not sure how worrisome this should be to Claudius Schettini.  No
one knows what levels of acids in the environment will have a
harmful effect on paper (Jean Tetrault of CCI and Cecily Grzywacz of
the Getty are each working on this question).  The situation in the
science library involves open shelving, not a closed environment
where vapours will build up.  The problem is not as worrisome, then,
as it would be if the plywood were used inside exhibition cases.
Perhaps low levels of acids won't damage the library materials.
Perhaps they will, but will take a long time to do so.

I hope a few more people will contribute their opinions on this
subject.

Karen Potje
Head, Conservation/Preservation
Canadian Centre for Architecture
1920 Baile Street
Montreal, Quebec H3H 2S6
514-939-7000 x 1236
Fax: 514-939-7020

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                  Conservation DistList Instance 13:7
                   Distributed: Monday, July 12, 1999
                        Message Id: cdl-13-7-001
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