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whose glues to choose?
My, my, there are suddenly lots of questions about adhesives in my
mailbox. Thanks to everyone who sent me anecdotal accounts for my
presentation in October. I've sent the draft off and will keep everyone
posted on its progress.
The response to a questionnaire I sent around was very interesting. Most
people use what they were told to use when they were very young and
impressionable; only a few have really researched the problems with
adhesives and consolidants. Most were very interested in finding out more
but did not know where to look for good information on the aging
characteristics of adhesives. Since I have glue on the brain (so to
speak) after teaching a class on This Very Subject to a TERRIFIC group, I
thought I'd answer a few questions and make a couple of suggestions.
Shelton's Adhesive Rules:
1. Just Because You Can Buy It Doesn't Mean You Should Use It
A lot of off-the-shelf preparations are very inferior for paleo work.
They are not made for this work; their viscosity and set time can be
difficult to control; and you may never know what is in the proprietary
mixture. Whenever possible, buy your desired polymer in solid granule
form and mix it to your specifications in small batches. There are no
weird solvents or fillers this way. Butvar and Vinac (polyvinyl butyral
and polyvinyl ACETATE) are both very useful this way. It's more
expensive; consider getting several people or labs together to share a
purchase. (More on this below)
2. Just Because It Was Described In a Publication in 1964 Doesn't Mean
You Should Use It
You wouldn't BELIEVE how many outdated references there are in the world.
If you work really, really hard, you can probably find some paper
somewhere that justifies using just about any nasty substance. (It may be
mimeographed and distributed only to a junior-high class, but it is a
paper...right?) Any references more than 5 years old should be looked at
with deep suspicion. Conservators and archaeologists have been pushing
this issue, and, as a result, there is a lot of terrific, tested,
scientifically valid information out there. Unfortunately, it has been
very slow in arriving in the paleo community, where people are still
recommending using Glyptal even though the problems with cellulose
nitrate adhesives have been thoroughly documented for over 20 years.
Avoid all cellulose nitrate adhesives and consolidants; their behavior
over time is just awful, and many of us have whiled away our golden years
picking at old, crystallized, failed adhesives, which brings me to the
3. You Will Look Different in 20 Years: So Will Your Adhesive
Adhesives age. Many of them do not age gracefully. Too many of us look
at a nice new adhesive join, put the fossil in a drawer, and never look
at it again. We think our job is done. It isn't. Some adhesives
cross-link and shrink drastically over time. If the adhesive is stronger
than the fossil/matrix, a shrinking adhesive can pull the fossil itself
apart. Others yellow and darken and may stain the fossil surface
irretrievably. The worst agers are any "white" or "wood" glue (polyvinyl
ALCOHOL or polyvinyl acetate emulsion) and cellulose nitrates. Avoid 'em.
They do not do the job over time that you expect them to do. Read the
label if you buy an off-the-shelf product. The preparation of Duco
changed but no one noticed until Bill Simpson pointed out that the stuff
that had worked without shrinking on setting is now shrinking enough to
break tooth roots on pin mounts.
4. Someone, Somewhere, May Have To Undo Your Work For Good Reason
You could be wrong. Or someone may need to study a fragment in isolation.
Or your adhesive may have failed and allowed the joins to sag and warp.
Or you may have selected a consolidant that is chemically incompatible
with a molding agent. Whatever. You should document exactly what you used
and when so that the process can be undone if needed. You should select
an adhesive that can be practically if not ideally reversed *WITHOUT*
soaking the fossil itself. Also, bear in mind that you do *NOT* make the
break un-happen by using adhesives or consolidants. You still have a
broken specimen. It is now a chemically different broken specimen with a
polymeric material holding the pieces in the best possible approximation
of its appearance before the break....but that break is still there. The
specimen is still vulnerable. Reactive, shrinking, or staining adhesives
affect the specimen in that vulnerable area. Your job is to select an
adhesive that will hold the pieces in places with as little dimensional
change and loss of stability over time as possible, and to write down
what you used.
Specific questions I have received lately: The jury is still out on
cyanoacrylates (the "superglues"). In small amounts, like pin mounts,
they seem to be holding up well. They may not be stable over time,
though, and they are proprietary compounds, and you should NEVER use them
without the resources to loosen the bond IMMEDIATELY in the case of
accidental skin-skin bonding. White glues are emphatically
disrecommended. Cellulose nitrates such as Glyptal, ditto (though there
is some interesting evidence from Johannesburg on Glyptal...stay tuned).
Duco is no longer cellulose nitrate, but the new compound is still
disrecommended. "Glue" is a term reserved for animal-protein-based
compounds, by the way, such as hide glue or rabbit glue. Avoid 'em; no
boiling down any rabbits to fix T. rexes. They form a weak bond and are
very unstable. Shellac should be made illegal RIGHT NOW. It is AWFUL over
time. Paraloid/Acryloid (acrylic co-polymer) is very effective to date.
Epoxies work, though they mnay be exothermic as they set, and often the
bond is stronger than the fossil if the fossil is small. Don't use an
elephant gun on a mosquito. Epoxies are also very irreversible.
Comment 1: I think we need a wall chart to summarize what is known about
adhesives/consolidants commonly in use in paleo. Anyone interested in
having such a thing if I can pull it together? Comments?
Comment 2: I think that some people are interested in bulk purchases of
sounder adhesives. Any comments or suggestions?
That's it for now. More as I think of it. Questions always welcomed here
at Adhesives R Us.
Director, Collections Care and Conservation
| San Diego Natural History Museum |
| P. O. Box 1390 |
| San Diego, California 92112 USA |
| phone (619) 232-3821; FAX (619) 232-0248 |
| email LIBSDNHM@CLASS.ORG |