First I would like to thank all who responded to my previous reply about
Bynes efflorescence, either through the list or directly to me. I'm
overwhelmed and will use the next many days to follow all the traces
I've now got.
Then I've seen the postings about foams etc., and I would like to throw
in my two cents of knowledge here:
Now, *in general* a housing material must fulfill the following
chemically criteria's to be suitable for use with museum objects:
1) It must not emit any corrosive/acid substances
2) It should be pH neutral, for some types of objects preferable pH > 7
(*)
3) It must be chemically stable and inert
(*) Actually one cannot talk about the pH of a solid/dry material, often
the pH value instead refers to a extract of the material in water...
For plastic materials it can be added that plastics containing
plasticizes or unknown surface films ("slip agents") are also not
suitable.
Besides this there is off course also some physical considerations, such
as the housing should not be abrasive or have sharp edges which can harm
the objects....
"Good plastics":
Concerning this, the plastic foam which is most often used to house or
support museum objects is polyethylene, like Ethafoam or Plastazote (two
different brands).
For small or flat items which can be housed in sleeves or folders,
polyester or polypropylene is used. Some brands are Mylar and Melinex
(both polyester sheets).
Boxes made of polystyrene or polycarbonate are often used, but one can
off course also choose archival quality cardboard boxes.
"Bad plastics":
The worst plastics are the ones which emits large amounts of acetic
gases, embrittles, or bleed plasticizers.
Foams like polyurethane are bad, sleeves or folders made of PVC are
(very) bad. Avoid also to store calcareous objects close to other
objects made of cellulose esters - especially cellulose acetate, which
emits large amounts of acetic acid. One such source can be an archive
with photographic negatives.
I can be difficult to find out which kind of plastics one already has
got in the collection, although some kinds are easily identified than
others. E.g.. is PVC easy to identify: Place a bit of the test specimen
on the end of a copper wire, and place the wire over a gas flame. If the
test material contains chlorides, the flame will turn green. Be sure to
glow the wire to red-hot before you do the test in order to remove
potential dirt, salts, etc., which can give misleading results. A green
flame is a positive test for PVC.
My recommendation is that one should only buy housing materials from
well established conservation material suppliers. Among others this
could be Conservation Resources http://www.conservationresources.com ,
or Conservation By Design http://www.conservation-by-design.co.uk
- please note that I have nothing to do with any of these companies,
this is not an ad. There will be other companies which can offer the
same materials.
At the Website of Indoor Air Quality in Museums and Archives you will
find the paper "Display Materials: The Good, The Bad, and The Ugly" by
Jean Tetreault from the Canadian Conservation Institute. This is a very
good paper which presents the basic do's and don'ts:
http://hjem.get2net.dk/ryhl/good-bad-ugly.htm
Finally, just yesterday I got a brand new book published by the National
Museums of Scotland: "Plastics, Collecting and Preserving" (eds. Anita
Quye and Colin Williamson), ISBN 1-901663-12-4. Even though its about
plastic objects and not sea shells, it has got an interesting chapter by
Scott Williams: "Plastics for Storage and Display".
- Morten Ryhl-Svendsen
--
Konservatorskolen (The School of Conservation)
Det Kongelige Danske Kunstakademi
Esplanaden 34, DK-1263 Copenhagen K, Denmark
Fax: (+45) 33 74 47 77
The Website of Indoor Air Quality in Museums and Archives:
http://hjem.get2net.dk/ryhl/
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