Photographic sleeves

There doesn't seem to be much information out there on the properties of
polypropylene. I'm sure there is someone out there (hey, Doug
Nishimura!) who I'm sure know a lot more than I do.
 
But, yes, there is a difference. What I do know--or have heard--
follows.
 
1.  Both polypropylene and Mylar (generically known as PET (polyethylene
teraphthalate) or polyester) currently are acceptable as housing
materials for photographs by ANSI (American National Standards
Institute).
 
2.  There are basically two types of polypropylene.  One is stretched in
a single direction (oriented) and is the softer slightly whitish stuff
(but not as white as polyethylene). The other is stretched in two
directions (bi-axially oriented). This stuff is somewhat more
transparent; has a more brittle feel (more like Mylar); and is reputed
to be somewhat more stable.
 
3.  All three of the "archival" plastics (polypropylene, polyethylene
and polyester) can be purchased coated or uncoated. You need to specify
the uncoated stuff for "archival" uses.
 
4.   Both polypropylene and polyethylene have less tendency to build up
a static charge that can be a problem with friable surfaces and flaking
emulsion.
 
5.  Polypropylene is a better choice (more stable) than polyethylene,
but the best choice as far as stability is polyester.
 
5.  Problems with whitish deposits on the inside of some polypropylene
slide pages have been reported. These were probably the softer stuff.
 
6.  Mylar, since it is used as film base, has been more thoroughly
tested for stability than polypropylene. It has been confirmed in a
number of tests to be exceptionally stable. On the other hand, it has
the static charge problem. It also scratches easily and the corners are
sharp.
 
7.  You may not need to use plastic sleeves or pages for your particular
photographs at all. Check into album formats that have a mylar cover
over corner mounted prints. They do make polypropylene pages (rather
than sleeves) for standard print formats. I'm not sure what kind of
photos were talking about here. It usually comes down to a compromise
between price and ease of access.
 
8.  If you really want Mylar, you should shop around. Also bulk
purchases split with a friend or two can save a lot of money.
 
9.  Get those prints out of that old sticky album!  :-)
 
10.  Plastic photograph housings as microclimate: There are pros and
cons to putting images in plastic. On the con side is the possibility of
dreaded condensation inside the sleeve if you reach the dewpoint.
Another problem is that the material you put inside the sleeve may
benefit from a more "breathable" enclosure (nitrate films, acetate
films, etc.). On the pro side is that the plastic can create a
microclimate inside the sleeve that buffers rapid changes in relative
humidity. I have noticed this phenomenon when working at an institution
with no climate control on a very dry "Santa Ana" condition day. When
prints were in the plastic sleeves, they were relatively flat. When
removed they rapidly desiccated and curled. Quite a stress on the
object. The other materials were glass plate negatives that had cracked
and lifting emulsions. When removed for reading densities, the cracks
opened and there was the awful cracking sound of desiccated emulsion
peeling back on its own! We stopped reading lifting plates on days below
50% relative humidity!
 
This is probably more than you wanted to know! Hope some of it helps.
 
Loren C. Pigniolo
Photographic Preservation Specialist
Photographic Preservation Services

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                  Conservation DistList Instance 6:60
                   Distributed: Monday, May 17, 1993
                        Message Id: cdl-6-60-001
                                  ***