Heat-set adhesive

At some point in the murky conservation past many of the paper
objects in the collection I'm working on have been adhered all over
to low quality board.  These objects are prints, watercolours and
posters from the 18th-20th centuries.  The poor things are now
incredibly flat, and the board is incredibly acidic.

The board doesn't present a great problem as it delaminates easily
and can be peeled and pared off.

The adhesive used appears to heat-set and is a thin, uniform,
transparent film, shiny and slightly yellow in colour. The previous
conservator believed it had been applied in the 60s or 70s, and was
called 'Ademco' film. There is no sign of it having caused staining
of the objects so far, but I am worried about how it may age, and
many of the objects would benefit from wet-treatment.  If only I
could get it off.....

So far I have tried (on a piece that wasn't adhered properly so
could be peeled off):

    *   heat; spatula and blower, up to 120 deg. C

    *   solvents; IMS, acetone, 2-propanol, amyl acetate, toluene,
        xylene, Stoddard's, 1-1-1 trichloroethane.

None of these has had any discernible effect on the adhesive film;
no sign of softening or becoming tacky, never mind releasing from
the paper from the backing board that it is adhered to.

Has anyone got any useful advise they could offer?  There are other
solvents in our store, such as Chloroform, that I haven't tried as
the health and safety risks of using them in any quantity seem
prohibitive. I also haven't tried mixes of solvents, as with none of
the ones tried so far having any effect I'm assuming that nor will a
mixture of any of them--is this a correct assumption?

I'd also appreciate it if anyone has any information on 'Ademco'
film, as I haven't managed to find any references to it.

Rebecca Cameron
Paper Conservator
National Museums of Scotland

                                  ***
                  Conservation DistList Instance 13:9
                  Distributed: Thursday, July 22, 1999
                        Message Id: cdl-13-9-019
                                  ***