Natural rubber boat

I am currently working on an inflatable natural rubber boat from
1845 that is composed of something called Macintosh Fabric which is
layers of cotton canvas which have been stuck together with naphtha
and natural rubber to form a waterproof fabric. Unfortunately due to
cross linking, the rubber has become vulcanized and is now rock
hard. I am looking for some way to soften the rubber not only for
aesthetic reasons, but also because there are objects which are
stuck inside the boat that we are unable to get out because the
material is so rigid. i.e. oars. They will have to be cut in half to
remove them, something which we don't want to do. Ammonia vapors
have been tried, as well as heat, but neither has had a desirable
effect. Any suggestions would be appreciated,

Heather Place
Conservation Technician
Manitoba Museum of Man and Nature
190 Rupert Ave.
Winnipeg, Manitoba R3B 0N2
204-988-0694

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                  Conservation DistList Instance 13:13
                 Distributed: Tuesday, August 10, 1999
                       Message Id: cdl-13-13-007
                                  ***