Dear Anne,
My wife is a conservator and I posed your question to her.  She said it is 
most likely plasticizers in the plastic migrating to the surface.  
Although you may be able to remove it with alcohol the problem is likely 
to re-occur.  Just the nature of some old unstable plastics- built in 
inherent vice.
Sincerely,
David Ryan
Registrar
Colorado Springs Pioneers Museum
www.cspm.org

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