I found a long post online by someone who sounded as if he knew what he
was talking about.
http://www.diychatroom.com/f4/removi...acement-30292/
(you have to scroll to the bottom)
He said apply a mixture of linseed oil and chlorine bleach with the
frame lying flat, for a day or so. Apparently the linseed oil will
penetrate into the putty and soften it, while the chlorine attacks the
calcite in the putty (which is made from high-calcite lime and linseed
oil) and does something. Anyone tried this?