SFIV1967
Venerated Member
I'd think any warm temperature is bad for celluloid and it would only shrink even more. I think you have to try to cold press it flat between two flat wooden boards and leave it there for a couple of days or better weeks. I might be wrong, you would have to ask somebody who understands old celluloid, I just understood that starting at 60 deg C (140 F) celluloid can already start such decomposing process.I wondered if very, very gentle heat between two metal plates over a long period of time would flatten it with a minimum of shrinkage.
By the way, I got such an overlay from the same seller I think, I keep it between two wooden boards for almost 2 years now as I had no time to do the restoration yet it is intended for. My one was at least flat when it arrived.
Ralf