[kmas55]

I have three '75 sets, one in three card panels, that have that inherent curl. I'd like to protect and store in flat sleeves; however, I attempted this once and had a bad outcome. About two years ago I slipped a curled '75 Ryan directly into a semi-ridged thinking it would "relax" it flat over time, but only to have it spontaneously crack after being fine for months. Has anyone found a way best to un-curl these and store while decreasing the risk of cracking? Thanks for any help.