Foxing in stamps: long-term effects of sterilization and treatment with NaCl

Summary A single exposure to gamma irradiation can prevent the appearance and stop the development of the foxing phenomenon in new or used postage stamps maintained in sealed polyethylene bags. No visible damage to the paper or to colours was observed seven years after treatment. Tyndelization of the stamps in an open autoclave is also effective but only for used stamps. Autoclave sterilization commonly decreases the phenomenon but does not completely prevent it. UV light has a destructive effect on colours of stamp and increases the foxing after short exposure times. There is a direct correlation between degree of foxing and phenol content. Soaking the stamps in 5–10 % NaCl solutions increases foxing, but with 30 % NaCl development of foxing stopped. These treatments cause no visible damage to stamps.