I believe Thymol Crystals were stopped being used for mold in books because in the long term it was thought to damage the paper of manuscripts ; not that it wasn't ineffective in killing mold. I have a Nikon F 135mm lens that had about 20 mm of mold in a corner when I bought it - in the interior , I cant get to it . I put it in a sealed box with said crystals and it has not altered one bit in 15 years !