The gum on 1 has a high sulphur content and often was washed off the stamp to avoid the (almost) inevitable damage that suphur

The gum on 1 has a high sulphur content and often was washed off the stamp to avoid the (almost) inevitable damage that suphuric gum produces over time. (See the note in Scott by Gerrmany B68 for their comments about sulphuric gum.)