one of these things is not like the others

This is a group of diplomas and a certificate that I recently treated. The certificate on top is paper while the others are parchment. The paper document is from 1939, while the parchment documents are 1921, 1918, 1916 and 1915. They are not too far apart in age, have been boxed together as part of the same collection and underwent the same treatment for similar issues, but what a difference in aging.