Sounds like it was/is an acid based product. I am guessing here. As my collection matured and I was aquiring more pricey notes I paid particular attention to the saftey of my notes. SO FAR I have had no problems. If a sheet is old I do replace it with a new one, mainly the age can be seen by scratches. I have not seen them 'yellow' or get brittle yet.

I was told the cracking is because the material is absorbing the oils in the note and causing it to shrink and crack. Mybe you need to "oil it up" hahaha