I might be totally wrong, but I remember one summer my folks schlepped the family to FDR’s house and they had some old money in the museum there. I think it was Depression era or before, and it was clear?
I think I have one or two green copper pennies. I remember those. The teacher used them to explain how the Statue of Liberty turned green.
The paper used on that money kinda degrades over time, so it becomes translucent.
And brittle.
We started using cloth based “paper” for our money at some point after the depression.
Don’t know precisely when.