What do you think would be some of the challenges in collecting and preserving impressions? Why?

Back before cameras, paintings were required and they took very long to create and holding a expression like smiling would strain the person. I might've been off topic but this is what i think it would be.

Some of the greatest challenges in collecting and keeping the impressions safe would be to get everything together and exact and to make sure you keep the evidence good and collect everything you can find from one object.