Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

Mary Moorman said that it was after the second time the FBI took her photo that it was returned with the thumbprint. It was weeks after it was taken when it was long stable.  

So, could a thumbprint do this to a stable Polaroid photo?

We can test that. We have Polaroids. I have Polaroid pictures. I have quite a few Polaroid pictures. Here is a Polaroid of my Great Uncle John, taken in 1972 in Portland, Oregon.

Now, here it is after I mashed my thumb into it hard. 

There was no effect whatsoever. It didn't change a thing. I maintain that short of having a volatile agent of some kind coating your thumb, it is impossible to do what we see in the Moorman photo with your thumb. A clean thumb could never do it, not to a dry photograph, and that includes a Polaroid. 

No mature adult has any reason to believe that the thumbprint on the Moorman photo was accidental.