Hello good people,

I have a theoretical question that is perplexing us at George Eastman House, regarding the contrast effect obtained with printing out paper. In this case, collodio chloride POP.

Here is the mystery: By slowing down the printing time, higher contrast is achieved.

A low contrast negative is printed in open shade or under diffusion, whereas a high contrast negative is printed under direct sunlight (historically). However, the diffusion aspect is not the culprit (and besides, wouldn't this tend towards lower contrast anyways?), because the same effect is obtained with a yellowed glass. The key here is slower printing, with light of less intensity.

This has been known for a long time, and is considered a standard control in the process. The reason for WHY however, has alluded us.

Some things to know about POP paper. It is pure silver chloride with an excess of silver. As the image prints, it is "self-masking" (that is, metallic silver is formed and thus blocks light from penetrating further).

Now why on Earth is this the case? If the negative has a fixed set of densities from high to low; why would only printing time, or rather the intensity of light, affect the outcome. Could this be due to some kind of reciprocity phenomenon? Could this be due to particle size, and in some way affected by the self masking quality of POP?

I'm interested to hear your ideas, and intelligent thoughts on the matter.