May be not exactly on topic, but related. I was watching the "Silver Footprint" DVD on Robin Bell last night and saw that one of his tricks was using used 4x5 negatives as masks by cutting the openings for the format required and black-taping the rest. He then sandwiched it between the negative to print and the carrier glass. He says it gives the 'dirty border' whilst preventing newton rings. Has anyone done something similar?