ISO
As a rule, paper ISO is 3. In a post on the Large Format Photography Forum blog (the best large format blog I have found) “A brief flashing (2 sec. I think at f22, 10 in height on the enlarger w/#0 vc filter) prior to loading” (jamesaz 2017). With preflash, the paper rating increases to approximately ISO 12.
Blue wavelengths
Photo paper is “sensitive to the blue wavelengths of light so any subject reflecting or emitting blue light”, such as blue sky. Pre-flash paper under the enlarger before loading into the camera to control contrast “to add some exposure to your paper but not enough to alter the pure base white” (Tweedie 2009).
Pre-flash
Tweedie states that a pre-flash of “2 seconds at f16 with the enlarger head set at approx 65cm from the base board”, advising that to experiment as the formula will vary depending on the paper type and enlarger.
Figure 1. Mark Tweedie, Pre-flash exposure chart, 2009
Keywords
Paper negative, Exposure times
Bibliography
jamesaz. 2017. "Exposure Time For Ilford RC Pearl Used As Paper Negative". Blog. Large Format Photography Forum. https://www.largeformatphotography.info/forum/showthread.php?140900-Exposure-time-for-Ilford-RC-pearl-used-as-a-paper-negative.
Tweedie, Mark. 2009. "Paper Negative – The Underestimated Medium". Mark Tweedie. https://www.marktweedie.co.uk/paper-negative-the-underestimated-medium/.