U of S | Mailing List Archive | alt-photo-process-l | Re: gum and humidity (was: Measuring humidity)

Re: gum and humidity (was: Measuring humidity)

At 8:06 AM -0700 9/29/06, Katharine Thayer wrote:
(This discussion seems to be ranging across several threads; I pity anyone who tries to follow it later)

On Sep 28, 2006, at 9:11 PM, Sandy King wrote:

Temperature and humidity are major issues for carbon printing. Sensitivity is much higher at 85F and 90% RH than at 70F and 35% RH. This absolutely has to be factored in when printing.

Yes, but this example confounds humidity with heat again. For gum at least, sensitivity is also higher at 60F and 90% RH than it is at 60F and 65% RH (35% is a range I never encounter, so I stayed within a range I'm familiar with so I can speak from my own observations).

Well, I don't see any reason not to confound them since they are both different variables. There is no question but that the RH with the temperature held constant has big effect on sensitivity. This can range from almost no sensitivity at RH of 20% or lower, to very great sensitivity at RH of 90% or higher. However, the oppose it also true. If you hold RH constant and vary the temperature you will have more sensitivity at 90 F than at 50F. When I first started printing carbon I carried out testing of this type to help me better understand how to get consistent results, and although I don't have the data any longer, I am reasonably certain the above is correct.