U of S | Mailing List Archive | alt-photo-process-l | Hunidity - RH

Hunidity - RH

  • To: alt-photo-process list <alt-photo-process-l@usask.ca>
  • Subject: Hunidity - RH
  • From: "Jeffrey D. Mathias" <jeffrey.d.mathias@att.net>
  • Date: Fri, 29 Sep 2006 09:36:02 -0400
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Keep in mind also that RH is just as much a function of pressure as temperature. That said, RH is what one wants to watch (as well as temperature and pressure.) This is because RH indicates the partial pressure of the water vapor which is what will determine if water is going into or out of the paper from or to the air.

Note too that the water vapor can move from or to the paper at a pretty good speed (a few minutes). I have placed a paper onto a scale and watched the weight change (heavier or lighter) as the water vapor came to equilibrium.

In being able to control and reproduce a pt/pd coating, one should record the ambient conditions, including RH, temperature and pressure. Also remember to take the time to let the dried coating and paper come to equilibrium.

It is also important to have a vapor barrier behind the paper in the contact printing frame, otherwise the heat from exposure can lead to uneven water in the coating resulting in blotchy areas. This is more important with high humidity coatings such as those using Li and AFO for a cooler, bluish color and POP.

Jeffrey D. Mathias