Re: Dark reaction
Etienne of course has it right; the "dark" reaction is a reaction that begins immediately when the paper is coated and dried, proceeds in the absence of radiation, and has the potential for fogging the emulsion. The speed of the dark reaction depends on a number of variables, humidity being a crucial one; someone in a very dry climate may be able to keep coated papers for days without the emulsion fogging due to the dark reaction, whereas in the beach climate where I lived until recently, the gum layer would completely harden within a few hours in complete darkness. The relationship between humidity and speed of the dark reaction is very steep. A graph illustrating the relationship between relative humidity and time in hours required to produce a specific degree of hardening in a dichromated gum layer, shows the dark reaction taking 1100 hours at 50% RH and 0 hours at 85% RH. (Jorgenson and Bruno, "Chromium Chemicals in the Graphic Arts" Chapter 14 in the two-volume tome "Chromium." )
The article Etienne links gives a brief general summary but there are much more detailed studies available. As noted, Kosar is fairly good on the dark reaction, except that the literature Kosar reviews and summarizes is almost entirely related to dichromated gelatin, which isn't precisely equivalent to dichromated gum. Studies on dichromated PVA give a closer approximation to the mechanisms of dichromated gum arabic, and very occasionally, one can even find direct studies of dichromated gum, as the one cited above.
On Oct 29, 2009, at 12:30 AM, etienne garbaux wrote:
Dichromated colloids, particularly after being coated and dried, begin to insolubilize (harden) -- even if not exposed to light (hence, "dark reaction" -- not *caused* by darkness, but happens even in darkness). This tends to fog dichromated-colloid media rather quickly. It is more rapid at higher temperatures, and there is some evidence it is more rapid at higher ambient humidity.