U of S | Mailing List Archive | alt-photo-process-l | Re: gum thickness and French check

Re: gum thickness and French check

On Nov 20, 2006, at 7:55 AM, Christina Z. Anderson wrote:

This weekend I accidentally forgot to add the water part of the equation to my gum/pigment/di mix (I don't use am di at 30%, but cut it with an equal amount of water so it comes out to a 15% am di). When the gum prints went into the water for development I finally observed "flagging". The layer, instead of flaking off as I usually see happen, came off in a chunk here and there, and at the edge of the chunk there were these little tears of wavy veils that waved like little flags!
Chris, am I remembering the proportions right, that you use 1 unit gum/pigment: 1 unit saturated ammonium dichromate: 1 unit water? If so, then by forgetting the water you just bring it down to the proportions I (and many other gum printers) use: 1 unit gum/pigment: 1 unit saturated ammonium dichromate. I don't think I've ever seen this "flagging" you describe, although I have seen something I call "frilling" (but only on glass and yupo, never on paper) where the hardened gum lifts off the surface as a veil and then breaks into pieces that curl up at the edges. But this effect, whenever I've seen it, has been a result of the surface being too slick or hard for the hardened gum to get a purchase on it, not of the relative proportions of the various components of the coating mix.

Here's an example of this effect on glass, with a fairly light pigment mix of some transparent blue pigment (pthalo or Prussian, I don't remember). Is this what you're talking about, or something else?


You recently said that you use a thicker gum, which might possibly account for the difference in observations, although you also said that you're using up some commercial gum; is this observation with the thicker gum or the commercial gum? Either way, the extra water (if my recall of the proportions is correct, your coating mix has 50% more water in it than mine, which is a significant difference) doesn't dilute just the dichromate; it dilutes the gum as well.