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

Re: gum thickness and French check

I actually use 1 gum/pigment: 1 am di--the standard formula.

My formula seems more complex than that because I use "stock" gum/pigment mix and dilute my am di.

My stock is 1 tube of 14/15ml watercolor paint in a combined total of 60ml gum/paint. That I mix 1:1 with plain gum.

Then, on the am di side, I "cut" my am di 30% in half with an equal amount of water so in essence I use a 15% am di. The only reason I add water instead of mixing my am di at 15% is then I would have to measure out my am di with a scale and I am lazy. I just dump the pound of am di in a jar and put water on top, adding more water when necessary until there seem to be few crystals on the bottom at which time I dump another pound of am di in the jar. I do it this way because my darkroom temp doesn't vary much from 70.

But when I say my formula it looks like this: 1 stock gum/pigment: 1 gum: 1 am di 30%: 1 water.

Why I did the mistake is this: since the gum arabic I am using up is so thin--it is either a Daniel Smith or Photographer's Formulary dark, thin gum--I have been doing the same formula of 1:1:1:1 but replacing the water with more gum. That makes a thick enough/easily spreadable layer that doesn't streak and fish eye on me.

However, I was mixing my formula with the THICK gum I just mixed, a 60/100 mix, and I accidentally used 1 gum/pigment: 1 thick gum: 1 thick gum: 1am di 30%. When I was brushing on my layer I realized my mistake right away--very difficult to brush on.

It was the third layer on a set of large gums, hot pressed and cold pressed paper. I have never observed flagging in my life so I am confident to attribute it to a thicker gum. I had chunks of areas of the print fall off (never observed that either, as usually if a layer falls off it "grains" off), and at the perimeters of those holes left behind the flagging occurred. I assume it is the same as your veiling or frilling--it looks like the top of the gum is hardened and sticking together nicely, like a polaroid emulsion lift, but the underneath, less exposed layer has floated off. What was so funny is that they waved back and forth in the water bath so I could immediately see why they were called "flags".

----- Original Message ----- From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@usask.ca>
Sent: Tuesday, November 21, 2006 11:01 AM
Subject: 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.