U of S | Mailing List Archive | alt-photo-process-l | Re: was Miracle size for gum now tonal range

Re: was Miracle size for gum now tonal range

Yes, phritz, this observation corresponds with mine and with the observations of observant gum printers through more than a hundred years; more dichromate= a longer tonal range (more steps), less dichromate= fewer steps and a more contrasty print. And I think you've got the right idea, at least part of it, for why that's so.

See, you aren't at complete odds with gum at all, even though it feels that way at the moment; you do have a sense of how it works.

On Oct 10, 2009, at 1:02 PM, phritz phantom wrote:

hi chris,
i did tests for tonal range about a year ago and again two months ago. i printed step wedges with saturated and 5% ammonium dichromate. since i don't have a proper step wedge (yet), i can't say how many steps, but for me i get about twice the tonal range with saturated than with 5%.
with the (uncurved) chart throb scales and 5% am-di there appr,2.5 lines between max. densitiy and white. while with the saturated solution there are 5 or 6 lines between white and black.
another thing i noticed is that the 5% solution does significantly clear better than the saturated (on the cheap paper i use). i wouldn't really call it stain, but the whites of the 5% sheet are noticably more brilliant. also i think the tonal range of the 5% does break off more abrupt in the highlights. maybe it's just the fine highlights that wash off quicker in the development, because of the lesser light sensitivity of the 5% solution.


Christina Z. Anderson schrieb:

Someone said in the literature recently that gum prints 2 stops (log .6). I was surprised at how low this was, as I always hedge my bets and say 4-6 stops, with 6 a stretch. Partly if you don't clear with pot metabi you can get a false read of maximum black because the brown stain of the dichromate can read a darker step when it is not really hardened anymore "goo" on top. So I always figured that the 2 stop person cleared and found that to be true. Or maybe was guessing it from the seat of his/her pants.

But it's all kind of a moot point I suppose once you fit your negative curve to the exposure/dichromate you use.

I use a 15% solution of am di (2 tsp to 100ml quick n' easy approximate).

So Marek is figuring a 3 stop range, you a 4-5 stop range....

You say that with weaker di and greater exposure you are getting a longer tonal range--do you have a picture on the web of that comparison somewhere?


Christina Z. Anderson