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

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