RE: Dichromate dilution and speed

Keith,

I don't recall seeing any of the negatives that Sam uses for gum
printing but I am going to assume that they look like mine in the
shadows but that they have much less contrast. I know that he has
been using plain OHP material on the Epson 2200 because he does not
need to build up as much contrast for gum as I need for my processes.

My negatives have a minimum amount of density in the shadows but are
in fact rather high in contrast. They typically measure from about
0.20 in the areas where there is some textured shadows to about 1.95
in the highest highlights that retain detail, for a density range of
about 1.8.

My printing times for carbon with the digital negatives are generally
about 1.5 - 2.0 minutes with a bank of BLB lights, and just slightly
longer with the NuArc 26-IK. I did some tests a year or so ago of
some gum papers that Sam coated and the printing times with carbon
and gum using a continuous tone step wedge were virtually identical,
about three minutes to get the first Dmax with both processes. I am
almost positive that he was using at that time the same method of
coating that he describes in the new article at unblinkingeye.com so
I am not at all surprised by how fast his papers print. The data from
my tests of Sam's gum is in the article on UV lights I did at
http://unblinkingeye.com/Articles/Light/L2/l2.html

So the question remains, why do Sam's gum, which use such a low
percentage of dichromate (about 3%) in the coating, print with so
much speed, while others need saturated solutions of 10X the amount
of dichromate. I could be wrong but I think there is really some
dramatic difference in technic rather than an accumulation of little
differences.

Sandy

>I forgot to mention an important factor: I'm using lith negatives. Which
>brings up a point: in the past, I've used digital negs, but never
>Pictorico. My negatives are rather contrasty and dense. Sandy, are Sam's
>negatives anythng like yours? When I saw your negatives in person, I was
>very much surprised that they produced the (very fine) results you obtain in
>your work. They seemed very thin - nothing like what I use.
>
>-----Original Message-----
>From: Keith Gerling [mailto:keith@gumphoto.com]
>Sent: Monday, December 01, 2003 12:23 PM
>To: alt-photo-process-l@sask.usask.ca
>Subject: RE: Dichromate dilution and speed
>
>
>Given the recent exchange, I thought I might as well offer my comments.
>Typically, I use a saturated solution of Potassium Dichromate for my gum
>work. But after reading Chris's account about using weaker solutions, I
>performed a series of (albeit un-scientific) tests. Working with a negative
>that normally requires 600 units under a Violux plate burner, I tried it at
>700 units with an emulsion using half the amount of Pot Di, making up the
>difference with tap water. After 30 seconds in the water, the emulsion had
>totally dissolved. All of it. Increasing the exposure to 1000 units
>produced a abominable result: very posterized with no emulsion on most of
>the print all of the emulsion remaining in the shadow areas. I was very
>disappointed.

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