From: Sandy King (sanking@clemson.edu)
Date: 06/29/03-10:10:32 PM Z
Chris,
>
> What I don't know and maybe you can answer is this: do two steps on
>the 21 step wedge equate to a stop? If so I would say am di is faster by
>about a stop to a stop and a half.
Two steps equal about log 0.30 density, which is indeed about a stop.
However, I think you are confusing speed and contrast. The speed of a
photographic emulsion is generally considered to be the point on the
step wedge where you have the first maximum density in the shadows.
Contrast is determined by how many visible steps you have. Less means
a printing exposure scale of greater contrast, more means less
contrast.
In your tests what you have found is that ammonium dichromate gives
less contrast, or softer results, than the other dichromate. This is
true even when the dichromates are compared at the same strength
solution.
Sandy King
> It's kind of worthless information, actually, but it was done in an
>effort to see if for any reason I would keep sodium dichromate on hand,
>assuming that the 100% saturation would be of benefit. I do think there are
>benefits to having pot and am both, but cannot see why I would ever use
>sodium di again (unless it is really cheap? Can't remember).
>Chris
> ----- Original Message -----
>From: "Joe Smigiel" <jsmigiel@kvcc.edu>
>To: <kthayer@pacifier.com>; <alt-photo-process-l@sask.usask.ca>
>Sent: Sunday, June 29, 2003 6:20 PM
>Subject: Re: lemon juice and gum printing
>
>
>> Chris & Katherine,
>>
>> I think what is being demonstrated here may be underexposure (or speed) of
>the Na & K flavors rather than any dichromate stain. The fact that numbers
>and letters remain with the ammonium exposure indicates that a chromium
>image, not a non-image stain, has resulted using that salt. A "stain" IMO
>would be a general overall fog and not an image as you describe here. The
>step wedge has blocked exposure in the dense areas (numbers) and formed a
>reversed image of them if I'm reading this correctly. That's an image.
>>
>> Also, I'm not sure what you mean when you say you cleared the print. Was
>this plain water or something like potassium metabisulphite? Water would
>leave the image/stain intact (assuming sufficient exposure) while the latter
>would wipe out any chromium image or stain.
>>
>> Joe
>>
>> >>> kthayer@pacifier.com 06/29/03 11:30 AM >>>
>> > Hi Keith,
>> > One thing I did today, too was to do side by side exposures of am,
>> > pot
>> > and sod di to see if there was a speed dif. I swear, am di is really
>> > fast! It gives the clearest, sharpest steps of all three dichromates, is
>> > the speediest, and sod is not much different than pot. I did this test
>> > with NO pigment, just side by side straight dichromate. Then I cleared
>> > to see what remained, and the am di was the only one that printed the
>> > numbers and the words and the steps of the tablet.
>>
>> Hi Chris and all,
>> I think what you're demonstrating here isn't how the three dichromates
>> print in normal gum printing so much as you're providing support for the
>> idea that ammonium dichromate "stains" (meaning dichromate staining, not
>> pigment staining) more than the other dichromates, which I haven't
>> observed myself but others have reported. If there's no pigment in the
>> coating and you still see the numbers, words, and steps of the tablet
>> after the print has been cleared, then you've got dichromate stain, which
>> in my experience generally results from overexposure.It may be that people
>> want to print with dichromate stain rather than with pigment, as in your
>> historical example, but we should be clear that that's something different
>> from the usual printing practice.
>> kt
>>
>>
>>
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