[Alt-photo] Re: GUM: problematic yellow?

[Remko de Graaff]

followed the thread with interest and jump in with my results I found 
with almost the same procedure.
first I noticed that the gum/pigment ratio is some where different as I use.
some details to compare:
my a stock solution consist of  15 ml (Schmiencke) pigment on 50 ml gum. 
(Luciano uses about the same (15+45))
I use then 3ml gum + 1ml stock solution + 4ml Potassium Dichromate(37% 
!!!) ) . ( against 1ml+1ml stocksolution + 2 ml Potassium dichromate 12%!!!)
although the strength of the dichromate the exposure times looks very 
long to me and from the  M and C images I would go for exposure times 
around 2 min as max. this is giving Dmax already.

It would be interesting to know what exposure unit you need and for 
later what printer you are going to use.

concerning the visibility of the yellow print  I scan the print and 
measure  then in Photoshop the density ore with an old density meter. 
This are not absolute values but the relative  values give you enough 
info to select the right exposure time.

remko
Loris Medici schreef op 5-2-2014 23:14:
> Thanks John, that's what I do because I'm incredibly incompetent in judging
> the yellow test prints. I just can't make sense of those yellow step tablet
> prints with naked eyes. (Blue rich light helps, though...)
>
> Did the trick on Luciano's print and I think 3:30 or even 05:00 will give a
> nice log 1.2 - 1.3 range, but that depends on his target DR. I don't think
> something over 05:00 will do good.
>
> Regards,
> Loris.
>
>
>
> 2014-02-05 John Brewer <johnbrewerphotography at gmail.com>:
>
>> That's a useful tip Loris!
>>
>> J
>>
>> Sent from my iPhone
>>
>>> On 5 Feb 2014, at 09:12 pm, Loris Medici <mail at loris.medici.name> wrote:
>>>
>>> Luciano try to evaluate the yellow test prints by splitting RGB channels
>>> and looking to the B channel. Since it's a very light color it's hard to
>>> judge it by eye...
>>>
>>> Regards,
>>> Loris.
>>>
>>>
>>> 2014-02-05 Luciano Teghillo <luciano at lucianoteghillo.com>:
>>>
>>>> ​...
>>>> So far no problem with blue
>>>> (http://www.lucianoteghillo.com/images/scan-blue.jpg) and red
>>>> (http://www.lucianoteghillo.com/images/scan-magenta.jpg) but yellow
>> it's
>>>> not
>>>> printing correctly, at least that's my impression.
>>>>
>>>> After the first test
>>>> (http://www.lucianoteghillo.com/images/scan-yellow1.jpg) with the usual
>>>> exposure times used for blue and magenta (from 1 minute to 3.30 minutes
>> in
>>>> 30 seconds intervals), it looked like I was not reaching maximum "black"
>>>> but
>>>> also the steps didn't look as defined as with the other colors. So I
>>>> decided
>>>> to print one more test with exposures of 16, 32 and 48 minutes
>>>> (http://www.lucianoteghillo.com/images/scan-yellow2.jpg). I think this
>>>> second test is event worst than the first.
>>>>
>>>> Now everything is the same: same gum, same potassium dichromate, same
>>>> process. I mix a stock solution with 15ml tube and 45ml of gum, which I
>>>> then
>>>> further dilute as needed. In the case of yellow, I mix 1ml of stock
>>>> solution
>>>> with 1ml of gum and 2ml of 12% potassium dichromate.
>>>> ​...
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