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

[John Brewer]

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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