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

[Remko de Graaff]

thanks Luciano. we almost have a comparable set up. the distance uv unit 
and UV bulbs here is about 15 cm and gives exposure times of 1 minute .
therefore I wonder about the long exposure times you think you might need.
remko

Luciano Teghillo schreef op 6-2-2014 9:51:
> Remko,
>
> By exposure unit you mean the UV printer? If yes, I use a self-built unit with a bank of UV tubes (Philips Actinic 8W with electronic ballast). For printer (to print my negatives) I will use an Epson R2880.
>
>
> Luciano
>
>
>
>
>
> -----Original Message-----
> From: alt-photo-process-list-bounces at lists.altphotolist.org [mailto:alt-photo-process-list-bounces at lists.altphotolist.org] On Behalf Of Remko de Graaff
> Sent: Thursday, February 06, 2014 9:37 AM
> To: alt-photo-process-list at lists.altphotolist.org
> Subject: [Alt-photo] Re: GUM: problematic yellow?
>
> 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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