[Alt-photo] GUM: problematic yellow?

[Luciano Teghillo]

After Diana suggestion (thanks again) I was able to source some Fabriano
Artistico soft-press and I was able to print some exposure tests. The paper
is great! I am not getting any stains, and finally I am printing something,
even if they are only step wedges without having to pre-shrink, size, etc.
etc.... The final test will come as soon as I can print more layers....
 
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.
 
The watercolor used for yellow is Maimeri Blue, Primary Yellow, PY97
(Arylide yellow, also known as Monoazo yellow).
 
This might be important or not, but I noticed some separation between gum
and pigment in the stock solution bottle. If I let the bottle rest for a
day, I can see a ring the color of gum 1/4" thick on top, something I did
not notice with the other colors.
 
Do you have any advice? I do not have other yellow tubes from other
pigments/brands but I could look for it. It just seems strange that I cannot
print this color!!!
 
Thanks,
Luciano