Meanwhile, a friend has asked for my comments about his near-gum-printing
experiences. I thought these might usefully be made to the group, where
perhaps others will have further insights...
> ---cut--- ...gum-14. Been using equal parts gum and dichromate. Was not
> using a foam brush for application. Been using a hake brush and having
> difficulty getting an even coating. Now the big question. How much
> pigment (in grams) to how much gum (in cc)? I suspect I am using too
> much pigment because my exposures are more like 30 min. with a FL source
> that will print cyanotype in 3 min.
>
Generally, with my light sources, a made-for-gum negative takes about a
third as long to print as cyanotype. However, if you're using the same
negative that you use for cyanotype, it could be too contrasty for gum and
hence slow you down -- although I doubt as much as you describe. Among
other variables that affect gum "speed" the following come to mind:
Which dichromate are you using? Potassium di, only 10% concentration,
is significantly slower than ammonium di at 30% concentration.... (can
anyone do the math on this? ie., how many times *slower* the k di is?)
As for the "correct" ratio of pigment to gum:
The short answer is as much as you can pile in until the emulsion flakes
off instead of making a continuous tone, that is, IF YOU WANT TO DO ONE
COAT GUM AND DON'T MIND JERKY HIGHLIGHTS. If you want nice smooth
highlights without brushing, the amount of pigment can only be determined
by testing each individual pigment *UNDER A 21-STEP*.
That's not just the color in question, because the covering power of
pigments and the difficulty with which UV light penetrates each varies,
but each manufacturer's version of the color may be different. The "same"
pigment can come from a different mine or a different factory. Besides
which, different manufacturers add different additives. This includes,
but is by no means limited to the fact that cheap paints have less actual
pigment in each gram, more fillers, like alumina hydrate, or gum arabic,
or whatever, and that these materials may impede light and enter into
reactions differently.
So you look at the test PRINT, and here, trust me, a 21-step tells you SO
much more than just your regular old negative. Notice how many steps of
tone (the denser the pigment the fewer the steps, all other things being
equal) and how smoooooth and long-scale the highlights are.
As noted, the first manifestation of over-pigmentation is likely to be
highlights flaking off. An option here of course is blasting the emulsion
with light, way overexposing for an ordinary automatic-development print,
and then BRUSHING OFF to reveal the picture. That way, you can use a
humongous over-load of pigment and still get a semblance of continuous
tone -- although the highlights will be very *GRAINY*. (Brushing takes the
pigment off the peaks of the paper, leaves it in the valleys.)
Normally, you don't get near as many steps with heavy pigment -- the sheer
amount of color stacks up in the bottom register, making all look the
same, as well as cutting off tones at the other end because light doesn't
get through the heavy coat.
And did I say different colors have different sensitivity to light....?
So, by the way, do different gums. I've tested a half-dozen gums and find
differences of "speed" by 100% or more among them. My first hypothesis is
that that's a factor of pH, the more *acid* being faster (uh oh, or was
that the other way around? got to check the file) but there are many other
variables; that's just a preliminary hunch....
But you want a ballpark figure: I seem to use more pigment than other
people. My series with Rowney Jet black gouache used maybe .8 g black and
from .5 to .7 g other pigments in 10 to 11 cc total emulsion (gum, am di &
usually water, because my gum is very thick). But note that some colors
like thalo blue and indigo are very intense & you'd use less of them.
> (Oh, and do you use cold water or warm?)
>
My "normal" development is cold water (ie., available temperature), with
warm water as a fall-back. I haven't tested this AT ALL, but my sense of
the situation is that warm water would lose delicacy in the highlights,
all other things being equal (which they so rarely are)....
For applying the emulsion, a foam applicator is best (wastes the least),
but you have to follow with a whisking smooth with a DRY hake brush (for
which reason it's handy to have several for a day's printing). Some folks
on the list have said they use a Daniel Smith "elephant brush," which is a
much coarser bristle, for that. My experience suggests that might grind
the emulsion into the paper (or some papers) too much, ie., lead to
staining -- or at least this semester when some students got heavy
staining, I told them to lay off that kind of brush & staining went away.
Judy