Re: Gum Chemicals - starting out question

Judy Seigel wrote:

>
> I found a dramatic difference in speed (exposure time) between two of my
> favorite gums... But I note that there are not, IME, "two types of gum,"
> but many.

I fear I've been misunderstood again. I say there are two types of gum;
this is not the same as saying there are only two gums. Individual gums
are all different, and will vary widely in speeds and other aspects,
but IME there certainly are two types of gum.

 The first is pure gum arabic, which has a quality all its own. If you
leave a bit of it to dry overnight it dries as hard as clear plastic and
as clear (but colorless only if it dries in a very thin layer).. If you
tap this dried gum with a fingernail it makes a clear crisp hard tapping
noise. It stays hard and dry even in very damp air, but if you drop it
in water, it dissolves within a few minutes. This type of gum is almost
transparent when liquid, (may have a slight cloudiness but no
discernable sediment) ranges in color from light yellow to deep amber
and has a nice fresh smell; it's not like anything else in the world and
I love working with this lovely substance.

Then there's this other stuff, that is usually called lithographers' gum
although not every gum that goes by that name is this type. It is dark
brown or even black and looks like crankcase oil; it is full of sediment
and very viscous. If you leave a bit of it to dry overnight it doesn't
dry to a hard dry substance like the pure gum but remains a sticky
tarry mess that is very difficult to wash out of a vial. It may perform
similarly in gum printing to the pure gum (although my tests show a
clear distinction between them in number of steps printed and how widely
separated the steps are, as I'll describe briefly below) but the two
types of gum are different substances IME.

In any event this is not something someone can tell you in
> advance. Have you got a 21-step? One little test with the 21 step will
> give you the range of a gum-pigment combo,
 

One little test won't tell you much of anything. I ran comparison tests
with six gums, again and again, and only after a series of tests did the
character of each gum emerge. If I had run only one test and accepted
the results as gospel, I may have given unwarranted rankings to gums
whose performance is statistically indistinct from each other. But the
two types of gum performed significantly differently from each other.
The dark lithographers' gums consistently printed 5 or 6
widely-separated steps, while the pure gum arabics consistently printed
7 to 9 steps that were more nicely graduated. I prefer the longer scale
with more gradual tonal gradations, but someone else might prefer a
shorter scale with tonal skips between the steps. The second kind makes
a really dramatic-looking step print, but I don't like the jerky tonal
jumps in actual prints.

Some gum printers believe that the lithographers' gums will muddy your
printed colors if you are using transparent pigments, and I once
suspected that to be true, but I now know that it is not. Only the
actual gum arabic in the "gum" becomes crosslinked, and the extra
material, the sediment and gunk, must leach out into the water during
the development soak. At any rate, the dark lithographers' gum prints as
clear a layer of gum as a purer gum does; I've demonstrated that by
printing them both unpigmented on a clear surface. So that's not an
issue. But there is still the disadvantage of the gunk in the gum
obscuring colors while mixing, which is a problem if you like to mix
colors by eye.
>
> I'll add that I never found the "purest" gum worked the best in
> printing. For instance, I found the Daniel Smith "Premier" (or maybe it
> was "Premium?) gum almost unusable.

You're having fun bashing my favorite gums, aren't you. I ignored the
"bad" label you gave to my old favorite in your last mail, but really,
this name-calling isn't very useful. Obviously the Daniel Smith Premium
is not "unusable" because it's my favorite so far of the candidates for
my new favorite gum. My only complaint about it so far is that my old
favorite would print on mylar, and this gum won't print on mylar unless
the mylar is scuffed. And by the same token, obviously my old favorite
Photographers' Formulary gum wasn't "bad" as you characterize it,
because it served me faithfully for years without ever doing me wrong.
I'm sorry, but I can't take seriously the results of one-time tests that
rate gums "unusable" or "bad" that I know from much experience to be
excellent gums.

That was very light, almost water
> white, and about $20 a PINT !

There's an apparent misconception here; you don't tell how pure a gum is
by how "white" or colorless it is, but by how clear of sediment and how
like gum arabic it is. A pure gum arabic can be a deep amber as well as
a lighter color. It's not the color but the clarity and the purity of
the gum.

It shouldn't be necessary to repeat this, but I will anyway just to be
sure there's no misunderstanding: I don't think one type of gum is
inherently better for gum printing than the other. I personally like the
pure gum arabic better, for several reasons I've already listed, but
it's a personal preference. Each to his own.

Katharine
Received on Fri Sep 10 01:49:02 2004