Re: About testing (long) (was: Re: The consistency of gum)

Every post I've opened in the last several days has required a response
from me, so it's clear that I'm never going to catch up.

 I did write a response to Charles, but decided that perhaps others may
be confused by my nomenclature. The distinction between "gum mixed from
powder" and "black guck" was not clear, apparently. So let me try again.

I distinguish two different substances that both go by the name "gum
arabic" and are both used in gum printing. I may be the only person in
the world who notices and cares about the distinction, but here it is:

By "gum mixed from powder" I mean a high quality pure gum acacia with
nothing added to it but water and a preservative. It's immaterial to me
whether I mix it myself or whether I let my supplier mix it, what's
important to me is that it if I don't mix it myself, it is mixed by the
person I buy it from and that person can verify to me what's in it, and
that it's mixed to specific gravity 14 degree Baume. This gum is clear
(meaning free of sediment, not colorless) translucent and light in
color, ranging from a honey color to an amber, something like Karo
syrup. A small bit of this gum left to dry in a measuring vial overnight
dries to a hard transparent amber disk which pops out of the vial clean
as a whistle. It stays hard and dry, like a piece of clear plastic, even
in humid air (of which I have plenty) but dropped in water it quickly
dissolves. To me a side benefit of gum printing is working with this
lovely, organic, fascinating, material.

The other kind of gum is generally called "lithographers' gum" although
not every gum labeled "lithographers' gum" is of this type. It is
usually, but not always, cheaper than the other gum. It is dark and not
very clear because it has so much sediment in it, more like crankcase
oil than honey or Karo syrup. Instead of drying to a clear transparent
amber it dries to a black gummy mess, that's why I call it "that black
guck." It comes to the supplier pre-bottled, so neither the supplier
nor I can be sure what it consists of.

I have no idea what accounts for the difference between the two types.
It could be that the black guck is a different variety of acacia than
the acacia senegal, or it could be, as I read somewhere, that it's not
acacia at all but the sap of some other tree. It could be that the
powder used to mix it is a lower grade sort of acacia. It could be that
there are additives in it. I don't know; all I know is that these
materials are not the same thing.

 Most people probably won't care about the distinction as long as both
types work, and many people will choose the lithographers' gum because
it's cheaper, and that's fine with me. My purpose isn't to try to
impose my preference on others, but to explain what the distinction is
that I'm making.
Katharine Thayer

ryberg wrote:
>
> I have been reading this thread with considerable interest. Please
> clarify a point. Some days ago you mentioned you were testing six gums.
> >From what you wrote, I was under the impression that one was the now
> unavailable premixed gum which you still had some of, and the others were
> powdered gums from different retailers which you had mixed with water
> yourself.
> In your most recent post I get the idea that I was wrong, and that you
> are really testing the old one and five other premixed gums from different
> retailers. Your use of "black guck" may be part of my problem--is this the
> new version of the old gum?
> All this is really because I was about to go to the trouble of mixing
> some dried gum in the hope of getting an extra step or two. But If I've
> misread you then I might as well wait and hope you will eventually find (and
> announce) the gum you like the most.
> Thanks
> Charles Portland OR
Received on Thu Dec 4 20:02:31 2003