... Paul Anderson's "Gum-pigment ratio test" has been reprinted as often
as the Neiman-Marcus cookie hoax on the Internet -- from Henney and
Dudley's Handbook of Photography, bible of the '40s, to William Crawford's
Keepers of Light, bible of the '80s. But it bears little relation to the
way the medium actually works...
On the premise that pigment particles must be surrounded by a certain
proportion of gum arabic to keep them from embedding in the fibres of the
paper, Anderson decided to find the exact amount -- enough gum, but not a
drop more than necessary.
Beginning with an inch of pigment, he added a dram (about 1.8 cc) of gum
arabic, put a dab of the mix on paper, added another dram of gum, put
another dab on the paper, and so forth, adding and dabbing 10 or 20 times,
until the final mix was mostly gum, very little pigment.
When the dabs are dry, the paper is soaked face down in water -- no
dichromate, no exposure -- for a half hour, then checked. The first dab
that washes away completely with no residual stain is supposed to be the
highest proportion of that particular pigment that can be printed without
staining on that paper by still development.
Only it isn't.
Dutifully running this test one day with fresh paper, on a whim I tried it
on preshrunk paper. Oops, the result was completely different. I
understood in a trice that any variable -- length of soak, order of coat,
etc. -- would also be completely different. Later it dawned on me that the
dichromated emulsion also changes the paper, because dichromate is a
tanning agent. And finally, with a little help from high places [eg, Mike
Ware] I understood that the process is pigment, gum, light, and dichromate
interacting, that paint and gum alone tell us very little.
So I tried some actual exposure and development situations. Doubling
pigment did not increase staining, even when the color got so strong it
looked straight from the tube. I tested several gums. The problem wasn't
extra staining, it turned out, but flaking, as the thick, opaque mix kept
light from getting through to the paper. (Such intense color will, in any
event, print only a couple of steps before tone blocks up entirely, but
that's a different point.) I shared my finding with folks doing the dab
test.... They did not, I think, believe me....
End of extract...
=======================
As noted, however, this myth has a life of its own...
cheers,
Judy