On Sat, 13 Jan 1996, s carl king wrote:
> Second, with regard to Judy's testing with gum printing,
> couple of observations:
> 1) The blue pigment used in the tests could give results
> very different from other colors. In three-color carbon
> printing, for example, the cyan tissue will always have higher
> contrast than the magenta and yellow.
Presumably this is the "internal filter" effect approximating light
of longer wavelength i.e. bluer light giving higher contrast.
> 2) It is important to note that in my tests
> the same strength sensitizer is not producing like results
> with the different tubes. The daylight and cool white tubes
> can produce useful exposures but only with higher strength
> sensitizers than used with BL. I believe that the kind of
> dichromate you are using in gum printing could make a big
> difference in terms of the results. Ammonium dichromate is
> more sensitive than potassium, and sodium quite a bit more
> sensitive than the other two. Since you are using saturated
> solutions in gum printing this could make a big difference. I
> believe that Davis used sodium dichromate for his test -- he
> does say in the article in Photo Techniques that this is the
> dichromate he uses.
I thought sodium was supposed to be the *least* sensitive (or else about
the same as potassium at the same concentrations). As I understand it,
Phil Davis uses it for everything (gumbi, photogravure, contrast control
for platinum, etc.) because it's cheaper, and he stocked up years ago
following Paul Anderson's recommendation. Sodium dichromate isn't
generally recommended because its deliquesence makes it difficult to weigh
accurately, although the problem may have been exaggerated. Contrast for
carbon (and presumably gum) has long been known to vary with different
sensitizers - potassium, said to be about half as sensitive as ammonium,
is also said to give higher contrast (again at the same concentration).
Another commonly cited generalization is that a 2.5% solution of ammonium
dichromate has the same speed, contrast and keeping quality as a 3.5%
solution of the potassium.
Sodium dichromate is much more soluble so if as Sandy says Davis is
using a saturated solution it would be much more concentrated and
therefore probably much lower in contrast. This might indeed explain
why Judy is getting unworkably high contrast with an ammonium or
potassium sensitizer.
Kosar notes that the spectral character of a light source strongly
influences the extent to which dichromate concentration affects
speed and contrast. Phil Davis and Sandy seem to have worked out
what's appropriate for their given set of conditions, but from
the sound of it the conditions Judy's working with are quite
different.
Philip (who's now going to give this up and actually do something!)
pjackson@nla.gov.au