Last week's bulb-fest seems like ancient history with all the action today
on getting the dichromate out & gum-by-enlarger (that was brilliant Rand,
you could make a fortune), but we non-mechanical types are still
worrying about "the best" fluorescent bulbs:
On Wed, 10 Jan 1996, s carl king wrote:
> I finally got my hands on a batch of GE F20 T12D (Daylight) tubes
>... did with the UV/Cool White tubes. The results were *spectacular*.
> The daylight tubes were both faster (if we measure sensitivity by
> the time required to produce the first maximum black on steps 1
> and 2), and of greater contrast than the UVs. I have not yet had
> assure you that for my money, with this particular tissue and'
> in my working conditions, the daylights are vastly superior to
> the UVs.
>
My tests showed Daylight bulbs contrastier too, but too contrasty for
gum printing -- and also SLOWER -- as follows:
Color was Rowney permanent blue, paper was Zerkal book (don't ask),
3 minutes exposure for the blacklight bulb, 6 minutes for the
daylight bulb (trying to equalize them, not yet there);
I also tested two different gums, which showed quite different results.
Regular "litho gum 14" from Philben, mixed equal parts with 26% ammonium
dichromate, one hour "development":
The Blacklight bulbs printed 7 more or less even steps of density
(Carson's "straight line"!) except for a fairly abrupt cut off at #7
(which is why I would normally use a 1 to 2 emulsion).
The Daylight bulbs printed only four steps, starting at #3, continuing
through 6, the last two steps being extremely grainy.
It's tempting to say "case closed," except for one peculiar thing: The
intensity of the *blue* was much greater with the Daylight bulbs -- as was
the dichromate staining. After the one-hour soak there was no visible
stain in the Blacklight sample (really -- this is wonderful NYC soft
water we're using here). The margins of the Daylight sample were so
stained as to appear nearly green.
Can I get an equally intense blue by just adding blue pigment to the mix
exposed by the BL bulb? Maybe, but adding pigment reduces the number of
steps. What about mixing bulbs? Maybe.
The other set of tests were with a gum that's also supposedly
lithographer's gum 14, but is actually noticeably more viscous than my
usual. It's from RBP company in Michigan, indirectly suggested by Steven
Livick.
Results were less satisfactory, and I doubt you could make a satisfactory
"photograph" with this bulb-gum-pigment combination. That is there were
more steps (up to 9 with the BL bulbs, 8 on the Daylight bulbs) but in
both cases the steps were much tinier, the ramp by the BL bulbs from step
#1 to #8 or 9 being almost one step, with a sudden drop off at the top.
The Daylight bulbs don't even have these teeny tiny steps, in fact show
only only 2 1/2 "steps" at the top.
I have, BTW, previously made tests with the RBP gum which showed a good
range. Will have to check them again, meanwhile
I don't understand it either.....
Judy