I've been so enjoying the thread on carbon -- it's simply great to see
people more baffled & more in pain than I am, not to mention a process
that makes mere gum seem like child's play -- I hate to break into it with
well, child's play, but who knows, maybe this is the great gum
breakthrough:
On Sat, 9 Dec 1995, Philip Jackson wrote:
> I'm not clear how the aluminum foil wrapped
> around the base of the fluorescent fixtures should have been destroying
> definition or why anybody would want to do this anyway.
The implication was that by the *reflector effect* the speed of the lights
would be increased. The *reality* was (I hypothesize) that the inevitable
inescapable crumpling of the foil created a thousand points of light, ie.,
the utmost *opposite* of point-light source. (I think that the speed of
the light may in fact be less without the foil -- haven't tested precisely
-- but it's still plenty fast enough.)
> > the limited tests I've done so far show that the pigment is
> > richer and shadow steps deeper with the NuArc. Does anyone have a theory
> > for that?????? (I have found no difference with cyanotype.)
>
> How about the theory of reciprocity effects? Perhaps there's a
> considerable difference in light output between the two sources causing a
> change in contrast, assuming an identical exposure time. Even if you
> increase the time to compensate for the fluorescents being a less intense
> source, it may not necessarily follow that you'll achieve the same
> exposure, because what's supposed to be the reciprocal relation between
Guess I wasn't perfectly clear. The EXPOSURE seems as near as I can tell
to be quite the same -- after some trial and error I've calibrated the two
light sources. 175 units on the Nu Arc equals 3 minutes on the
fluorescents (at 1 1/2 inches from bulbs). And, as a side remark, I'll
note that the lapsed time is virtually identical -- 175 units on the Nu
Arc probably takes about 3 minutes. In any event, the top couple of
highlight steps on the Stouffer 21-step are identical -- they reach the
same number, are virtually the same density and texture.
But as you go on down the scale to midtones & shadows, there is, depending
on pigment-to-gum ratio (more evident with less pigment) a richer pigment,
that is it just looks like the gum held more paint when exposed by the
NuArc, and when you get way down to the bottom steps, there's better
shadow separation than with the fluorescents. I haven't tested this
exhaustively (life always intervening) but have seen it *distinctly*
enough times to consider it a fact.
Maybe it's not the light source, but because contact is better with the
NuArc -- due to vacuum frame -- or because position is 20 feet north in
magnetic field.... Who knows? Any theories will be gratefully accepted
and acknowledged. So far the best/only theory offered is that the mercury
light penetrates better (is that it?), but then why are the highlights the
same?
yours from tundra country,
Judy