[pure-silver] Re: old papers and new papers
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Sun, 30 Nov 2014 13:43:30 -0800
----- Original Message -----
From: "Jeffrey Thorns" <puresilver@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Sunday, November 30, 2014 11:30 AM
Subject: [pure-silver] Re: old papers and new papers
Can you give us an idea of how much slower Lodima is than
'normal'
enlarging paper? 2 stops? 5 stops? 10 stops? Or is it more
related to UV
output?
I have no idea what the speed of Lodima is but I can
speak for Azo. The difference in speed between a medium
speed _enlarging_ paper and a medium speed contact printing
paper is quite substantial. At the time Kodak was listing
relative speeds the speed of Azo was given as 16, the
slowest Kodak contact paper of the time was Athena, a very
warm tone paper at 5. Velox, a fast contact paper was 32.
Enlarging papers varied from Kodabromide at 1000 to Opal, a
warm tone dual purpose paper at 160. Most modern variable
contrast papers would rate around 500 on this scale.
Contact printing papers were meant to be used in either
a printing box with an array of fairly powerful lamps, or in
a printing frame with typically a 60 or 100 watt lamp about
four feet away. A typical printing box for 8x10 had four to
six 60 watt lamps in it. Exposure times were anywhere from
around 5 seconds to maybe 15 seconds.
Azo was a neutral tone paper used mainly for commercial
photography. It was commonly used for mass production
printing of pictures for advertising or fan photos of
actors.
Every maker of printing paper made a similar paper and
from the old sample books I am not so sure that some were
not better than Azo but we will never know.
Kodak also made a paper called Commercial Azo mainly
aimed at making originals for half-tone reproduction. It was
not the same emulsion being a warm tone paper.
Tone rendition of papers depends on the curve shape of
the emulsion. Experienced makers like Kodak could control
this to a great extent. It is mainly a function of the
distribution of sensitivity of the silver halide crystals in
the emulsion which, in turn, is controlled by a number of
factors such as the speed with which the silver nitrate is
added to the emulsion mixture.
Unfortunately, much of the emulsion making technology
for both film and paper is hidden in the secrecy of the
processes. While Kodak and others came to rely increasingly
on patent protection most of their processes were protected
as trade secrets. Trade secrets yield a considerable amount
of legal protection and have the advantage over patents of
having essentially infinite life. So at least some of the
exact processes for making some of the famous products of
the past are probably lost.
My understanding of the use of cadmium compounds is that
they contributed to the shelf life of the finished products.
They may well have had other effects. I was told that they
contributed to the color of warm tone paper but have only
anecdotal evidence of this. It seems certain that the shelf
life of especially paper but also film was shortened when
cadmium use was banned by the EPA and others.
Note that the tone rendition of paper can be measured as
easily as that of film. There is no need to guess at the
relative Dmin or Dmax or the relation of the density to the
exposure, it can all be measured by very well established
methods.
Dmax, as I noted earlier, is a function of the emulsion
but is also strongly affected by the nature of the surface
and texture of the paper. Dmin is also a function of the
surface and texture but is also affected by the reflective
layer under the emulsion. In papers on untreated support,
sometimes called "fiber paper" the reflective layer is
usually a coating or multiple coatings of very hard gelatin
with a suspension of barium sulfate in it. This is often
called the Baryta layer. In resin coated papers the
reflective layer is a coating of plastic with a suspension
of titanium dioxide. Titanium dioxide is a slightly better
reflector than barium sulfate but under some conditions,
such as exposure to UV light, will emit oxygen. As a result
early resin coated papers had a limited lifetime since the
oxygen would attack the image and also could cause a break
down of the plastic surface in which it was embedded. The
addition of oxygen absorbent materials seems to have cured
this. In any case, the maximum brightness possible from a
print illuminated by reflected light is a combination of the
reflectance of the substrate and the loss through the
emulsion with the addition of losses due to scattering from
other than glossy surfaces. The maximum brightness can be no
greater than the incident light. Because of this and because
the maximum black of the best paper on glossy surface still
has some residual reflection the contrast of a reflection
print is seriously limited. Because the light must pass
through the emulsion twice paper emulsions are usually very
thin in comparison to a negative emulsion. Nonetheless, the
Dmax of a glossy paper is usually still greater than can be
used. This can be proved by making a good print and looking
at by means of transmitted light. Usually one finds some
detail in the shadows that is not visible under normal,
reflected, illumination. In fact, a print rivaling a rear
illuminated transparency can be made by mounting a
reflection print in a light box where it is illuminated from
around the periphery by strong light which is prevented from
leaking out around the edges. The print must be especially
made, usually of considerably higher contrast and will look
very dark under reflected illumination. The appearance of a
print is a matter of the choice of the parts of the negative
that are to be printed in the range a reflection print can
reproduce. The range of brightness a negative can record is
substantially greater than a reflection print can handle.
Normally, some control is had through control of the
negative contrast but the eye tends to interpret tone
rendition as the rendition of the mid-gray scale and will
tolerate a lot of compression of both highlights and
shadows. If the mid-tones are not reasonably linear the eye
will interpret the image as either too high or too low in
contrast.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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