Sandy King (sanking@hubcap.clemson.edu)
Sun, 03 Jan 1999 13:56:24 -0400
>On Sat, 2 Jan 1999, William Laven wrote:
>> .... if we found the difference to be only
>> 10%, I'd write it off to other variables, but when the difference is so
>> dramatic (in both exposure times and contrast) then the clear thing that's
>> making the difference is the spectrum of the bulbs, not the differences in
>> bulb age or possibly minor differences in same-wattage ballasts.
>
>However, a different emulsion, even nominally of a "platinum" print may
>well have a different sensitivity. For example, we know from tests a
>couple of years ago, that carbon sensitivity is different from gum, even
>though both use ammonium dichromate. Sandy (as I recall) did tests with
>carbon where a daylight fluorescent improved contrast, but in my tests
>with gum the same bulbs were 3 times slower & much too contrasty.
>
>Judy Seigel
I am very interested in hearing of reports such as those of Bill Laven
regarding the use of light sources other than the 350 nanometer BL tubes
for alternative printing. As has been previously pointed out, there may be
other variables, including those of emulsion differences, which account
for the significant difference in exposure times between the 420 super
actinic bulbs and the 350 nanometer bulbs of Bill's friend. I know for
certain that with carbon priting there are several variables which could
cause a difference in apparent emulsion sensitvity of one or even greater
steps, independent of the light source.
It is true that several years ago I carried out some tests on emulsion
sensitivity of carbon tissue with several different light sources,
including BL, GE Cool White and GE Daylight tubes. What I found was that
with an appropriate strength sensitizer the GE Daylight tubes (which peak
somewhere over 500 nanometers) were on the whole as efficient as BL tubes
in exposing for carbon. I have since returned to use of BL tubes because I
have a 15 year data base of exposure information related to their use which
is basically useless with the Daylight tubes (for making reprints, etc).
Nonetheless, my successful use of the Daylight tubes leads me to believe
that overall the best source (weighing image contrast against sensitivity)
for carbon printing (and possibly for gum printing) would be a bulb in the
400 nanometer range.
Regarding the sensitivity of dichromated colloids there are points of
information which may be of interest to some persons on the list.
a) Research indicates that the sensitivity of dichromated colloids
(gelatin, gum, fish glue, etc.) is influenced more by the spectral
absorption of colloids than by the nature of the colloid. That is, the
radiation absorbed is actually what determines the eventual hardening
(sensitivity), of the colloid. In turn, absorption is largely determined
more by the dichromate, and its concentration (which absorb ultraviolet,
violet and blue radiation), than by the colloid.
b) Dichromated colloids have their greatest sensitivity at about 210
nanometers and are considered totally insensitive above 600 nanometers.
However, glass absorbs virtually all of the radiation below 300 nanometers,
and the actual practical sensitivity has been found to be between 350 and
450 nanometers, with a maximum at 360 nanometers. Within this range it will
be found that contrast will range from a minimum at 350 nanometers to a
maximum at 450 nanometers.
c) The concentration of dichromate in the sensitizing solution has a very
major influence on sensitivity. Tests have shown that sensitivity increases
almost proportionally with the dichromate concentration, that is, a 2%
sensitizer will be twice as sensitive as one of 1% sensitizer, which in
turn will be twice as sensitive as one of 1/2%, and so on.
>From the above I conclude that the difference in sensitivity previously
observed between different dichromated colloid systems such as carbon and
gum, using similar light sources, can be explained by other variables
(concentration of sensitizer (*and accuracy of measurment of same*, age of
materials, etc.) other than the system itself. Of these, the most
significant would seem to be the actual strength of the sensitizer.
Sandy King
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