In order to Compress the Values of your digital images as shown
above accurately, you will need some place to start! You first need to test your minimum exposure time with your particular
gum printing setup. This is the shortest time it takes to fully expose a layer
of gum bichromate emulsion after UV light passes through a clear part your
transparency film. We will use this determined time as a starting point for all subsequent
testing and printing.
>> To begin, you need to coat a prepared sheet of paper with a
typical layer of gum emulsion. Typical meaning exactly
as you would for a normal image. I have always used Charcoal Gray pigment for
my tests.
(Other pigments may in fact print with different results and you can
test those later if you like, but for now, this will do)
>> I then make a file in Photoshop with bands of 0%, 10% and 20%
and print it on a transparency sheet.
(In
this example I also printed 100%, 90% and 80% looking for a maximum
exposure time, although it is not really needed).
It is not
completely necessary to print these bands of value, it just helps to
judge what your minimum exposure time will be at the end of this
step.
>> Then I cut a strip of the gum emulsion coated
paper at that width and set the remainder aside (in the dark) for later.
>> Tape the transparency (with or without
bands of value) and paper down to the glass so they do not move
around during testing, disturbing your results. Then use a ruler to mark off
half inch or 1 inch segments to the side with a pencil. Each segment
will be exposed with an additional amount of time.
>> You will need two pieces of cardboard
(I use the backs of the water color paper pads) One piece will go
under the glass above the negative and print, aligned with one of
your measured pencil markings. Call this piece of cardboard Piece A.
>> The other card board will be used
like a shutter, completely blocking the light until you are ready to
expose.
You will place this Piece B on
top of the glass. Slide the glass, cardboard, print, and all beneath
the exposure unit.
>> Get your timer ready and set it to 15
or 30 seconds, or whichever time interval you choose. I am using 30
second intervals in this example.
Flip your UV light on, keeping Piece B in place for the moment.
Piece B will block the light as the lights flicker on for a
second or two.
>> Once the lights are shining steadily after a second or
so, start your timer and remove Piece B simultaneously. When your
timer finishes, turn your light source off as quickly as possible.
>> Open your glass and shift
the edge Piece A
down to the next pencil marking on your paper.
Repeat the process exposing each
increment of the paper strip with an additional 30 seconds (or
whichever increment you are using) until you are finished.
>>
Finally, expose a strip for 10 minutes on the side using piece A under the glass to
reveal only about a half inch of the emulsion. (You can see this
totally exposed strip at the top of the image to the right.)
Develop that test strip normally, let it dry and you should end
up with something like the image to the right.
>> To see the relative values better,
scan the dried minimum exposure test on a flatbed scanner
and load it into Photoshop.
Go to Image>Adjust>Levels. Drag the
shadow point to the very bottom of the histogram values, and drag
the highlight point to the very top. This helps to clarify what we
see in the test print.
>> This is what the scanned print with adjusted levels looks
like. We want to find out the shortest exposure time that will
completely expose our emulsion to a solid color.
In my example to the right it looks like 3.5 or 4 minutes is
where my minimum exposure time falls.
To be safe, and for ease of
memory, I simply choose 4 minutes to be my minimum exposure time, based on this test.
