From: Sandy King (sanking@clemson.edu)
Date: 08/24/02-02:24:22 PM Z
Contrast control with Van Dyke
Although the Van Dyke process is one of the simplest processes around
and gives good results with a wide variety of papers I have been
quite vexed by an inability to control contrast. The problem is that
the classic Van Dyke formula, when mixed as described in most of the
literature, has a very long exposure scale that requires a negative
with a density range of well over 2.0.
Two methods of contrast control are described in the literature: 1)
mix a few drops of a dilute dichromate solution with the sensitizer
and 2) add dichromate to the first washing bath. I have tried both
of these methods and found that neither is effective. Adding the
dichromate to the sensitizer before coating appears to work, but when
one examines the scale of a print made from a step wedge it become
immediately apparent that the addition of dichromate serves only to
decrease sensitivity, with the exposure scale remaining the same. And
adding dichromate to the first water bath in my tests served no
purpose at all.
So what is one to do? OK, thanks to an initial suggestion from Liam
Lawless that the use of ferric citrate in combination with ferric
ammonium citrate could increase contrast from 3-4 steps I began
experimenting with various mixtures of the two citrates. I have
finally developed something of a method for controlling contrast. The
key is a magic solution which I will call Van Dyke B, which when
mixed in different proportions with the classic Van Dyke formula,
hereafter referenced as Van Dyke A, will increase contrast from 2-5
steps over what one gets with the classic formula, if used by itself.
With this method there is no loss of printing speed (as measured by
the first maximum density on the step wedge print, or Dmax), and as a
bonus there appears to be a slight increase in maximum density, or
Dmax.
Sounds great, you say. And it is, but there is the issue of getting
ferric citrate into solution. Turns out that it is very difficult to
get ferric citrate to go into solution. Based on advice provided by
Philippe Monnoyer I tried adding acetic acid, citric acid and sodium
citrate to the ferric citrate solution in an effort to get it to go
into solution. Turns out this works, though it is still a slow
process. Philippe describes the resulting solution in this way.
"At this point you have a mix of
-sodium ferric citrate
-sodium ferric acetate
-acetic acid
-citric acid (reaction of acetic acid + citrate)
but all these species are separated in ions in the solution. They are Na(+),
Fe(3+), citrate(3-), acetate(-), H(+)
The color of a salt solution depends on the presence of other species in the
same solution. It make complexes that absord the light differently. For
example, a solution of iodine crystals in water or in acetone won't give the
same color. But it's still iodine.
For your process here, what matters is that you have both citrate(-) and
iron(3+) present. The citrate (or oxalate) ions are the ligth sensitive
part. Iron is the species that will take electrons from the citrate and give
it back to the nobel metal, until it is a "black" metal. The smaller the
metal particles, the warmer the tone. In chrysotype, it is even red and
blue. RH% during exposure determines the size of the particles, and the
tone. More RH% gives a faster reaction and bigger particles, and finally a
more neutral tone."
So how to proceed? First, I will assume that you already know how to
mix the classic A-B-C Van Dyke formula. If not, here are instructions.
A. 35ml of water
10g ferric ammonium citrate
B. 35ml of water
1.5g of tartaric acid
C. 35ml of water
4g silver nitrate
Mix A, B, and C separately and allow to dissolve completely. Then add
B to A, then C (slowly) to the mixture of A and B.
The Van Dyke B solution is mixed as above except for solution A. The
basic formula is:
A. 35ml water
10g ferric citrate
10 g citric acid
2.5g sodium citrate
B. 35ml water
4g tartaric acid
B. 35ml water
5g silver nitrate
Here, instead of the 10g of ferric ammonium citrate we use ferric
citrate, to which we add citric acid and sodium citrate. The
procedure is as follows.
1. First, add about 10ml of water to 10g of ferric citrate in a
pestle and grind for a couple of minutes. Then add 25ml more of
water; transfer the solution to a beaker and heat in a microwave
until the mixture boils.
2. Next, add 10g of citric acid to the above mix, stir, and heat
again in the microwave until the solution begins to boil.
3. Finally, add 5g of sodium citrate to the above solution, stir, and
then heat again in the microwave.
4. Mix 4g of tartaric acid with 35ml of water, and when dissolved,
add to the above solution.
5. Mix 5g of silver nitrate with 35ml of water and add, slowly while
stirring, to the above solution.
Allow the solution to stay overnight before using.
Contrast Control by mixture of Van Dyke A and Van Dyke B
10 parts VD A 17 steps, Exposure Scale = 2.55
9 parts VD A + 1 part VD B 15 steps, Exposure Scale = 2.25
8 parts VD A + 2 parts VD B 13 steps, Exposure Scale = 1.95
5 parts VD A + 5 parts VD B 12, steps, Exposure Scale = 1.80
2 parts VD A + 8 parts VD B 12 steps, Exposure Scale = 1.80
(With some loss of printing speed.)
10 parts VD B 12 steps, Exposure Scale = 1.80
(And more loss of printing speed.)
As will be apparent from the above the maximum contrast increase is
found by a mixture
Of approximately equal parts of VD A + VD B. Adding more VD B at this
point does not increase contrast but does make printing times longer.
I have noticed that increasing the amount of sodium citrate in the A
solution from 2.5g to 5.0g increases contrast by about another two
steps, or down to about an exposure scale of 1.50 when VD A and VD B
are mixed in equal amounts.
This may sound complicated but in fact the procedures are fairly
simple once you have mixed the Van Dyke B solution.
Sandy King
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