Now to establish the physical parameters of these tests .I decided to
employ only colloid's with no pigment present, so that the pigment would
not cloud the issue. The colloid, paper and dichromate being the main
study.
The colloid's chosen where, gum arabic[W&N] ,Gloy[PVA],and Egg[Fotempera].
these colloid's where mixed to standard proportions, with a saturated
solution of Amm,Dichromate.
The papers chosen were, Rives BFK, Fabriano Artistico fina, and Synteape plastic
paper. the Rives was chosen as a favourite for gum printing, the Fabriano
as my favourite for most Photo -Alt processes , and the Synteape as an
impervious substrate compared to the other two .The papers did not receive
any additional sizing, and were coated with a 4inch plastic foam roller
Controlled exposures were given using my exposure system employing a
Starna metal halide lamp 400 watts @ 20inches. When printing Fotempera this
system gives me an exposure of 45secs minimum to 3mins maximum 90secs being
the norm.
Unfortunately I do not own or have access to a reflection densitometer, so
a comparative visual method was used, to measure the staining. this system
was based on the use of a calibrated Kodak reflection step wedge .the
samples were
punched with an office punch to give a hole in the centre of the sample
then this sample with hole was run along the wedge to find a match . A
crude method but reasonably effective for the purpose of these tests.
The tests had five (5) distinct stages which are as follows ;--
(1) --: to find the darkest value through exposure
(2) --: to relate this value to a Stouffer wedge to find the maximum tonal range
(3) --: to remove the stain by a number of differing methods then compare
results
(4) --: to repeat (3) at normal exposure times
(5) --:to compare the results of (3) to that of (4).
Stage One
First thing was to establish how much darkening could be caused by exposure
to the dichromated colloid . A series of doubling exposures were given to
the three
substrates in the classic photographic manner. with each colloid, in turn.ie
1+1+2+4+8+16 mins Nine test's in all.The maximum exposure being 32 mins
the two papers went almost black on print out, but when soaked for 5 mins
reached a
max of Comparative reflection density [CRD ].of 1.00. the plastic only
reached 0.50
Each of the colloid's behaved in the same manner darkening at the same rate
but the colour differed.
The egg had a rich red tone
The gum had a slight green tone
The gloy had a slight brown tone
Although the maximum exposure was 32 mins, the maximum required to reach a
maximum brown was only 10 mins, I found it impossible to reach a dark brown
on the Synteape with any of the colloid's.
Stage Two
At this stage I decided to take the Synteape out of the test as the
staining was comparatively minor and could be removed by a very short
wash.This might be why
I get such clean colour on this base .
Now was the time to relate the stain to the exposure of a normal negative a
Stouffer wedge was used to simulate this.I found with further tests that an
exposure
of approx 7mins would give a range of tones, showing 15 steps on the scale,
and the max density was 0.70. ie mid grey.This time seemed constant for all
of the tests
Stage Three
It was now time to do the actual clearing tests I used
a) 5% sol pot alum 2 hours
b)5% sol sod metabisulfite 2 to 5mins
c) 1% sol sulphuric acid 2 to 5mins
d) long soak 12 hours
I cut a small sample's of each paper tested in stage two with the three
colloid's printed down on each paper, twenty four tests in all.
The results for (a) was a minimum of 0.10 on the Fabriano all three
colloid's were the same.The Rives held more stain at 0.15 .The results for
(b) was a minimum of 0.10 with the gloy 0.15 with the gum and 0.30 with
the egg, on the Fabriano .The Rives behaved in the same manner
The interesting feature of this test was the colour a beautiful blue/green/grey
colour.
The results for (c) on Fabriano all three colloid's were the same
but the colour was different , gloy yellow, gum neutral grey, egg, slightly
green,
the value was 0.050. the results on Rives was the same,these readings are
the minimum for this stage tests.
The results for (d) were for Fabriano all three colloid's were the same 0.30
the readings on Rives were gloy 0.20, gum 0.30, egg 0.50, .There was also
a change in colour gloy yellow grey. gum yellow brown, egg red brown.
these readings were the maximum for this stage tests .
Stage Four
All the tests conducted in stage three were now repeated at normal exposure
times
of 3mins, 90secs. and 45secs. Each of the four methods reduced the maximum
density to 0.10 and below, tone however was still visible.
Stage Five
The main difference between stage two and three was the the degree of stain
caused by the length of exposure as density increased to a maximum of 0.70
the stain became more difficult to clear the long soak became least
efficient, on the other hand the 1%sulphuric acid worked quickly and
efficiently at all densities.
General comments
I think one of the most interesting thing to come out of these tests is the
surprising long tonal scale possible with just dichromate alone, one would
only have to add a
very small amount of pigment of a complementary hue such as say, ultra
marine blue to achieve a full tonal scale in one hit. Another interesting
fact is that the clearing bath could be used to control both density and
contrast. This tonal variation could go a long way to explaining the
apparent fickleness of the processes.The colloids worked in a very similar
manner as did the two real papers
the similarities were far greater than the differences, it was only the
Synteape which seemed to have a life of its own.
pete