>The mechanics of the reaction are that there is a transfer of an atom from the
chrome salt, which is broken down by the energy of light, to the long chain
molecule of the gum and that causes the molecule to stiffen and the gum to
become insoluble in water. As far as I know that transfer involves no change in
colour. Once the transfer has taken place all the remaining chrome salt can be
dissolved and washed away<
>This is my understanding of the process arrived at after discussions with those
who have made their careers in the chemistry of colloids but expressed in a way
that I can understand so that I may pass the information on to those in my
workshops, and so that I think I know what I am doing.<
Pete talking
Although I have a basic understanding of chemistry relating specifically
to the photographic processes, it does not extend to a chemical
understanding of the inner mechanics of the dichromated colloid, and
further more I do not have your advantage of help in understanding, by
friendly chemists. therefore I cannot make informed comment, on their
findings .
However I do now what I observe, so please forgive me for
repeating and extending my previous statement, but I have conducted
numerous tests over the years to determine what ie going on in respect to
Dichromate stain, which is in fact an image forming mechanism.
If an emulsion consisting of gum and Dichromate alone with no pigment
included is exposed in step wedge fashion, the first discernible tone
will be a light beige colour, this colour will slowly change to a warmer
and darker tone, with increasing stepped exposure to light, until a rich
chocolate brown colour is reached, and however one increases the exposure
no further increase in density will occur for the system has reached a
maximum brown and stabilises. An image which is very beautiful, I start
with this exercise with my students when we enter the world of the
dichromated colloid.
Now this image can be lightened and the colour changed by placing it in a
bath of the following solutions ;-
a) 5% solution of bisulphite
b) 5% solution of metabisulfite
c) 5% solution of pot alum
d) 2%solution Ilford acid hardener
e) 1%solution of sulphuric acid
Other methods that I have yet to test
f) extended wash or soak { Judy 1 hour } { Luis 2 hours } {Terry 6 hours +}
g) Sodium dithionite {Philip Jackson }
When the above are tested I will post to the list but I suspect that all
of these methods will be very similar
What happens in (abed) is the brown coloured image changes to a green/ blue
grey pastel tint the highlight tones lose their colour and become
practically invisible, the middle tones lighten slightly,
shadow tones are fully visible and are impermeable . However, e) converts
the brown stain to a neutral grey but follows (abed) in all other
respects.
Terry talking
>Honestly Pete, water will wash it away. it is only the pigment in the different
thicknesses of gum that should be making the image.<
>Surely it is the hardness and the softness that give the tonal range and they
are the result of the reaction of the Dichromate and the gum to light.<
Pete talking
I do not doubt your sincerity Terry, it is just that you do not share my
view of the nature of the gum image.In my* opinion * there are three
distinct imaging mechanisms at work simultaneously.This could account for
the" will of the wisp"nature of the process.
Firstly the Hardening action of the the
dichromated colloid. This is the basis of myriad photo- alt and photo-
mechanical processes, and can be seen as a simple mechanism, whereby the
colloid is made insoluble by the action of light, this is a very contrasty
image lacking tonal value in a thin film image such as direct carbon [a
Luis Nadeau term ] there is image, or there is not image, in a similar
manner to a high contrast photographic emulsion such as[ although the
imaging mechanism is of course totally different ] lith. This emulsion
layer is almost uniform with probably a slight variation in the highlight
tones,generaly different thickness in the emulsion layer do not exist, as
they do in in say a carbon transfer image.
Secondly the stain or chrome oxidation layer which gives a soft contrast
short scale tonality the contrast of this image can be modified, by adding
pigment to the emulsion and carefully adjusting the pigment to colloid
ratio this is why it is possible to achieve a reasonable tonal range gum
print in one exposure although this balance is only achieved by precise
measurement of colloid to pigment quantities, as the amount of pigment is
increased the contrast goes up until we reach the lith like quality of the
prime imaging processes.
Finally the third and most difficult to detect is the effect where the
paper plus sizing layer can have an effect on the tonality of this
combination and can give a Fresson type tonality, where the paper fibre
gives a graphic random screen quality, I am now straying beyond the remit
of this topic , so I will stop here.
Terry talking
>We have the analogy of the change in state of the silver salts in gelatin
>where the remaining salts are washed away after the light has done its job and
>the film has been developed.In a bromoil print we bleach the silver and wash
>it away, again using chrome salts to change the state of the gelatin. When the
>print is fixed and washed
>only the colourless atom transferred from the chrome salt is left in the
gelatin. In neither the bromoil nor the the gum print, if they are exposed and
washed appropriately, is any colour left behind.<
Pete talking
With due respect I am not talking about a silver- salt suspended in
gelatin, there may indeed be similar mechanisms at work, but a dichromated
colloid such as in gum is a different case to an emulsion such as gelatin.
Terry talking
Knowing what a mess even a 0.5 % solution of HCl can make of platinum prints I
would hesitate in putting a print in a 1% solution of H2SO4. Which makes me
wonder about the H2SO4 in the acid amidol developer.
Pete talking
Again what has platinum got to do with it, Remember you asked Bas van Velzen
in Paris when we were having our meal in the cafe whether he thought my use of
1% H2SO4 as a clearing bath was safe, he replied that as long the print
was washed with several changes of water no harm should result from the use
of this solution
Terry talking
My banner says 'No Nasties & No mysteries'.
Pete talking
I agree mines" lets work together "
Well thats it mate for time being
pete