"Terry wrote
>To Gummists
I find that if one washes a gum print in gently running water for six hours, it
will clear to the effect that there is no discernible difference in colour
between that of the print and my undichromated test swatches.<
Pete talking
This is no doubt correct for your test swatches but we do not know what
colour they are,however the stain is still there whether it is apparent or
not and is a fundamental component of the imaging system and which is not
necessarily a bad thing, this is what gives the soft tonal range, other
wise we would be dealing with a lith type image,it is just this combination
of a hard and soft tonality in the same emulsion, that invokes the
intriguing character of the medium."
Terry talking
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. 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 as the gum is porous. We have the analogy of the
change in state of the silver salts in gelatine 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 gelatine. When the print is fixed and washed
only the colourless atom transferred from the chrome salt is left in the
gelatine. In neither the bromoil nor the the gum print, if they are exposed and
washed appropriately, is any colour left behind.
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.
I do not know what the mechanics are when the print is grossly overexposed as my
concern with these prints fades rather after I have put them in the waste paper
basket; ( any rescue missions not being worth the cost other than the ocasional
' I wonder what would happen if ?' or use as backing for the inking up of
bromoils. The change in colour of the gum in an overexposed print may be
because the gum has been made impermeable as well as insoluble so that surplus
salts cannot escape, or the structure of the gum has changed to the extent that
it refracts light differently. I do not intendf to pursue the matter unless one
day, in a flash of inspiration, I can see a practical application for the
knowledge. Someone else on the list may be able to answer.
Pete talking
Let me put this way 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 get slowly
warmer and darker in tone, with increasing exposure until a rich chocolate
brown colour is reached, when a kind of maximum black is reached,and
however one increases the exposure no further increase in density occurs
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.
Terry talking
But if you wash it the dichromate will disappear. This is what Mungo Ponton
found in 1839.
Pete talking
It is this image that underlies the pigment, however the the first tones on
the edge of underexposure can be lightened to a degree by using so called
clearing techniques, but as far as I know there is no method of actually
bleaching ,or dissolving this stain from the tones in the middle and shadow
regions.
Terry talking
Honestly Pete, water will wash it away. it is only the pigment in thediffernt
thicknesses of gum that should be making the image. In my early gum years I
used to use the stain as an extra colour until I found there were more stable
methods that did not involve the misuse of the chemicals.
Pete talking
To use the old saying a leopard cannot changes its spots but what can be
done is to arrange them in a more pleasing form.
this is just what the previously mentioned methods do, bisulphate or
metasulphate
will change the brown stain to a lovely soft green / blue grey, I have not
yet tried Terry extended wash but I seem to remember doing something
similar in the distant past by accident and got a result similar to the
metabisulphite method however I will try again, testing testing it never
ends !
Terry talking
All the sulphite does is speed up the process of clearing the dichromate but
with water you do not have to wash the water away he sulphite afterwards. My
tests consist of making a print and seeing if it works and if it does not
working out why.
Pete talking
It is not that I am green [ and who would say anything different in
these days of political correctness,] but lazy, my method is quick
and uses very little water. A 1% solution of sulphuric acid clears the
brown stain to a neutral grey, in two to three mins then a brief wash in a
dish, with three or four changes of water and *bingo* problem solved
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
I have said all this somewhere before, I am beginning to have some sympathy with
Judy it seems I have my head stuck down a well as well ;-)
Terry talking
My banner says 'No Nasties & No mysteries'.
Terry
PS I think that it would be helpful to copy this to the list. Would you like to
do it as it is your original.