RE: Renewed interest in gum processes

Hi John,
 
I am not sure if the process offers any advantage in terms of the final
image though. Sensitizing through immersion is easier for carbon than for
gum because, as we have mentioned in the past few weeks also, gelatin does
not dissolve in cold water but gum does. So if you do sensitizing of gum by
immersion, it must be done quickly and there shouldn't be too much
sensitizer remaining on the paper after the immersion, otherwise the gum
will start to dissolve and might be messy.
 
But I suppose after some trials and practices, an even sensitizing can be
obtained; but even then the advantage seems to be that you can prepare lots
of gum+pigment-coated paper in advance to be sentized only when needed. The
advantage seems to be in the preparation process rather than in the image
itself.
 
 
Dave S

  _____

From: John Grocott [mailto:john.grocott403@ntlworld.com]
Sent: Friday, April 14, 2006 3:13 PM
To: The List
Subject: Renewed interest in gum processes

Hi List folk,Just over a week back, it seems much longer due to the pile up
of postings, I submitted the following outline of this interesting
variation of gum printing. Having been inspired by the discussions and
practical information from many members, I , now,really must give this a go.
On studying the process it will be seen that the most significant difference
to the traditional gum technique is in the method of sensitizing which
reminds one of the carbon transfer method of sensitizing.

I must say that if the results are promising I might re- name the
''Bichromated Wash - drawing'', with a totally different label, after making
some variations and try to publicize it as something of my own
invention................or would this be considered unethical ( hijacking )
?

Thanks, to all concerned, for the encouragement in the recent debates.

As ever, John Grocott - Photographist - London.

SKYPE(video) name CARBONS999

................................................

>From ''Photography: Theory and Practice''

Chapter on Pigment Processes

by L.P.Clerc, Hon.F.R.P.S. Pub. 1954 (3rd edition) Edited by A. Kraszna -
Krausz

BICHROMATED WASH - DRAWING

''This interesting variation of the gum-bichromate process is particularly
suitable for obtaining large pictures, by printing under large negatives.

A drawing - paper is first sized with a very thin film of weak gelatine
solution. The beginner should choose a paper with fine grain. For this the
following mixture is used -

Crystalized sugar 4g

Soft photographic gelatine 4g

Water 100 ml

prepared by allowing the gelatine to swell in the sugar solution, and
dispersing it over a water bath below 115 F. The sizing is preferably done
in a room at a temperature of at least 70 F; the paper should then be at the
same temperature as that of the room. It is fixed with drawing pins to a
drawing - board, placed level and the gelatine solution poured on the centre
in the proportion of 1 ml for every 16 squ '' of surface to be covered. The
solution is spread with a swallow - tail brush, previously impregnated with
the gelatine solution, and squeezed out in its edges. The solution is driven
onto the pores of the paper by vigorous operation of the brush, spreading it
evenly until the sizing has a mat appearance. The sheet is then passed
through the steam from a pan of boiling water until the gelatine becomes
uniformly glossy. In the interval between the two sizings the brush is held
in the steam to prevent the gelatine in it from setting.

Now, put 1 g of degreased lamp black ( choose a black of brownish tint ) on
a piece of glass placing the powder in the form of a crater. Then pour into
this 2.5 ml of a 50% solution of gum arabic; mix thoroughly with a flexible
knife, adding gradually 2 ml of water. Grind with a glass muller for about
10 minutes, until the mixture becomes perfectly even. To cover a sheet of
paper 20'' X 16'' about 2 g of this paste is placed in a clean cup, and
about 0.75'' of the strip delivered by a tube of sepia water colour is
added. This gives a warmer tone, and at the same time, improves the adhesion
of the coating. Now add from 15 to 18 ml ( the smaller quantity in cold and
damp weather, the larger in warm and dry weather ) of the following mixture
prepared with boiling water -

Crystallized sugar 2 g

Starch 2 g

Water 100 ml

After mixing thoroughly, this paste is placed at the centre of the gelatine
coated sheet and spread with the swallow-tail brush previously charged with
water. When the colour begins to thicken, a flat goat- hair brush is used to
finish off the coating, the first stroke of this brush being at right angles
to the last stroke of the swallow - tail brush, so as to break up the
streaks. Continue in this way until the sheet is surface - dry, and then
leave to dry thoroughly by hanging from stretched cords.

The sensitizing is done, as required, by immersion in a 1% solution of
ammonium bichromate, with the addition of 1% of neutral sodium or potassium
citrate.

The negatives best suited for this method of printing have a density range
of about 1.3.

The exposure should be equal to that required to make a P.O.P print for
toning and fixing.

Before development, the print is immersed face downwards for about ten
minutes in a dish of water at 115 F, taking care to avoid adhering air
bubbles. As the water slowly cools down, the print is moved about from time
to time. The print is then fixed to a rigid support, sloped to about the
angle of a painter's easel, and development is begun by squirting the
surface with water at a temperature of about 18 F above that of the soaking
water. This may be done with a toilet spray or an air brush. The spray is
held from 12'' to 16'' from the print, and the water is squirted all over
the surface. The development may be localized as required by bringing the
vaporizer nearer. Development takes about 20 minutes for a surface 9'' X 5''
which has been correctly exposed. ''
Received on Sat Apr 15 20:06:27 2006