Technique on the eraser bichromatée.

Proceeded discovered by Poitevin in 1855, that deposited a patent.
Used from 1876 to 1920. Replaced then by the process gélatine/chloro-bromide of money
Named as" Coal ". The processes cousins of the eraser, used to this time, are gelatin with and without transfer, the process to the powders, Ozotype, to the fat inks, to oil, etc..
Principle: A colloid, (Name given to all substance that cannot be dialysée: Gelatin, eraser arabique, albumin, etc.) added of a bichromate, after drying, change under light and becomes insoluble.

Necessary products for the realization of tests in eraser bichromatée :
 Of the eraser arabique (in pieces, useless that she/it is purified).
 Of the potash bichromate (as orange crystals)
 Of formalin (to clarify the solution of eraser and to prevent mildew)
 Of the good quality paper.
 A pigment: (the LINHEL gouache gives the best results)

Preparation of the eraser solution :
 In a container in glass (pot of jam)
 Water distilled 100cc.
Eraser arabique 35g. (To put the eraser in a sock or a low in suspension)
 Pieces of sugar 2g.
To wait for about fifteen days that the eraser dissolves itself and becomes pasty. To press the sock to extract the eraser of it, the impurities must remain in the sock. To add 12 drops of formalin (3ml.). The solution is as good recent that old.

Solution of potash bichromate:
In the tepid water, to introduce 15% of bichromate. After dissolution, the bichromate no dissolves remains in the bottom of the solution. It is about a solution to saturation.

Choice of the paper :
All papers are good, but for some one must improve them by an encollage in order to get better results. For that to make, gélatiner needs the leaves. (To operate this encollage on several leaves in order to win of the time).
In the same way for other supports as wood, the glass, etc.
To foresee: leaves of gelatin (food leaves)
Acetic acid
Alum of potash
Starch of corn
In 200 cc of water, to put 2,5 g. of gelatin. To make melt gelatin while carrying this  solution to 40o, then to add 1cc of acetic acid and 40cc of a solution of potash alum to saturation.
It is possible to add a spoon of starch of corn, that will facilitate the étendage there and will return the surface of the mat paper.
With a large brush (Tail of cod), well to spread this solution in all senses, until the moment where the bubbles and the streaks will have disappeared completely. Let dry the leaf completely.

Preparation of the sensitive solution:
To prepare in a bowl: 10cc of bichromate
5cc of eraser
1cm of retiring gouache of the tube
Very mixed with a short and hard brush. He/it is possible for a better display, to add a milk of starch.
It is good to take the eraser with a pipette in the center of the jar, bus to this place erases it is purer.
Étendage of the solution:
To load a tail of cod slightly and to pass in all sense on the leaf that one wants to smear.
To finish with a dry and thin badger until the moment where the layer will be well regular.
This formula can agree for a realization in several successive layers, to carry up the density or to vary the colors in the shades. In this case, this first layer should contain little color, because only the half-hues will be definitive. In the contrary case, he/it would be able to he would produce some extortions and the grain would be coarse.
To let dry in the obscurity 1 hour.

Pull:
In a frame, behind a leaf of glass, to put the framed (set the margin) negative, then the sensitized paper. To cover with a felt. So possible to press the whole so that the negative and paper are well in contact in order to avoid fuzzy parts of the picture. It is possible to press the rear with a plywood, in this case to fix the negative and paper with the help of bugs, that would serve to a location for another layer.
It is possible to expose the picture in light of the day (but to the shade). For an industrial use, it is possible to use an U.V lamp. What permits a more precise exhibition.

Spoliation:
To immerse the test in a cold water pan (ever to use hot water);
To swing the pan slightly, and to change water from time to time. The picture won't linger to appear. More the spoliation will be led slowly, more matter will be fine.
To finish, to make sink in the margin an abundant spray of water; this one activates the spoliation in the blacks.
Finished once, to make dry in full day on tilted blotters to 45o about.
For the other layers: Eraser 5cc
Bichromate 5cc.
To put more pigment slightly.
To make dry the leaf.
The time of exhibition must decrease to every layer.
If it was necessary 60 minutes for the first layer, it will be necessary 45 of them only for the following and 5 for the last. This one will be able to be charged in color, because she/it must only give some blacks. And must be reserved absolutely for the end.

After complete drying in light of the day, the colors are not very quick because of the sallow hue of the bichromate.
He/it will be sufficient to revive the colors to plunge the test in a solution of sodium carbonate bisulfite to 5%. until complete elimination of the bichromate, then a washing of 5 minutes, and the test will find all his/her/its freshness.
It is possible to do a fast test of this process while using a white of .uf, a solution of bichromate and a pigment.
the test can be pulled on different supports: paper, canvas, wood, etc.
It is not necessary to discourage itself, but to persist, because the eraser as the other processes demand one knack and a good experience. Good courage! 

Haut de page

Michel Caron.