The production of a bichromated gum succeeds within narrow technical margins below or beyond which it is missed, for example when the exposure time is too short or too long.
The artist exploits the spaces between these margins to express his intentions. It also relies on different qualities of the materials it uses, such as paper, for example.
I noted a dozen parameters presenting margins of maneuver or differences of qualities that the gommist tries to control in a combined way to lead to a work.
It is well known that for pictorialist photographers, the bichromated gum process is not only a technique of photographic printing among others, nor is it only a means of imitating painting. Let us also note that it is not just a means of offering freedoms of interpretation in the photographic process.
When Demachy or Puyo describe in a chapter of the Art Processes in Photography entitled « Beauty of the pigmentary matter », how to obtain « transparency and depth, power vigor and intensity » or « fat blacks », « modeling » or « Soft degraded in the half-tints », they are not far from seeing in these processes, and gum in particular, a medium in its own right.

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The paper :
A thick grammage (200, 300g) allows good paper holding through multiple baths.
The choice of the grain makes it possible to play on the rendering: a fine grain, even satin, produces a detailed photograph and a towel can be chosen to make the masses of a landscape, the details being lost in the coarseness of the paper.
The regular paper textures resulting from a mechanical manufacturing process did not hinder Kühn to draw Lotte picking a flower (PHO 1984 26) for example, and Demachy made oil carryovers on laid paper (Paris PHO 1979 62 ), But ordinarily papers without regular textures are preferred.
Cotton papers tend to absorb the emulsion more than cellulose papers, resulting in a « burial » of the pigment and less pure whites. Unless these are desired effects, it is remedied by a paper sizing, usually gelatin.
The papers evoked by Demachy and Puyo in The Processes of Art in Photography p. 53 (Tochon-Lepage, Cassagne, Lalanne …) seem not to be manufactured in our time.

Most often made from animal gelatin, it allows the sensitive emulsion to remain on the surface. The commercial papers (in our time as before) are for the most part already glued.
A good gluing increases the fineness of the details of a photograph, preventing the emulsion from diffusing into the fibers of the paper. For this purpose, it is possible to re-glue a paper which has been glued to a certain extent. However, diffusion of the pigment into the fibers may be aesthetically attractive.
The sizing processes practiced in the papermaking factories are not disclosed (no more now than in the 19th century) and these degrees of absorption are appreciated.

The sensitive emulsion:
It is composed of gum arabic, potassium dichromate and pigment.
The process is based on the discovery, perhaps by Poitevin, that a colloid becomes tanned to light if it is added to certain metal salts such as potassium dichromate, for example.
This sensitive mixture, which is spread on the sheet of paper, presents the appearance of a more or less liquid varnish. The display of the mixture is the most technical gesture of the gum draw and the one that leaves the least room for subjectivity: it is a question of obtaining the thinnest layer possible, in a fastest time possible Because otherwise the gum takes it and it fluff. This gesture takes into account the viscosity of the gum, the ambient temperature and even the nature of the pigment mixed therein (for example, smoke black tends to dry the gum faster). Up to three brushes of different fineness can be used successively during the minute that the operation lasts.
If we make a black and white, the result of the mixture spread out is a dark gray, a semitone to a tone below the black.
The emulsion is sensitive to light only when dry and must be used within 24 hours; Beyond it it loses in sensibility.
The « French-style » pictorialists prefer to perform the test at once, striving to obtain the nuances between the deep blacks and the pure whites (the half-tints) in a single draw. Demachy / Puyo are quite critical of the gommists who take the proof several times, that is to say who get the half-tints by superimposing several pale exposures on the same paper. (Some contemporary gommists repeat up to 11 times the procedure layer sensitive to exposure under the negative of scraping to drying.) Puyo dances the jig in his tomb at Morlaix.
Gum arabic can be concentrated between 30 and 50%. More concentrated still, it becomes difficult to spread. However, the more rich it is, the more it retains the pigments at the time of skinning, giving the fat and dear blacks dear to Puyo; The more the artist will have material at hand for his interpretation. Conversely, a less concentrated gum will give paler grays and more detail but these will tend to disappear at the slightest intervention during the skinning.
The gum is acidified after a few days and becomes more spinning, which increases its ductility; Thus making it easier to spread and more flexible to skinning. To prevent it from rotting, a drop of formalin or some salicylic acid is added.
The bichromate of potassium causes the colloid (the gum here) to tan under the action of the ultraviolet. The bichromate crystals dissolve about 14% at a temperature of 20 ° (ie 140 g per 1 liter of water). If the artist’s studio is cold, the dichromate is recrystallized and its concentration decreases. So I missed a few eras in the fall before I knew what was going on.
The proportion of dichromate in the mixture is 1/3 to 1/2 with respect to the amount of gum, depending on the desired effect. More dichromate will allow shorter exposure and more contrasting images, but, by the way, less nuanced.
To cover an A4 sheet, the proportions can be 1 g of dichromate per 3 g of gum and 0.03 g of black of smoke.
All pigments can be used although Demachy and Puyo usually prefer those which are soluble in water, such as black smoke. The ivory black (calcined bone), which is sparingly soluble, gives, however, to my taste, interesting effects. I sometimes even use bronze powder to warm the blacks; It is sufficient to spread the emulsion rapidly so that the pigment powder remains in suspension, mixed in the gum.
Destitute of precision balance, the pictorialists measured the quantities of pigment in judged, often using the colors out of the tubes of watercolor.

The negative:
Better it be sweet. If Puyo says it is too « clashing », the time it takes for details to appear in the highlights (the dark parts of the negative) will cause the dark parts of the image (produced through the light Negative) become too exposed, will become difficult to dissolve.
The thickness of the glass plate of the negative, added to the thickness of the glass of the chassis-press (up to 4mm) can obstruct the UV and force to increase the exposure time. (The negatives that I obtain by digital printer on transparent pvc support of 140 microns require 22 minutes of exposure, against 12 minutes for supports of 100 microns).
Demachy / Puyo envisage the hypothesis of scratching or coloring a negative to modify the image, but on this point they send the photographer « to his conscience ».

The exhibition :
There is a minimum exposure time below which the emulsion does not cure in a desired manner and beyond which it hardens too much, becoming difficult to soluble in the stripping.
This exposure time is defined by the artistic will to obtain a cast or uncut image, contrasted or diffuse, and also takes into account the concentration of the gum and dichromate, the color and the type of the pigment or the way Which one wants the image, subsequently, to deprive quickly or slowly.
The sun illuminating differently in the year and even in the day, one used to measure the time of exposure by means of a tongue coated with dichromate which was exposed gradually to the sun at the same time as the chassis-press Containing the proof. The bichromate browns in the sun to a maximum (brown caramel); When this maximum is reached, an additional part of the tongue is exposed, and so on. A complete browning of a portion of the paper tongue is called an « Artigue degree ». Thus, if we had seen that a combination gum-bichromate-pigment-negative-chassis made an acceptable image after three degrees Artigue, we had a reference to work in summer as in winter (the three degrees being browned in 6mn under a light Sun in June, and in almost 2 hours under a December sky).

Counting :
A rather frenetic and creative moment where the operator exploits the multiple properties of water: its temperature, thickness, fineness, strength, softness or strength, to which it can add brush strokes and even spikes Of wood, for example to place a burst of light in a glance.
The test is first soaked in a more or less warm water to be softened, then it is fixed on an inclined plane. Water streams of varying degrees of thickness, more or less warm and more or less high, are then poured into different places. E bottle or a fine spout. The test can be turned upside-down or diagonally so that the water only impacts the chosen places. The proof can be rewound (up to a point beyond which the image diffuses) or on the contrary let it dry a bit to work more precisely details.
Touching with a brush, by which the layer of pigmented gum is more strongly removed, tends to disorganize the structure of the pigments artificially and is therefore too visible to Demachy / puyo, which the modifications effected by Water alone.

Drying :
It must be checked. It can be accelerated by placing the test on a dry blotter and facing a soft fan, or slow it down to accentuate the famous « sunk » dear to the pictorialists, this time placing the test on a damp blotter.
I happened to lose a nice image to the drying because the fog had fallen at night on my unheated workshop: the humidity had made all the details get confused.

The last baths:
After taking care to expose the image dried for the last time to UV, to harden the residues of gum if necessary, it can be soaked in a bath of alum of potash which will eventually tanner or sulphite of soda The paper of the last traces of dichromate.
Personally, I dive the test in a bath of baking soda or it gains a little sharpness and clears a quarter of a diaphragm. I take the opportunity to retouch some areas if necessary and when it is still possible, because despite the humidity of the bath the gum has become practically unalterable.

Finally, polishing the test with a mixture of white wax and benzene makes it possible to restore « fat » to « buried » images. Demachy / Puyo sometimes used it.
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Out of a hundred eras that I made this year, I keep less than ten, others are either technically missed (gum rotated, dichromate too little concentrated due to the cold shop, emulsion poorly spread, bubbles to the counting , Fog …), or artistically unpleasant.
The wonders of the Musée d’Orsay by Kühn, Steichen, Puyo or Demachy demonstrate not only their technical skill but also the artistic possibilities allowed by this medium when manipulated by their care.