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Carbon printing is a photographic process the foundation of which was laid in 1855 by Alphonse Poitevin.
The process relies upon the ability of an organic material (gelatin) to become insoluble in water when sensitized with dichromates and exposed to sunlight. A substrate (carbon tissue) is coated with a layer of gelatin mixed with pigment, and sensitized with potassium dichromate. A contact print is made under a large negative. The gelatin layer is then transferred to a carrier, and developped.
This development is done by rinsing away the gelatin that was not hardened by light. The result is a relief layer of gelatin, showing the full tonal scale of the negative, from paper white to maximum black, on the final paper support.
Carbon printing is one of the few processes rendering all the details of the negative in the final print.
Explanations of the above will be based on texts, pictures and drawings. Don't be impressed by the sometimes complicated equipment I built for this; everything can be carried out in a simple way, using simple tools. I have been working for months on the whole process, but considerable time was spent on the more or less successful building of several pieces of equipment. The taking of pictures, their digital processing and several drawings also took quite some time.
I did thoroughly enjoy this, and am very pleased to share my findings with all those interested in the process. Questions are welcome at any time, and I'll try to answer them to the best of my abilities.
The process will be explained in seven stages, each of them being presented with its own description, formulas, pictures and drawings.
The seven stages are:

1. making a large negative
2. sizing watercolour paper
3. mixing and coating a pigmented gelatin layer
4. sensitizing
5. exposure
6. development
7. drying

In the early days of carbon printing, large negatives were not a problem, as the cameras in use were mostly large format.
Currently, large negatives can be computer made, but I still have to see the large digital negative fitting my needs for various alternative processes and keeping their cost (printer, inks) at a reasonnable level.

The urge for sticking as much as possible with the traditional methods was so powerful, that I decided to build my own 4'x5' camera.
(fig. 1)

In order to be able to make even larger negatives, I bought some old cameras on used equipment markets. I refurbished these cameras and adapted their film holders to the modern sheet films
(fig. 2 and 3)

Using these cameras is the easiest way to get a large negative; let's see the hardest way now..
After some years of activity, every amateur photographer is owning a serious number of negatives, either 24x36mm or 6x6cm. How to make large negatives from these small ones is shown below.

Under my camera and its bellows, I built a sliding support where bellows and negative holder can be mounted solidly and free from any vibrations. (picture #4).

I completed the system with a small lightbox and with a negative holder that can be moved in order to allow pictures to be taken at 1x1 ratio.


Using this lightbox ensures that the light intensity always is the same, and therefore also the corresponding exposure time, as long as the density of the negatives remains the same. (see sketch 1)

To record the positive, I use Rollei Pan 25 film (25 ISO). After some testing with a step tablet ( Stouffer ) it is possible to determine the exact exposure which will produce a good positive showing full details in the shadows as well as in the highlights, provided they were in the original negative.
The film is processed in ROLLEI RHS developer, diluted 1+7, during 5min. 30sec. at 20°C. That is about 10% more than the manufacturer's recommendations, in order to get full details in the whole negative. These positives are then enlarged under the enlarger, in just the same way than making a paper print. The only difference is that the film is exposed on a black background rather than a white one.
Some years ago, continuous tone films similar to the variable contrast Gevarex were still available. These are gone now. It was possible to manipulate them under red safelight. The currently available orthochromatic films should be adequate for this use too, but until now I haven't done any testing with them. I use ADOX CHS (25 ISO) panchromatic film for making my large negatives. This film is a normal film for picture taking purposes, and therefore rather fast. So I had to equip my enlarger with a shutter between negative and lens, as very short exposure times are necessary (pictures # 5, 6 and 7).

Every single light leak has to be eliminated on the enlarger, as the film has to be manipulated in complete darkness. I use to process these large negatives in trays, just as prints; but sometimes I use a self-built container which makes it possible to process several negatives together (see picture # 8).

Contrast can be steered by adapting the developer concentration and development time. It is recommended to use a printing frame, preferably equiped with registering pins, on the baseboard of the enlarger for exact positioning of the film, as all the work has to be done in complete darkness.

I built two such frames, one being somewhat more sophisticated than the other. Sketch 2 shows the first one, which has a base that can be fixed with pins to the enlarger's baseboard. This base is fitted with four registering pins for the positioning of masks and films. A pivoting glass plate provides the necessary pressure on the whole.

Unlike photographic paper, film has to be exposed on a black background. The usual white base of most enlarging easels is inadequate. Finally, I have cut various masks corresponding to the film formats I use (see picture # 9). These accessories make it a snap to position precisely the film under the enlarger.

Finally, I keep the exposed films in a light-tight drawer while working. The second, somewhat simpler frame basically is a board on which four registering pins were glued. The surface was painted black. For focusing, I use a white sheet of pvc that fits on the registering pins.

Depending on its format, the large negative is placed in the corresponding mask, which then is placed on top of the surface to be exposed for contact printing. The masks were cut in 0,2mm thick aluminum sheets that I got from a printer.

Remark: The support papers displayed in picture #9b were sized with Gesso, which guarantees good flatness. As for the carbon tissue, I do often use clear pvc sheets sanded on one face to make it matte, instead of paper. Pvc has the advantage of being indefinetly reusable, and of staying perfectly flat.