Wednesday, June 19, 2013

How did Talbot take the negatives?

The first ancestor photographic process that I have tried was the daguerreotype. With some success, disregarding my bad method of coating copper plates with silver metal, which leaves, after some time, the entire surface divided in small fractions, although the image persists. I learned about the process, but if I wished to improve my photos, I would need to invest more time and money to get the right tools and materials, and not just 'scratching' as I did.

The second historical process that I am trying since a few days is the salt print, invented by Talbot to make the copies of the negatives. My first results have been very promising and easy to obtain. The process is particularly simple, you just need things easy to find as some tools and trays, it is in a very closed relation to modern paper photography. The main difference is that it doesn't take advantage of the latent image, it uses only the image formed gradually by the incident light, which converts the silver halide in silver. This may take a few minutes, the process is not therefore suitable for taking pictures with a camera in fractions of a second, or in a few seconds, like with the daguerreotypes.


The above picture is an example of salt print, with a small difference, instead of common salt, sodium chloride, I used the potassium bromide, which works the same way. After several minutes of exposure to light, the paper is well washed and then fixed in common fixer. In this very early stage of photography, Talbot used as fixer common table salt in a warm and concentrated bath.

When making salt or bromide prints like this, the printing paper is given a thin coating of salt or bromide by means of an aqueous solution and leave it to dry. Then a thin layer of silver nitrate is added with a brush or a glass rod and it will combine with the chloride or bromide, or halide, to give the silver halide, which is sensitive to light. After further drying, a transparent negative is placed in contact with the paper and exposed to light, bright sunlight is ok! The paper will become more and more brownish, and when you think it is enough, it is washed very well and fixed as I said before.

Photographic modern B&W films or papers are different, they are coated with a layer of an emulsion containing silver halide. They use the latent image and need, therefore to be developed with an alkaline bath, and then washed and fixed.

But how did Talbot managed to take the pictures in the camera? He also used the laten image in a very complex process called calotype or talbotype, see description here, for instance.

But I discovered, by almost pure chance, a simpler gelatin dry process which uses latent image and is developed like the daguerreotypes, using Becquerel's method of exposing the latent image to red light. Here is a picture made in that way.

Edgar Alan Poe

A lot of work must still be done to produce more dark and contrasty pictures, but the idea works. Meanwhile, I already made more positives using the latent image which is projected for 20-30 secs with fluorescent ligh on the sensitive surface and then developed during about one hour by exposition to red light.

My older grandson

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