3 Negative and copies. First improvements.

1. Negative and copies: First improvements
Calotype paper and colodion wet plates were the techniques
that first produced photographs by generating a negative
image that could be subsequently positivated as many times
as desired. In addition, it was much more economical to use
paper of glass instead of metal as support. In this way introduces
two very important characteristics for the subsequent
development of photography: the multiple image, and its very
economic cost.

2. Exposure to light altered the chemical structure of the silver salts
which upon contact with the developer (gallic acid) became black.
After achieving a negative image it was later fixed using sodium
hyposulphite.

3. This negative photograph was placed on a new photographic
paper and acted as a filter. When exposed, the areas that in the
negative picture appeared white would allow to pass the light
generating a positive image.

4. In 1851 the british sculptor and photographer Frederick Scott Archer
introduced the method of wet glass plates impregnated in collodion. The
exposure time was reduced by about 15 times with respect to the
daguerreotype, but it had the disadvantage that the negatives had to be
exposed and revealed while they were still wet. After 10 minutes the
collodion would dry out and lose its sensitivity to light.

5. The process required great skill and forced the photographer to
reveal the photo at the time which made it necessary to use a field
laboratory.
The process included the following steps:
- Wash the glass plate very well.
- In full light pour, collodion with iodine or bromine on the glass plate
so that it covers the whole surface.
- Bring the plate to a room with "safe light" (red) and immerse the
plate in silver nitrate so that it becomes light sensitive for 3 to 5
minutes.
-Remove the plate from the bath clean the leftover and mount it in a
plate box protected from light by a black curtain.
- Charge the plate box in the dark chamber, run the black curtain for
a period of 1 second to several minutes.
- Reveal and fix.

7. Mathew Brady, a american photographer, and his 20 assistants
made in 1861 thousands of photos of the battlefields during the War
of American Secession. For this they used negatives of wet collodion
plates and tents as development chamber: field studios.
Many of the images were photographs of corpses, being the
first time that the reality of the war was shown firsthand. Brady
photographed many portraits of Union officers. He also
photographed some confederate soldiers and had the opportunity to
photograph Abraham Lincoln several times.
Mathew Brady and the Civil War

8. Fotografía del Presidente Lincoln. Brady, Mathew.1861

12. This an iconic image as it highlights the fundamental aspects of
what photography was going to mean from now on:
- Democratizes photography, not only important people will be
portrayed and remembered in images.
- In addition to the historical aspect, the daily side of reality is
documented.
- These are images that will inform us of a social situation that
will generate a great impact on the public.