[alt-photo] Re: tintype

[etienne garbaux]

Cor wrote:

>So the plate stays wet all the time, I do not see why the pores 
>should/could close and plate would be dry. That occasionally can 
>happen if you life in a very dry climate or used to much time 
>between exposure and development.

Jalo wrote:

>Collodion is not non-permeable during the process because it is kept 
>wet. That's why the process is wet plate collodion, it doesn't work 
>if the collodion dries. The plate is not "essentially dry" during 
>fixing -- it is very wet.

There are two drying mechanisms at work, and both are continuums 
("more" and "less," not "wet" and "dry").  The collodion itself is 
drying by the rapid evaporation of the solvents (alcohol and ether), 
and the remaining water from the silver nitrate sensitizing bath is 
also evaporating.  It is the former, more rapid, drying that changes 
the permeability of the collodion.  The collodion is still quite 
"gooey" when it is sensitized, so the silver nitrate solution 
penetrates the collodion and forms silver halide not just on the 
surface but to a moderate depth.  However, the collodion solvents 
evaporate rapidly, so the collodion is considerably drier by the time 
it has been exposed than it was when it was sensitized, and the 
developer and fixer have a much more difficult time reaching the same 
depth notwithstanding any remaining water from the sensitizing 
bath.  Hence the need for a very aggressive fixer.

A collodion plate should not be "very wet" when it is loaded into a 
plate holder -- one would properly describe a plate that is ready to 
expose as "quite damp," and plates are usually fairly dry by the time 
they have been exposed and are placed in the developer.  (Of course 
they are "very wet" when they go into the fixer, having just been 
developed and washed -- but that has no bearing on the previous 
drying of the collodion.)

Best regards,

etienne