Re: Sources for wet plate collodion chemicals

Richard wrote:

> Ammonium thiosulfate is a better fixer for Silver Iodide, is it not
> good enough for wet plate? Secondly, I am puzzled by the term
> "brightening", does this refer to the negative or the print? Is it
> actually another term for contrast[?]
>
> Cyanide is a solvent for metallic silver so, it seems to me, that
> excessive fixing in it would destroy part of the very fine grain wet
> plate image. Is this slight reduction what is being referred to, and
> if so, it is considered desirable or undesirable?

The collodion coating, even when still wet, is only semi-permeable to
water. Thus, diffusion into and out of the collodion is very limited
and the most aggressive agents available are generally employed.

"Brightening" is the term used for methods that create a whiter-looking
silver deposit, which is desirable when the image is a positive (i.e.,
tintype, ambrotype, and daguerrotype). As you know, modern
silver-gelatin materials tend to produce large "fuzzballs" of silver
filaments that absorb light and look black. The positive silver
processes depend on much smaller, reflective deposits of silver that
look whitish. Brightening accentuates this effect.

Best regards,

etienne
Received on Sun Aug 28 17:20:35 2005