I would use <physfacet> (a child of <physdesc>) for that information.
From the tag library

<physfacet> Physical Facet
A <physdesc> subelement for information about an aspect of the
appearance of the described materials, such as their color, style,
marks, substances, materials, or techniques and methods of creation.
It is used especially to note aspects of appearance that affect or
limit use of the materials. It generally should not be used for
aspects of physical description that are covered more directly by the
<extent>, <dimensions>, and <genreform> elements, although use of
<genreform> may be appropriate for further specification within some
<physfacet> instances.

I hope this helps.


On Wed, Jan 19, 2011 at 5:47 PM, Elizabeth Russell <[log in to unmask]> wrote:
> I'm hoping to get some expert input on a tagging question specific to
> finding aids for photograph collections.  What would be the most appropriate
> tag to describe the chemical process or materials used to create a
> photograph?  Examples of descriptions that we would like to include are:
> -Blue-toned Chlorobromide print
> -Chlorobromide print on Opal G. paper
> -C.B. toned selenite
> The definitions of both <genreform> and <phystech> do not seem to offer a
> perfect match for this kind of description.
> Additionally, a more general question for those with experience in EAD and
> photo collections.  Are there resources that you would recommend as EAD
> guides for those working primarily with photographs?
> Thank you for any suggestions,
> Best,
> Elizabeth Russell
> Visual Materials Assistant
> Special Collections
> University of Washington Libraries
> MLIS Candidate
> iSchool
> University of Washington