Some time ago I heard about another possibility of development, but I don't
remember every detail: A copper plate is put into mercury so an amalgam forms on
its surface. If you take this plate and put it at a few millimeters distance
over the exposed daguerreotype plate and seal the whole thing, you can then warm
this sandwich up, let cool down and get the image, without exposing you to the
mercury vapor. If your sealing shuts tightly, you can even do the development
outside.
Klaus Pollmeier