The "American Hand Book of the Daguerreotype" by S. D. Humphrey, describes a
way to develop daguerreotype without mercury.
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Daguerreotypes without mercury.
The following process possesses some interest, and is worthy a trial from
operators. M. Natterer, of Vienna, discovered a process for obtaining proofs
on iodized plates with the chloride of sulphur, without the use of mercury. A
plate of silver is iodized in the usual manner, and then placed on the top of
a vessel six or eight inches high, having at the bottom, in a small cup, a few
drops of chloride of sulphur; it should remain exposed to the action of the
vapor until the sombre yellow color is changed to a red, after which it is
brought to a focus in the camera, where it is exposed to the light in the
camera, for about the time necessary to produce an ordinary daguerreotype.
The plate is then taken out and examined in the camera by the light of a
candle. It often occurs that no trace of the image is as yet perceptible, but
if the plate is heated by placing over a spirit lamp the unprepared side, or
if left for some time in the dark, or, lastly, if exposed only a few seconds
to a weak, dimmed light, the positive picture then appears with all its
shades. Of these three modes of bringing out the image, the second is superior
to the others.
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Philippe Moroux
http://www.knoware.nl/users/philippe/