> I recently attended the George Eastman House's daguerreotype workshop. If I
> remember correctly, it was asserted at the workshop that one of the reasons
> that daguerreotypes have such astounding resolution is that the layer of
> light sensitive silver is only one or two molecules thick. It was suggested
> that other processes, relying on perhaps *hundreds* of layers of light
> sensitive molecules, would therefore have a "dispersion factor" to deal
> with, and would therefore be that much less "sharp" as a result.
>
> This made good sense to me, but a friend recently asked me this: If that is
> true, why did Southworth and Hawes reportedly "double plate" all their dag
> plates with an extra layer of silver? Wouldn't that effect the relative
> resolution of their dags, vis a vis their peers?
>
> Further, how would this double plating of the dags effect the sensitization
> process which took place over crystals of iodine and then bromine? How
> would the sensitized layer over a double layer of silver render a benefit
> over the single layer if it's only the top few layers of molecules which are
> being sensitized anyway?
Well I really am not an expert in this, but will hazard a guess. First of
all, the vapor sensitization process isn't likely to penetrate deeply into
the silver layer. Only the exposed silver atoms at the surface of the
film are likely to be converted to silver halides which means that the
thickness of the silver probably has little direct influence on the
sensitization process.
There are several mechanical benefits that having a thicker silver layer
might give you. One is that the silver plating would be more robust. A
slight scratch would be less likely to penetrate through the silver layer
to the baseplate. With a very thin silver layer you may also be able to
actually see some of the copper baseplate through the film. A thicker
silver layer would prevent this. Also, there is a solid solubility issue
when you put two materials in intimate contact. The two materials will
diffuse into each other somewhat. A thicker silver layer would minimize
the amount of copper atoms that could migrate through the silver to the
surface. Having a thicker silver layer would improve the purity of the
surface silver, and perhaps help preserve image brightness. However,
there should be a point of diminishing return beyond which you aren't
getting any more advantage, and are just making the plate more expensive.
Hows that for a reply from someone who doesn't really know what he's
talking about! <grin>
- Wayde
(allen@boulder.nist.gov)