How Daguerreotypes Work: in depth

[amilne]

Hello,

Some of you may have seen my post peddling mercury in the classifieds.

I have no intention of ever making dags myself, because of the danger, but, I was reading something elsewhere and realized I do not actually know the chemistry behind daguerreotypes.

So, who knows it? And I do mean, really, really knows it.

As is usual in hobbyist literature, most of the sources I was able to quickly draw up don’t go into any detail whatsoever. I was able to piece together.

1) Elemental(?) halides are carried via H20 vapor up to burnished silver plate, where I presume they create a one or two molecule thick film of light sensitive silver halides.

2) The halides are exposed to light energy.

3) The halides are exposed to mercury vapor. Those halides that were exposed to light turn into a thin film of elemental silver and mercuric halides.

4) The burnished silver that has been revealed after thiosulfate fix reflects off of a dark matte background angled appropriately to create a positive image. So this is not a “true” positive a la halide diffusion or E-6 “reversal.”

But is it the silver that forms on top of the burnished silver that changes the scattering of light that forms the image, or do the mercuric halides have something to do with it? They probably are also removed by the fix?

[amilne]

No one knows this?

[petecarney]

Here is my understanding of the process:

1 No H20 vapour, just elemental halogen vapour which reacts with the silver to make halides.
2 Silver halides exposed to light are reduced to tiny crystals of metallic silver.
3 When exposed to mercury vapour, blobs of mercury grow at the sites of the tiny silver crystals. The mercury does not bind to areas of the plate still coated with unreduced halide.
4 The thiosulfate wash removes the halide leaving a matt white image made of tiny drops of mercury on a silver mirror. Yes not really a true positive – it has to catch the light correctly. No mercury halides involved.

[amilne]

Pete, thank you so much. That clears some things up. But there are still things that aren’t checking out. The tiny drops of mercury you describe are probably not mercury, per se. My guess is that the mercury vapor is depositing on the silver as an amalgam. Which is a cool thing that happens with mercury and some metals. And silver halides to the best of my knowledge don’t reduce spontaneously. They need to complete a redox reaction with another chemical. And this is where it gets weird for me, since it seems that the development step is missing. That being said, the AgX film on the silver plate must be miraculously thin. Like, only a few molecules thick. (Dag resolution BTW must be incredible) . And I am no chemist but I believe films this thin can behave in strange ways indeed.

[petecarney]

Agreed – the drops of mercury I mentioned are correctly described as an amalgam. It’s the exposure to light which causes the Silver halides to be reduced to Silver. The Mercury treatment is the development step – in which the latent image is amplified to something which can be seen. Agreed, again, on thin films and dag resolution. Cheers.

[amilne]

Ok, so, I forgot that in gelatin emulsions silver halide crystals do form very small bits of elemental silver on exposure to light. This is actually where the crystal ends up forming around. On a Dag, either the amount of the halide that does reduce on exposure to light is enough to generate a visible film of amalgam, OR, there is no crystal lattice on the plate at all, rather individual molecules, and the silver halide just goes straight from halide to silver at the touch of light. Fascinating all the same.

[Author]

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