How does the daguerrotype give directly a positive image?

Question

I have already asked this question in the Chemistry community, but since daguerrotype is an old, obsolete technique, I hope this question can be considered as on-topic also here, to increase my probability of getting an answer.

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I have read the wikipedia entry about the old photographic process called daguerrotype.

It says there

The image is on a mirror-like silver surface and will appear either positive or negative, depending on the angle at which it is viewed, how it is lit and whether a light or dark background is being reflected in the metal. The darkest areas of the image are simply bare silver;

As I understand it, to make a daguerrotype one first exposes a the silver coating of a copper plate to iodine fumes. This creates a layer of silver iodide which is white, and light-sensitive. When exposed to light the silver iodide is affected, but this is not directly visible. To get an image some more chemical reaction is needed.

In "modern" BW photography (that is, the one used for a long time before color photography and of course before digital) the silver iodide that was exposed (light part of the initial image) was reduced to metallic silver that appeared dark, while there was no silver in the part of the image that was not exposed to light (dark part of the of the initial image) which remained clear. Thus a negative image was obtained and had to be reversed by a second exposition to light.

But daguerrotype does not have a second exposition. Anyway, it is not transparent because of the copper plate. It is directly positive. Granted, wikipedia says it can be negative "depending... ". But mainly it is positive as is obvious when one looks at old daguerrotypes, for instance the two that illustrate the wiki page I mention above. And the wikipedia does say "the darkest areas... are pure silver", meaning those that were not exposed to light, in a "positive" image.

The chemical process involves mercury fumes. I would like to understand how this works to create an image which is directly positive at least for the more usual way that

it is lit and whether a light or dark background is being reflected in the metal

Answer 1

This creates a layer of silver iodide which is white, and light-sensitive.

The iodine (or later iodine and bromine) turn the plate yellow.

The chemical process involves mercury fumes. I would like to understand how this works to create an image which is directly positive at least for the more usual way

The photoactivated silver more readily reacts with the mercury. The silver mercury amalgam that is formed is described as milky white.

I believe the halide is then removed by the hypo fixing process. The silver that remains is highly reflective. If untarnished, it is a good mirror rather than being white or black.

To view the image as a positive, you need to hold the plate at an angle so that you have something dark reflecting in it. If you instead hold it so that you have a bright wall or other light, the silver will appear brighter than the amalgam and the image will appear negative.

There's a video of a daguerrotype from different angles. The video is not particularly sharp, but you can still see how the image changes from an apparent positive to an apparent negative when a bright area is reflected.

But mainly it is positive as is obvious when one looks at old daguerrotypes

I'd guess that most photos were taken with lighting conditions set to maximize the contrast as a positive.