B-dags, how do they work?

[corey r]

OK, I know the process and know what I have to do to make them work, but what is the photo-chemical reaction taking place with low spectrum light to get them to develop? This is still eluding me, what makes the latent image come forward?

[RonF]

As long as you pose this question, I still don’t know what a regular daguerreotype is :<)

What I mean is that years ago I read that the white area on a daguerreotype plate is an amalgam, but I never thought that was right. I have come to think that perhaps the white on a dag and/or a b-dag is pure silver redeposited in a crystalline form; but I am by no means sure and I still don’t understand the mechanisms involved.

[RonF]

I wish that I could delete that last post, I waited too long.

OK, I was wrong, the highlights on a regular dag are in fact amalgams or contain amalgams.

I’ll have to go dig up my copy of “The daguerreotype: nineteenth-century technology and modern science” By M. Susan Barger, William Blaine White. I was just looking at it online, but I own it.

[corey r]

How does the chemical composition or molecular structure of silveriodide change when developed with low spectrum light to become insoluble to fixer? anyone? The m-dag silver+mercury amalgam makes sense to me, but the change in b-dags does not.

[photolytic]

When a total of 3 photons of high energy blue light have hit a molecule of silver iodide, it splits into silver metal and iodine. This happens in the camera exposure for both mercury and Becquerel Dags. The mercury forms amalgams only with these silver atoms formed during the exposure (latent image) plus those formed during the plate fuming process while the lights are on (photolytic silver). Mercury does not “react” or reduce any silver iodide as happens when ordinary film is developed with a reducing agent such as hydroquinone.

In the light spectrum the wavelength of red light(625-740uM), is higher than the wavelength of blue light (435-500uM). In the Becquerel process, exposure to high spectrum red light does not have enough energy to cause more silver iodide the break up, but it does have enough energy to somehow cause these molecules of silver to vibrate and move around. When the silver atoms collide they form the larger aggregates of pure silver that make up the image. The unaffected silver iodide dissolves in the fixer as with any silver based film.

[CasedImage]

what a great and succinct post John, its not often this gets discussed, many thanks

www.CasedImage.com

[Andy Stockton]

It is very clear and helpful. Your explanation made it it easier for me to understand why Becquerel Daguerreotypes look different from mercury developed ones. I didn’t understand that before.

[Pobboravsky]

I don’t know the exact mechanism for Becquerel Development but my best guess appeared in Study of Iodized Daguerreotype Plates published in 1971, p.40.

[/quote]Unexposed silver iodide is normally insensitive to red or yellow light. However exposure in the camera produces latent-image silver specks large enough to act as optical sensitizers so that yellow or red light can selectively photolyze the exposed crystals, leaving the unexposed crystals unchanged. The image consists of finely divided, powdery silver instead of the mercury-silver amalgam found in conventional daguerreotypes. …

Quote:

What is optical or dye sensitization?

Photo-sensitivity to green, yellow and red light is produced in silver halide crystals by coating the crystals with a dye, or dyes that absorb light of these colors.

All silver halide (AgI and AgBr) photographic processes inherently absorb only UV radiation and blue light and therefore are only sensitive to ultraviolet radiation and blue light. This is because they do not absorb green, yellow and red light. If these colors are not absorbed they cannot produce a photochemical reaction; in other words they cannot produce a latent image. Manufacturers extend the color sensitivity of films into the green, yellow and red by adsorbing one or more dyes onto the surface of the silver halide crystal. The dyes absorb green, yellow and red light and transfer the absorbed energy to the silver halide crystal.

When an iodized daguerreotype plate is exposed to an image in the camera a latent image is produced consisting of a small cluster of metallic silver atoms, a so-called silver speck on the surface of the silver iodide crystal. The number of silver atoms in the speck is roughly proportional to the intensity of UV/blue light striking that silver iodide crystal. The highlights receiving the highest intensity have the largest silver specks and the speck size decreases to zero for those silver halide crystals receiving no exposure; that is, the extreme shadows.

My best guess for the mechanism of Becquerel-development is that the silver specks produced by the camera exposure can now absorb light in the green, yellow and red parts of the spectrum. It is as if the silver iodide surface has been dye sensitized image-wise by the camera exposure. Exposing the plate to a uniform red light during Becq-dev’ment reduces to silver only those silver iodide crystals that have latent image specks of silver. In other words, only those that have the Mark of Zorro are zapped.

[photolytic]

Wiil this also work with invisible infrared light?

Take your best shot here because I already know the answer.

 

If you give an iodized plate a second camera exposure, the yellows and greens in the subject should also start to should show up more strongly in the image.

Has anyone given such a pre-exposed iodized plate an exposure to a color chart to see if this happens? I don’t mean hypersensitizing which presumably affects all the colors equally.

 

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[corey r]

@photolytic

this image is tagged “All IR after hypo” do you mean that it was not developed until after a fix bath!?!?!

whet you refer to a second exposure in camera, what exactly do you mean? exposing the plate evenly to a specific wavelength of light and then to and image? twice to the same image?

[corey r]

and again thanks for the clear and concise answer to my first question, a great help in furthering my understanding of how the process works

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