Why is silver black?

[geyes30]

I was looking at my negatives and can’t get this question out of my mind - Why is it that silver metal looks shiny, but silver in film black? I have some ideas, but would like to hear from experts. Thanks!

 

[MFstooges]

Because it is not metal but in the form of salt. Just like why Gold Bromide is dark red/black and Iron Oxide is red.

 

[BobD]

See this:
https://www.inesbouwen.com/en/blog/why-does-silver-turn-black-or-green/

 

[JensH]

Hi,

in fact it isn't.
This is a microscope photo I made of developed TMY-2 film:

If we have a bright background the silver looks black because it is not translucent.

Jens

 

[JPD]

It has something to do with how the extremely small particles of silver metal suspended in the gelatin, absorb and reflect light that is different from a sheet of pure silver.

It is silver metal, though, not oxidised nor blackened by sulfur. Sulfur toning turn prints sepia or brown, but would blacken a silver spoon.

Googling didn't give me perfect answers, probably because it's both chemistry and physics, but I hope I'm at least partially correct.

 

[reddesert]

IMO, JensH and JPD are correct: the silver particles are metallic silver, not a salt or a tarnish, and the reason the film appears black rather than shiny is that you are looking at a suspension of particles. In the emulsion, either transmited or reflected light undergoes multiple scatterings. Because some light is absorbed at each scattering, if the emulsion is thick and the density of silver particles is high enough, a large fraction of the light will be absorbed, and you get a partially or fully opaque layer.

If you hold a B&W negative at the right angle with some room light in front and behind, you can sometimes get enough reflection off the negative image to see it as a positive. This unfortunately seems to work best with underexposed thin negatives IME.

This is somewhat related to why smoke is black and puffy clouds are white (ever wonder about that)? Both are suspended particles in air, and there's multiple scattering in both cases. Soot particles are larger than the wavelengths of visible light, like 1-10 microns, while the water vapor droplets in clouds are somewhat smaller than visible light and so the scattering is different, which makes clouds more reflective and smoke more absorptive. Depending on the illumination and the water content, the cloud can turn from white to gray or dark as the droplets change, and then it rains on you.

 

[Vaughn]

Hi,
...
If we have a bright background the silver looks black because it is not translucent.

Jens

And with prints, we are looking at the light that can make it through the silver containing gelatin (emulsion), and then be reflected off the white paper and make another trip through the silver and gelatin to our eyes. Light gets blocked twice by the same silver when looking at a print compared to just once when looking at a transparency,

 

[john_s]

It's like the grease on your bike chain: the metal is silver grey but the used grease has very fine particles of the metal in it and it's very black.

 

[250swb]

Add some silver polish to your developer.

 

[Ian Grant]

Because it is not metal but in the form of salt. Just like why Gold Bromide is dark red/black and Iron Oxide is red.

It's a salt in undeveloped films, which are not black It is Silver metal after development & fixing.

Ian

 

[geyes30]

My idea is very much the same as Jens’. I think small metal particles are opaque, and when taken in aggregate, they block/reflect/scatter enough light to appear black. In ambrotype, the light scattered/reflected by the particles makes the area appear lighter than the backing surface, whereas the clear areas allows light to pass through, and be absorbed by the dark backing material.

 

[guangong]

It’s silver. Once feature movie films completed their distribution route, they were processed to retrieve their silver content. This is the primary reason there are so many “lost films”.

 

[Nicholas Lindan]

As the silver salt is developed a thin thread of metallic silver is 'extruded' that then forms a tight little ball of silver wool. And like steel wool it doesn't look metallic but looks like a dark blob from a distance, and a few thousandths of a mm is a distance at the scale of these blobs.

If you printed on Agfa's resin coated paper and framed the print you have probably seen the silver migrate through the emulsion and form metallic looking silver/bronze areas on the print. Rockland Halo-Chrome toner will do this on demand.

 

[xkaes]

Why is it that silver metal looks shiny, but silver in film black?

Pick up a piece of aluminum foil and it looks shiny. Place it in front of a light source and it looks black. It simply depends on where the light is coming from.

 

[geyes30]

Pick up a piece of aluminum foil and it looks shiny. Place it in from of a light source and it looks black. It simply depends on where the light is coming from.

Seems so obvious when you put it that way. I wonder if it captures all the nuances of the situation.

 

[snusmumriken]

If you hold a B&W negative at the right angle with some room light in front and behind, you can sometimes get enough reflection off the negative image to see it as a positive. This unfortunately seems to work best with underexposed thin negatives IME.

Also best with silver-rich emulsions like Tri-X, and the kind of developer that re-deposits silver on the emulsion. I’ve not been able to do this trick for about 20 years now. It was convenient, though.

 

[MFstooges]

It's a salt in undeveloped films, which are not black It is Silver metal after development & fixing.

Ian

My bad. Let me limit my knowledge to kitchen salt.

 

[xkaes]

Silver iodide is a pale yellow, insoluble crystalline salt that is used in photographic emulsions -- also for cloud seeding!!! Think sodium chloride.

 

[reddesert]

Pick up a piece of aluminum foil and it looks shiny. Place it in front of a light source and it looks black. It simply depends on where the light is coming from.

That is unfortunately not a full explanation, because 1. negatives aren't shiny and reflective when you shine a light on them from the emulsion side, and 2. the explanation doesn't handle partial transmission well.

It does have to do with the light taking multiple bounces off the silver particles. This is also a reason that textured surfaces are "darker" than very flat surfaces - including steel wool as mentioned above.

Part of the multiple-scattering explanation is that the emulsion is thicker than a wavelength of light, and that there is enough density of silver particles that light takes multiple bounces (in physics terms, the mean free path of a photon through the emulsion is shorter than the thickness of the emulsion). Sometimes, as with silvering-out of a print, a thin layer can form on the surface of the print that is reflective: https://archivesandspecialcollectio...of-photographic-degradation-silver-mirroring/ This is of course undesirable. I think dichroic fog that sometimes forms on the surface of negatives might also be a thin layer, though I've never had to look at it closely.

 

[xkaes]

That is unfortunately not a full explanation

While true, sometimes a simple explanation is all that's needed. As geyes30 responded -- "Seems so obvious when you put it that way."

 

[geyes30]

Thanks all for your contributions. I think we have arrived at a reasonably complete explanation. Special thanks to reddesert.

 

[koraks]

Silver iodide is a pale yellow, insoluble crystalline salt that is used in photographic emulsions

It's a minor constituent of photographic emulsions. The bulk is silver bromide (most B&W papers and film) and/or silver chloride (color paper, AZO-type papers, POP papers). Silver bromide and silver chloride are opaque white in appearance. Dispersions have a milky white appearance. They turn purple and black quite rapidly under actinic light as the silver salt 'prints out' into metallic silver.