# Chemistry of Film Photography?



## tjc45 (May 10, 2013)

I'm doing a project for Chemistry on photography. I understand that silver halide crystals in the emulsion layer of the film are sensitive to light, but I don't understand what happens after that? What happens to the grains after the photons hit it? Also, what does this formula mean:

 Ag[SUP]+[/SUP]Br[SUP]-[/SUP] (crystal) + hv (radiation) --> Ag[SUP]+[/SUP] + Br+ e[SUP]-[/SUP]

THANKS!


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## timor (May 11, 2013)

Hi. This might help ?
[h=1]_"Latent Image Formation
_[/h]_[FONT=arial,helvetica,sans-serif]The  light-sensitive part of a photographic emulsion consists of a myriad of  tiny (about a micron) crystals of silver halide (mostly bromide).  These  crystals are suspended in a medium consisting mainly of a very pure  form of gelatin (the same stuff that is in Jell-O).  The resulting  emulsion is thinly spread on a supporting substrate such as a glass  plate or plastic film.  Actual photographic products have additional  constituents and features, but this is the essential form. 	 		[/FONT]_
_[FONT=arial,helvetica,sans-serif] 		  The atoms in a silver halide crystal exist as ions.  Each bromine is a  negative ion with an extra electron.  Each silver is a positive ion with  one electron missing.  The combination, of course, has no net  electrical charge.                      	[/FONT]_
_[FONT=arial,helvetica,sans-serif] During an exposure to light, a photon event occurs when an incoming  photon knocks off the extra electron from one of the bromine ions.  This  photoelectron is now free and mobile, and it wanders about in its  crystal until typically it finds a dislocation or flaw in the crystal  lattice, where it becomes trapped.		[/FONT]_
_[FONT=arial,helvetica,sans-serif] A few of the silver ions are not locked in the crystal lattice, and  these interstitial ions are also mobile.  The negative electric field  set up by the trapped photoelectron draws one of these positive silver  ions over to it.  When the two meet, the ion and the electron combine to  form a neutral silver atom.  This silver atom is the beginning of a  latent image.[/FONT]_
_[FONT=arial,helvetica,sans-serif] As the exposure continues, the same process is repeated several times  until, in at least one place in the crystal, a compact group or  aggregate of about four to six neutral silver atoms is eventually  created.  When this happens, a threshold is crossed.  This clump of  silver atoms is now large enough that it has become a fully formed  latent image.		[/FONT]_
_[FONT=arial,helvetica,sans-serif]    A silver halide crystal containing a fully formed latent image has  become developable.  Developable means that when an exposed plate or  film is placed in a developing solution, the latent image aggregate acts  as a development center.  Starting around the clump of silver atoms,  the chemical action of the developer progressively converts the whole  crystal into a grain of metallic silver.  The developer can be thought  of as a chemical amplifier that multiplies the size of the original  clump of silver atoms formed by light.[/FONT]"_
A citation from Camerabooks.com
How Photographic Emulsions Work: - CameraBooks


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## gsgary (May 11, 2013)

You can also use coffee, washing soda and vitamin C


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## tjc45 (May 11, 2013)

timor said:


> Hi. This might help ?
> *"Latent Image Formation
> *
> 
> ...



Sorry, I don't completely follow. Can someone put this in a simpler form?


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## amolitor (May 11, 2013)

Probably not. All that can really be done is to define all the terms, which is a lot of definitions.

Anyways. The point of a silver halide crystal is that it is inherently unstable. Hit it with light, and it breaks down a little into pure silver and pure halide. Just a little, usually (there are cases like "printing out paper" where it breaks down more thoroughly in normal usage).

Developer also breaks down silver halide crystals into pure silver and either pure halide of some other halide compound.

Developer has an easier time breaking down the crystals where light has given it a head start by breaking it down a little already.

So, if you develop for the right amount of time, the developer will mostly break down crystals that got hit with a bunch of light, and will mostly NOT break down the crystals there were left in the darkness.

Now you've got lots of pure silver where there was light, and very little where there wasn't.

Fixer dissolves silver halides, but not silver. Fixer leaves just the silver behind.


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## tjc45 (May 11, 2013)

Basically, a photon reacts with a crystal and forms a few pure silver  atoms, then the developer amplifies the silver to make it visible and  the fixer removes the remaining crystals?


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## amolitor (May 11, 2013)

Yep. That's pretty much it.


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## tjc45 (May 11, 2013)

So this formula is correct? 

Ag[SUP]+[/SUP]Br[SUP]-[/SUP] (crystal) + hv (radiation) --> Ag[SUP]+[/SUP] + Br+ e[SUP]-[/SUP]


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## amolitor (May 11, 2013)

I think that's implied by the discussion timor posted, yep.

That happens, and then later on tAg+s and the e-s get together to make Ag, which is sort of the point.


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## compur (May 11, 2013)

None of the above is correct. Images on film are produced by magic.


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## gsgary (May 11, 2013)

tjc45 said:


> Basically, a photon reacts with a crystal and forms a few pure silver  atoms, then the developer amplifies the silver to make it visible and  the fixer removes the remaining crystals?



Who gives a ****ing **** as long as it has nice tonality


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## Overread (May 11, 2013)

gsgary said:


> tjc45 said:
> 
> 
> > Basically, a photon reacts with a crystal and forms a few pure silver  atoms, then the developer amplifies the silver to make it visible and  the fixer removes the remaining crystals?
> ...



You will when you move on from film and move to wet slide photography or something 

Though as an aside lets keep the silly "who cares about the science" comments out shall we. We might not all enjoy chemistry and physics, but lets not discourage those who wish to know more from doing so.


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## amolitor (May 11, 2013)

The anti-intellectual streak in TPF gets awfully damn tiresome at times.

Sometimes, I just want to tell these bozos to grow the **** up and put in a god damned sock in it, because while if you want to be a ignoramus that's ok it's not ok to try to stop other people from not being ignorant fools.

But I don't, because, basically, I am a nice guy.


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## compur (May 11, 2013)

amolitor said:


> The anti-intellectual streak in TPF gets awfully damn tiresome at times.
> 
> Sometimes, I just want to tell these bozos to grow the **** up and put in a god damned sock in it, because while if you want to be a ignoramus that's ok it's not ok to try to stop other people from not being ignorant fools.
> 
> But I don't, because, basically, I am a nice guy.


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## tjc45 (May 12, 2013)

meanwhile I STILL DON'T UNDERSTAND


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## Josh66 (May 12, 2013)

tjc45 said:


> meanwhile I STILL DON'T UNDERSTAND



Which part?


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## amolitor (May 12, 2013)

This is not high school chemistry, it turns out, it's pretty subtle stuff.

Quite a bit of it can be understood in terms of high school chemistry, but the business of latent image forming is not.


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## timor (May 12, 2013)

That, cause it only seems to be just a chemistry, but is molecular physics.


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## amolitor (May 12, 2013)

Well, there's a fair bit of chemistry, but most of THAT is about solubility of things in other things, which is a bloody mess as near as I can tell.


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## tjc45 (May 12, 2013)

Yes the latent image! All that business about photons moving electrons into lattice interstitial imperfections in the crystal surface and attracting silver ions and forming atoms blah blah blah WHAT DOES IT MEAN


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## timor (May 12, 2013)

It means, that creation of a latent image has some randomness in it. Something beyond regular human control. And here comes the QC of a maker to keep that randomness steady. Not every emulsion made even by Kodak had ever seen the store shelves, every production batch is slightly different and has own number. In production of a movie is essential to use film with the same emulsion number. In the past photographers were buying large quantities of film from one production run and tested it before shooting for keeps, important for the zone system but not only.


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## compur (May 13, 2013)

tjc45 said:


> Yes the latent image! All that business about photons moving electrons into lattice interstitial imperfections in the crystal surface and attracting silver ions and forming atoms blah blah blah WHAT DOES IT MEAN



You are not the only one who is confused about how latent images are formed and developed as this excerpt from the book _The Science of Photography_ by H. Baines & E.S. Bomback explains:









As this excellent book further explains, when it gets right down to it, it isn't fully known for sure how this process works and the scientific explanations are really only theories and best guesses based on available evidence. (This is why I made my earlier comment about "magic.") What is known for sure is that light produces changes in the silver halides contained in a photo emulsion and developers exploit and magnify those changes. The book contains quite a lot of information about the theories concerning the changes and their development, far more information than could be posted here and it is worth reading. If you are curious I recommend you get a copy and read it.  It is available used for pennies as you can see here:
Science of Photography: Harry Baines, Edward S. Bomback: 9780852422106: Amazon.com: Books


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## Compaq (May 14, 2013)

One way to think about the process is the following:

The film emulsion consists of silver halide crystals. When these are exposed to light, defects in the network/lattice are formed: metallic silver clusters. More light => larger clusters. These clusters are reaction centres when developing the film; the forming of the image is catalized by these clusters. Large clusters => black/dark area on negative. Small clusters (or non at all) => light grey (or blank) area on negative. Between different sized clusters, tonal ranges between black and light grey show after development. 

This was a very simple explanation, without technical terms and jargon.


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## table1349 (May 14, 2013)

tjc45 said:


> Yes the latent image! All that business about photons moving electrons into lattice interstitial imperfections in the crystal surface and attracting silver ions and forming atoms blah blah blah WHAT DOES IT MEAN


It means two things.

1.  Were not going to write your paper for you.

2.  You need to spend some library time studying chemistry so you do understand and can complete your project.  



*Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime.*  Chinese Proverb

There are plenty of Chemistry teachers that have written books from which you can learn.  Good luck.


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## Compaq (May 14, 2013)

If you really want to learn this in detail, you need to understand chemistry. There are written many textbooks in general chemistry. Such book covers a little bit of everything, and explains basic terms such as "ions", "solution", "lattice", "salt", "electron", "acid", "base", "pH", "reducing/oxidizing agent" and much, much more.

I have been studying chemistry for three years, and plan on continuing for at least two more years, so these terms are second nature to me. However, chemistry can be "self taught" provided you get good books and spend time learning.

Again, I am not sure how deeply you want to go into this.


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## EstherC (Jul 18, 2013)

amolitor said:


> Probably not. All that can really be done is to define all the terms, which is a lot of definitions.
> 
> Anyways. The point of a silver halide crystal is that it is inherently unstable. Hit it with light, and it breaks down a little into pure silver and pure halide. Just a little, usually (there are cases like "printing out paper" where it breaks down more thoroughly in normal usage).
> 
> ...



just wondering, sorry if it's a stupid question, do what you discuss here only apply to b&w film or color also?


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## danielklaer (Jul 22, 2013)

tjc45 said:


> So this formula is correct?
> 
> Ag[SUP]+[/SUP]Br[SUP]-[/SUP] (crystal) + hv (radiation) --> Ag[SUP]+[/SUP] + Br+ e[SUP]-[/SUP]



yes but all that says is that silver bromide (a silver halide) breaks down in the presence of radiation to a silver ion, a free bromine atom and an electron. The free electron than meets your silver ion and forms a silver atom (this part of the reaction is Ag[SUP]+[/SUP] + e[SUP]-[/SUP] --> Ag).

You will need to do a bit of reading on your basic chemistry again as you need to at least understand what an equation says. Once you have reminded yourself of the basics there are plenty of resources, including this forum, where you can find out more specific info. It is very obvous when a student is just regurgitating what they have read without actually understanding it. Such an assignment would be marked accordingly.


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