Figure 1 – Silver halide photographic grains, from the Wikicommons and in the public domain. Originally from Plate VII from Robert James Wallace, “The Silver ‘Grain’ in Photography” by Robert James Wallace, The Astrophysical Journal, Vol. XX, No. 2, Sept. 1904, pp. 113–122, Chicago.

Today, let’s continue with our discussion of the silver halide process.  In our previous blog, we discussed the latent image, how it was made, and how you cannot see it.  We discussed how a silver ion in the silver bromide, AgBr, grain, Ag+, gained an electron and became free silver, Ag.  This process of gaining electrons in chemistry is called “reduction.”  Now, there are lots of chemicals that can contribute electrons and, yes, you guessed it, they are called “reducing agents.”  In photography they’re also called “developers.”  Usually the developers used in photography are organic compounds, but that’s not a critical point.

Now the key to all of this is that a crystal of silver bromide will not be reduced to free silver unless it already has some free silver in it.  Remember the latent image?  The free silver kind of primes the pump for the reducing agent.  There are two fancy phrases used in chemistry for this pump priming process.  We say that the free silver catalyzes the production of free silver in those grains were it already exists.  Alternatively, we say that the free silver acts as a nucleation site for the production of more free silver.

In any event, the effect of all of this is that those grains which had free silver, that is are part of the latent image, undergo reduction to produce more free silver.  In contrast those which had no free silver are unaffected by the developer.

This is great! Right?  We now have lots of dense silver grains (see Figure 1) on the parts of the emulsion, which were exposed to light.  So can we turn on the lights now and see the image?  Absolutely, not.  The problem is that there is still lots of unexposed and, therefore, still light sensitive silver bromide in the emulsion.  So there are a few more steps in the development process.

First, you need to stop the action of the developer.  While this can be done by rinsing in water, when you are looking for good control of the process, it is more typical to use an acid stop bath that stops development in its tracks.  This makes a lot of sense, since, by definition, an acid provides lots of positively charged protons ready to steal away any remaining electrons.

Then the remaining silver bromide needs to be removed. This is generally done using sodium thiosulfate, otherwise known as hypo.  This solublizes the free silver bromide out of the emulsion by converting it to bromine and silver thiosulfate ions.  These can then be safely washed away, and if you do a good job with your washing, the image stays clean for a hundred years or more.

Figure 2 – a modern color photographis film showing the different layers. 1. Film base; 2. Subbing layer; 3. Red light sensitive layer; 4. Green light sensitive layer; 5. Yellow filter; 6. Blue light sensitive layer; 7. UV Filter; 8. Protective layer; 9. Visible light. From the Wikicommons, by Voytek S under creative commons liscense.

This is pretty much all that there is to it.  But let’s take a moment to reflect.  This brilliant concept was created and refined by hundred of chemists, physicists, and engineers over the course of more than a century.  The wonderful complexity is hidden in what seems very simple.  Therein lies the magic!  The gelatin, for instance, has to allow all the chemistry to occur, allow the removal of unreacted silver bromide, but then offer some reasonable level of mechanical stability for the silver grains for a hundred years or more – and we’re doing this with what? – egg whites and animal bones.  It’s really truly marvelous.  Figure 2 shows a modern color film with all its complex layering required to produce a subtractive color image as we have discussed previously.  And remember, at the root of all of this is the basic silver halide photgraphic process.

FIgure 1 – Schematic showing the creation of a latent image upon exposure to light in a silver bromide emulsion. (c) 2013 DE Wolf

You will recall that photography was invented in 1838.  So the light sensitivity of silver salts, such as silver bromide, was recognized for a very long time.*  However, it was not until 1938 that a real theoretical-mechanistic explanation of this sensitivity began to be developed.+  We have already discussed the concept of valence and conduction bands in silver halides and how light can raise electrons between these two bands.  Recognize however, that when the electron is elevated in energy to the conduction band it leaves behind a positively charged region in the crystal lattice, referred to as a “hole.”  In fact, what light actually creates is an electron-hole pair.

In a perfect and pure crystal the electron will hang out a bit in the conduction band and eventually will recombine with one of the available holes.  So there should be this continuous up-down, creation-recombination process going on in the presence of light.  However, our photographic grains are not perfect crystals.  First, of all they are finite in size.  Second, they have impurities.  And third, there can be breaks or physical dislocations in the crystal lattice – like cracks in a driveway.

All of these imperfections can lead to the process shown in Figure 1.  Here, I’ve shown it as a single bond between a silver ion and a bromine ion, but we know that in fact the ionic interactions are three dimensional and more complex.  We have, for starters, a silver ion (Ag+) in close association with a bromine ion (Br-).  Light comes in and liberates the bromine’s electron.  So now we have a silver ion (Ag+) and bromine atom (Br) and an electron (e-).  The electron then combines with the silver ion to create atomic silver (Ag).  So we are left with silver (Ag) and bromine (Br).  It is the free atomic silver that we refer to as the latent image.

In the emulsion there are thousands of randomly distributed silver bromide crystals or grains.  These are the pixels of analogue photography.  Where light struck the emulsion some of these grains now contain free silver.  The more intense the light the more likely the grain is to contain free silver. The distribution of silver reflects the distribution of light and is what we refer to as “the latent image.”  You can’t see it and if you turn the lights on to try to see it, you will over expose the film – “Catch 22!”

The latent image is there waiting to be developed.  That will be the subject of my next technical blog.

*Perhaps the earliest reference to the concept of silver-based black and white photography is that of J. H. Schulze who observed in 1727 that a mixture of silver nitrate and chalk darkened on exposure to light.

+Gurney, R. W.; Mott, N. F. (1938). “The theory of the photolysis of silver bromide and the photographic latent image”. Proc. Roy. Soc. A164: 151–167.

Figure 1 – Women separating egg whites and egg yolks as the first step in the manufacture of silver albumen photographic paper from Josef Maria Eder’s Ausführliches Handbuch der Photographie, Book IV, part 1, 1898 edition. In the public domain in the US. See hyperlink in text for more information on the process.

A photographic emulsion is a fine suspension of insoluble light-sensitive crystals, for example silver bromide, suspended in a colloidal solution.  Basically, this means that while the crystals are not themselves soluble in water, they can be suspended in a material, such as gelatin, which is then allowed to harden thereby suspending the crystals permanently.

The emulsion is layered onto a substrate such as glass or a film, e.g. nitrocellulose or plastic.  In the nineteenth century the emulsion material was most often egg albumen, and this created an entire industry of manufacture that started with the separation of thousands of egg yolks and whites as shown in Figure 1.  Modern photographic emulsions are made from gelatin, which is partially hydrolyzed (boiled) extract of animal cartilage and bone – hence the classification of silver-gelatin print.

The light sensitive emulsion is generally manufactured, as suggested in my previous blog by dissolving silver nitrate and potassium bromide in a hot gelatin solution and allowing the crystals of silver bromide to precipitate out in the gelatin.

So now, we have the fundamental elements of a photosensitive material.  You can do this yourself at home, if you are industrious, and there are even commercial kits to help you.  If you want to do albumen printing you can purchase kits from Bostick and Sullivan. As should be obvious from our discussion of color photography and color movies, the fundamental silver halide process is ubiquitous.

Curiously, how it works, what the fundamental mechanism of light sensitivity is, was largely a mystery for the first century of photography.  That mystery is the subject of my next technical blog.

Figure 1 – The Periodic Table of the Elements.  The halide group is shown in yellow. From the Wikicommons and in the public domain.

In considering silver halide chemistry, the first question is exactly what is a silver halide.  A silver halide is a compound formed from silver and a halogen.  That didn’t get us very far.  What’s a halogen?  If we look at the period table of the elements (be sure to click on Tom Lehrer’s version) in Figure 1, the yellow group 17  are the halogen elements: Fluorine, Chlorine, Bromine, Iodine, and Antimony.

Figure 2 – The crystal structure of silver bromide. The silver atoms are the red spheres the bromide atoms are the pale blue spheres. Form the Wikicommons and in the public domain.

A silver halide then is a crystal formed between silver and one of these Group 17 elements.  Without getting into too much detail the silver metal gives away some of its electrons to become positively charged.  These are scarfed up by the halogen to become negatively charged.  The halogens ions all have a charge of -1 (electron’s worth of charge).  That’s why they are grouped together. Since opposites attract the silver and halogen ions attract one another and form very compact and structurally well-defined crystal structures.  These crystals typically take on a cubic form as shown in Figure 2, for the important photographic material silver bromide, where the red spheres are the silver atoms and the pale blue ones are the bromine atoms.  The other common photographic silver halides are silver chloride and silver iodide.

Silver bromide is not soluble in water.  This is convenient.  If you mix a solution of silver nitrate in water with potassium bromide in water, the silver and bromine switch partners to form silver bromide and potassium nitrate.  The potassium nitrate stays in solution.  The silver bromide precipitates into fine silver bromide crystals, ready to be washed and put into a photographic emulsion.  What an emulsion is will be the subject of my next technical blog.