Silver Halide

Okay, so, when you start to get into the nitty-gritty of silver-gelatin photography, the term "Silver Halide" will pop up a fair bit. Frequently used but infrequently explained, today we'll find out what it is. The simple explanation you'll often encounter is that silver halides are the light sensitive part of the silver-gelatin photographic emulsion. These silver halides are suspended in the gelatin that make up our film and our paper. Easy-peasy, right?

But what the hell is a halide?

Well Halogens are a group of chemical elements:

• Flourine (F)
  
• Chlorine (Cl)
  
• Bromine (Br)
  
• Iodine (O)
  
• Astatine (At)
• Ununseptium (Uus)
  

And those of you who remember your periodic table will conveniently find them here, under Group 17. Ununseptium is one of those quirky newly discovered elements that we don't know much about, and Astatine is a product of Radioactive decay. The other four elements are what we're interested in.

Now, the salts of these elements, known as Halides, are what we're concerned with:

• Flouride (Halide of Flourine)
  
• Chloride (Halide or Chlorine)
  
• Bromide (Halide of Bromine)
  
• Iodide (Halide of Iodine)
  

Flouride you'll have heard of in toothpaste and being added to your water supply by communists. So, it's water-soluble and thus not suitable for photographic emulsions. The other three are.
When you combine Potassium Chloride, Potassium Bromide or Potassium Iodide with Silver Nitrate they form silver halides.

To make film, a combination of Silver Bromide and Silver Iodide are used, as in Kodak's AJ-12 Technical Document and many other silver-gelatin emulsion recipes.

(When you make a salt print, you are combining Silver Nitrate and Potassium Chloride on the paper to produce light-reactive Silver Chloride.)

In photographic paper emulsions each of these halides lends a different quality to the final print. Silver Bromide emulsions tend to be faster, reacting more quickly to light and producing larger grain. The tone of the paper is generally colder, black to blue-black.

Silver Chloride paper emulsions were generally too slow to make enlarging papers, and were instead reserved for contact printing purposes (gas light papers), such as Kodak's AZO and the contemporary Lodima Fine Art and Foma Fomalux (Available from Blanco-Negro supplies). Silver Chloride emulsions are finer grained and generally a warm brown-black colour.

Now, what if we could combine both of these materials to make a fast, fine grained paper emulsion? Well, that's exactly what they do. Most paper emulsions use a combination of both Chloride and Bromide in varying ratios to give either Bromochloride papers (More Bromide than Chloride, for a neutral tone) or Chlorobromide papers (More Chloride than Bromide, for a warmer tone). Silver Iodide may also be added in smaller amounts to change the properties of the paper even further.

So really just saying Silver Halides is a gross over-simplification, and especially in regards to photographic papers the different halides in play clearly have a great impact on the finished image.

(Alex Bishop-Thorpe is a Senior Photography student at the South Australian School of Art, Architecture and Design)