Darkroom Digest: Understanding Silver Halides for Darkroom Photographers

August 07, 2017  •  Leave a Comment

It feels impossible for me to think about darkroom photography without at least being curious about the role of silver halides.

While it isn't necessary to know the scientific details about silver halides to create standard darkroom prints, I think understanding the underlying chemical architecture will make you a better printmaker and open doors you never knew existed before.  

SILVER HALIDES 101

A silver halide (or silver salt) is one of the compounds which are formed between silver and one of the halogens – silver bromide (AgBr), chloride (AgCl), iodide (AgI). As a group, they are often referred to as the silver halides and are often given the pseudo-chemical notation AgX.  

All you have to know about silver halides is that it is the light-sensitive chemicals used in photographic film and darkroom paper.  This might seem a little boring or irrelevant, but you will discover in the sections below, having a clear understanding of the silver halides can help you move into and explore new areas in your darkroom printing. 

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PUTTING SILVER HALIDES IN CONTEXT

It is fairly straight forward, even for the non-chemist to understand the concept of silver halides in darkroom photography.  However, I want to walk you through a little more detail to help you understand the bigger picture and role of the halides.  

When creating a traditional darkroom print, you begin with an unexposed sheet of silver gelatin paper that is coated with light-sensitive silver halides.  The silver halides (emulsion) is effectively suspended in gelatin which sits on the paper substrate.  

I should mention that silver halides are used in darkroom papers as well as film.  When the silver halides are exposed to light, they are reduced to metallic silver, which forms the image on your film and darkroom paper.  

In darkroom photography, there are three relevant halogens (Bromide, Chloride, Iodide).

You may hear some of us "old timers" talk about silver bromide (AgBr) papers and these were the cold tone emulsions.  

Silver chloride (AgCl) produces a warmer tone and is most famous in Kodak's long-ago AZO contact printing paper, and the remake by Michael Smith, known as Lodima paper.  Lodima is amidol spelled backward.  Michael has a good sense of humor.

There is a new paper from Adox, called Lupex, that is a grade 3 silver chloride paper in case you want to explore this emulsion in your creative endeavors.  As a large format contact printer, I have found that using a silver chloride paper with amidol produces a print that is similar in tonal scale and values to my platinum prints, but with a glossy finish.  By varying the two-bath amidol development bath, I am able to control my tonal values from deep blacks to rich and warm brown tones.  You may have also heard silver chloride papers referred to as gas light papers because the speed of the emulsion is so slow that the historic printers were able to work under gas torches.  

Iodide (I) is used by manufacturers of darkroom papers, but you won't find any pure silver iodide (AgI) papers.  If you like the look of Kentmere papers, it is probably because they are known to use iodide in their emulsions. 

Modern darkroom papers are a mix of halides.  You won't find many silver bromide papers any longer, which are known to work very well for Lith printing.  It is difficult to know the exact iodide formula for the modern papers because most of that information is considered to be the intellectual property of the paper makers.  

Based on my knowledge and experience, I think most darkroom papers today would fall into the chlorobromide emulsion category. As the name implies, they are a mixture of the faster bromide and slower chloride papers. Chlorobromide papers are usually slower than traditional bromides. The percentage of bromide to chloride can allow manufacturers to create either warm or cold toned papers with a variety of sensitivities. Examples of chlorobromide papers include Ilford MG IV and Warmtone, Fomatone MG Classic VC and Slavich Bromportrait.

I hope this was helpful, or possibly a refresher for some of you.  

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-Tim Layton 

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Tim Layton
Darkroom & Large Format Photography
Video Workshops/eBooks/Guides: www.timlaytonfineart.com/workshops
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