In this article, I share some important details on the scientific aspects of darkroom printing that are good to understand in order to take advantage of more advanced techniques and methods of darkroom printmaking.
PHOTO PAPER EMULSION
Contact and enlarging darkroom papers have a light-sensitive coating called, emulsion. The emulsion that you can’t see with your eye is silver halides suspended on a gelatin coating. Photo paper actually contains a mixture of silver halides and this is what responds to your light source and darkens when the paper when exposed. Halides are a group of elements consisting of bromine, chlorine, iodine and fluorine and their corresponding salts are what we refer to as “halides”. These halides are referred to as bromide, chloride, iodide, and fluoride. I will explain this in more detail below, however, the key part you should take away if nothing else is that silver halides are the magic ingredients in your photo paper and films.
The silver halides used in photographic papers are silver bromide, chloride, and iodide. Silver bromide emulsions are considered to be “fast” meaning they react very quickly to light. Silver bromides are associated with large silver grain and contain a cold black or a blue-black color range. You are not likely to see too many pure bromide papers any longer other than a few niche products. Freestyle Photographic Supplies has historically stocked Kentmere Silver Bromide fiber glossy papers in several grades.
The light-sensitive silver halide emulsion layer has a silver content of approximately 1.7 g/m2. The emulsion is covered with a gelatine supercoat which protects the emulsion from stress fogging and physical damage and also contains a small amount of developing agent.
Silver Chloride papers are considered to be very slow papers and they are not suited to be used as an enlargement paper. Kodak AZO is the best-known paper and is no longer in production. Chloride emulsions are fine-grained and are considered to be warm and brown-black in color. This fact alone may direct or steer you away from this paper based on your artistic goals. I create emulsions by hand and coat my own paper negatives and printing papers. The knowledge in this article is critical to understand in order to be able to do these types of things.
Historically silver chloride was combined with silver bromide and therefore called “bromochloride” for neutral tone papers they were “chlorobromide” for warm-tone papers. The only known silver chloride paper available today is Lodima Silver Chloride available exclusively via Michael A. Smith via his website. The other option is to make your own emulsions and coat your papers by hand like I do.
Silver iodide is used in very small amounts in some papers in combination with bromide and chloride to modify the paper’s properties and characteristics.
LET THERE BE LIGHT
When you expose your light-sensitive photographic darkroom paper via your light source you may have guessed that the light-sensitive silver halides will darken in various shades. Modern-day photo papers don’t require nearly as much light as the papers in the past because there is an electrical charge affecting the halide in the paper. Once you expose your paper and negative to light there is actually an image on the paper but you can’t see it yet. This is referred to as the latent image. Now you will need your first chemical in the print development process called “developer” to darken the image on the paper.
The latent image that was exposed with your negative and light source is now ready to be developed and darkened on your paper. The development step is where the “magic” happens for many people. You can literally see the image appear in front of your eyes. Your developer chemical is actually changing the invisible silver halides into grains of metallic black silver resulting in the image you see with your eyes. As you may have figured out the developer only acts on the silver halides that were exposed to light. Now you have those magical particles of silver suspended in the gelatin emulsion. We need to stop the development from continuing.
The stop bath does exactly what it sounds like, it stops the development process from continuing. It literally neutralizes the development from continuing. Some will argue that you don’t need a chemical stop bath and just use plain water at 68F/20C. I typically use a stop bath from Ilford that is considered to be an acid stop bath. Besides obviously stopping the development process the use of stop bath drastically extends the life of your fixer which we will discuss next. By using the stop bath I am finished with the stop bath process in 30 seconds versus having to thoroughly soak the paper in a water bath and rinse frequently turning over the water supply. I also use it because I don’t want any of the developer getting carried over into my fixer.
Remember that only the silver halides that received enough light were converted to metallic silver by the developer as discussed above. For example, your highlights and shadows received much less light because your negative was denser in these areas. Because of this your paper still has undeveloped silver halides and if you have white borders on your print from a frame, mask, etc. then there is a significant amount of silver halides still on the paper. We must remove these halides or they will expose or print out over time. Removing these halides is the job of your fixer. Your fixer will dissolve the halides but now you need to wash them away.
DOING THE WASH
After we have completed the fixing step we need to literally wash away the dissolved silver halides. Depending on the type of paper and archival intentions the washing time and technique will vary.
I hope this article has given you a better understanding of the science that is taking place when you are printing your masterpieces in the darkroom. This knowledge will serve you well in many different ways. When you read the technical specifications on your darkroom paper of choice this information should make more sense to you now. There are many other more advanced variables to discuss such as toning, bleaching, redeveloping and others at a later time.