I've noticed that developed colour negatives usually have an orange layer.
Is this an artifact of an old printing process, required by the chemistry, or is there some other benefit to it?
Looks like photo.net has a really complete answer:
Why do negatives need an orange mask? The simple answer is "impure dyes." This is generally true of all chromogenic photographic materials, where the dye molecules are made of a color coupler that is built into the emulsion, combined with the by-product of the development of silver by a color developing agent. With this kind of thing going on, the choice of dyes is a bit limited, and we end with dyes that are not as good as some others... (more)
Questions about the orange mask of the C-41 and earlier C-22 negative color film keep reoccurring. Maybe I can shine some light on this subject.
As you know, black & white film photography generates an image by chemically depositing a layer of metallic silver on film. This silver laydown is in proportion to scene brightness. This film thus displays varying translucency that acts as a “mask” to control how much light can traverse the film at any given location.
Color photography is a subset of black & white technology. The color is obtained by substituting dye for the silver. Basically, three emulsions are layered, one sensitive to red, one to green, and one to blue light. These are black & white emulsion. They develop up as three silver images.
Kodachrome, a slide (positive) color film was the first successful color film. Three dyes replaced the silver during the developing process. Cyan (blue + green) in red sensitive emulsion – Magenta (red + blue) in green sensitive layer – Yellow in the blue sensitive layer. It takes 4 different wettings of developer, all and all about a dozen steps to develop Kodachrome. The results are lovely however the process is too complicated for home of even the professional photographers’ shop.
A search was on following World War II to make color film and an easier to process both negative and postive types. The answer was to put colorless dye in each of the three emulsion layers. To get a colorless dye, it must be incomplete. Further, all three were prepared missing the same ingredient. If they somehow received this missing component they blossomed into a brilliant dye. Such a strategy greatly reduces the pool of dyes that will work.
The Kodak E-6 (color slide film) and Kodak C-41 (color negative film) use this missing ingredient idea. They are called incorporated color film because the dyes are placed in the film at the factory. The developer used is a black & white formula plus it contains the missing ingredient.
Basically, the film is placed in a color developer. A black & white silver image forms. As the silver image forms, it acts as a catalyst to unite the missing ingredient to the dyes. A cyan dye image is overlaid atop the silver image in the red emulsion. A magenta dye image blankets the silver in the green emulsion. A yellow dye is deposited in the blue emulsion. Now all three needed dyes are in place but the image is veiled by the three silver images. A bleach bath renders the silver resolvable by a fix bath. These can be separate or combined baths. The film emerges, the dyes have blossomed, a color image results.
The image is not faithful. The need to find three dyes missing the same ingredient is responsible. A yellow dye should pass red and green light with little interference and it should block blue light. This is what happens, the yellow dye is acceptable. The magenta dye should allow blue and red light to pass without restriction and stop the passage of green light. The magenta dye is lacking, it leaks some blue light. The cyan dye is meagre; it leaks lots of green light.
In a slide film, the only way out is to live with a less than faithful image. However a negative film is just a means to an end, we don’t look a negatives, we use than to make prints or slides etc. We can use alternate ways to improve the dye’s light transmission.
It was Wesley Hanson of Kodak Labs who figured out a countermeasure. Hanson added a touch of yellow to the incomplete magenta dye and an a touch of magenta to the incomplete cyan dye. This coloration bolstered these dye images and gives rise to the orange coloration you see when looking a C-41 negative (or its cine counterpart). These colorations create two positive image masks that improve the accuracy of the resulting positive image made from a color negative.
A tip of the hat to Hanson!
You have to remember that if we are referring to color negatives, such as a 35mm or 120mm stock like Portra, the orange-brown color is caused by the yellow and red color layers of the film. If you place a yellow lens filter or transparent vinyl over a red one, simple color theory will tell you that the resulting color will be an orange. Standard color negative film (not motion picture stock like Kodak Vision) works with three color sensitive layers. All that is happening when darkroom printing a color negative is that the colors are being inverted (flipped to the opposite color), thus it is called a negative. Just as vice versa a slide (or reversal) film is positive because the colors show up as our eyes see them and not flipped/backwards/inverted.