Grain Clumping – Fact or Fable?

By Dick Dickerson & Silvia Zawadzki Back to

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Coarse-grained images are often claimed to suffer from something called “grain clumping,” which in turn has been ascribed to too long a development time, too high a development temperature, too alkaline a developer, and too long a total wet time, among other reasons. Detailed rationales of clumping describe how individual emulsion grains swim together in the emulsion to form these clumps (doesn’t happen); others claim the origin is development by-products from one developing grain diffusing out and rendering nearby unexposed grains developable (also doesn’t happen except with lith films in lith developers). Still others confuse extreme graininess with reticulation (an altogether different “dry lake bed” effect resulting from a temperature shock between solutions that shatters the gel matrix).

The many references to grain clumping do have a valid premise in that they recognize that what we term graininess in a print is not a visual response to individual emulsion grains. These are far too small to resolve in even a very big enlargement. What we are reacting to is indeed aggregates of several grains. So call these aggregates “clumps” and there is surely some driving force responsible for their formation. Right? Nope. The clumps are of purely stochastic origin: The distribution of developed grains in a uniformly and moderately exposed area of a negative is completely random, and this randomness guarantees the presence of clumps.

Ever notice how a sidewalk turns dark when it gets wet? Imagine that the dry sidewalk represents clear film base and the wet portions represent developed silver. (Fellow upstate New Yorkers may relate better to an analogy of snowflakes on their driveways.) Step outside and watch the sidewalk as it begins to rain. Individual dark dots appear with the first few drops. They may seem to be uniformly distributed when they are very few in number, but as the rain continues and a larger fraction of the sidewalk’s surface is wet, drops will begin to overlap here and there. A little more rain and groups of partially overlapping drops may start forming random aggregates. Some will be nondescript clumps, others may form lines, worms, and assorted geometric patterns. Our brains are so good at pattern recognition you may even find an occasional face. Rain falling as a fine mist is an excellent model for a fine-grained film; rain falling as large drops mimics a coarse-grained film. View the pattern of the large drops while looking out a second-floor window, then run down and look again as you stand on the sidewalk. What you see is mathematically equivalent to prints representing lesser and greater degrees of negative enlargement.

Or try this: Open a new Excel spreadsheet. In cells A1 and A2 enter: =RAND(). Highlight the two entries and drag them down, copying them into the next thousand or so rows. This provides two long columns of random numbers. Go to the chart wizard and make an x-y plot of one column against the other, specifying round, black data markers maybe a quarter-inch in diameter. The result will look something like the illustration shown here. Every time you hit F9, Excel will recalculate and display a new random pattern. Make a big enough pattern and do enough iterations with F9, and you might even come across an image of your mother-in-law.

Developer activity, time, temperature, and a host of other things, most of which we have discussed in earlier columns, affect graininess. But they do not drive some mysterious clumping phenomenon. The raindrop analogy is more than just visually similar to film grain. The same probability functions and mathematical treatment apply equally to both, and allow pretty exacting calculations of both film density and film granularity, the fundamental variables being only two in number: the cross-sectional area of the average individual silver particle (raindrop) and the number of them per unit area of film (sidewalk).

Summarizing the origin and appearance of “grain clumping,” it is sufficient to remember this:

The grain in Spain
(and everywhere else)
falls mainly like rain.


About the Author

Dick Dickerson & Silvia Zawadzki
Contributor
Dick Dickerson and Silvia Zawadzki are retired Kodak black-and-white product builders who have authored numerous articles for PT. They can be contacted at querybw1@aol.com. Dick and Silvia reside in Rochester, NY.