We grew up in a jaded era. Back in the Sixties, everything from toothpaste to gasoline had a magic, secret ingredient, identified only by meaningless initials. Everyone understood claims of such components were not necessarily a good reason to buy a product. When Kodak introduced its “T-Grain technology” in the T-Max line of films, we shuddered a bit at the sense of déjà vu it prompted. And we in fact continue to this day to meet people (largely our own age) who ask, nudge-nudge, wink-wink, if the concept isn’t just so much marketing hoopla.
The basic claim with this line of black-and-white films, ascribed to the advent of “T-Grains,” was that they afforded access to a heretofore inaccessible speed-grain position. Decades of experience had taught that faster films were grainier films and vice-versa. The parameters of film speed and graininess were inextricably linked. This had been true in pre-T-Grain times precisely because of the symmetry of (pre-T-Grain) emulsion crystals. Put any manufacturer’s film in an electron microscope and the individual emulsion crystals always appeared as spheres, cubes, octahedra, or perhaps just misshapen lumps—but always as particles with a high degree of three-dimensional symmetry: length, width, and height being quite similar. The reason this enforces an unfavorable speed-grain relation goes back to high school geometry.
Why it works
Take a lump of clay and roll it into a ball about an inch in diameter. This is a good model for any of the old-style emulsion grains. If it really were an emulsion crystal, two of its characteristics would be of paramount importance: its surface area and its volume. Spectral sensitizing dyes, which impart speed to an emulsion, work only if they are adsorbed to the surface of the individual crystals: More surface area allows adsorption of more dye makes for a faster film. The volume of a crystal determines how many of them are contained in a unit mass of emulsion, and it is the number of crystals in a unit mass that determines graininess. Fewer crystals, more grain; more crystals, finer grain. (See our earlier column in PT, January/February 2008). So we want a faster emulsion? Pull off a larger lump of clay and roll it into a ball. It has more surface area than our first clay ball, so it is indeed a model for a faster emulsion. It is also more massive, so there will be fewer of them in a given amount of emulsion: This new “crystal” will be faster, but also grainier. And here is the rub from high school math: As we make progressively larger clay balls, the surface area (read: speed) is going up as the square of the radius, but the volume (read: graininess) is going up as the cube of the radius. This is not a winnable situation.
Well, not winnable if we insist on clay particles that are roughly symmetric in three dimensions. Get a brick and smash the smaller clay ball into a disc. Its surface area is greatly expanded. The thinner you pound it, the greater its surface area, the more dye it can adsorb, the more light-sensitive it is—but its volume remains unchanged. The “emulsion” is becoming ever faster the thinner it is, and graininess is not changing. Free lunch! Or try this: Break a piece off your flattened disc. Even this fragment may have as much surface area (speed) as the original ball it was derived from, but it contains only a fraction of that original ball’s volume: Same speed, finer grain. Another free lunch!
Okay, the lunch is not altogether free. The tabular grain also has a much larger projected area than the spherical one, so, fully developed, it blocks more light, thereby creating more density. To get the density back down to a useful value would require removal of many of the grains, meaning graininess again suffers. Fortunately most camera film developers are so-called partial grain developers, meaning there is no need to develop a crystal in its entirety. Use the entire (dyed) surface area of the disc-shaped silver- halide crystal to capture exposing light, but develop only a portion of it so the resulting silver particle doesn’t stop much viewing (printing) light and many are needed to generate any particular density. This is the path to improved speed-grain that T-Grain technology affords black-and-white films. Marketing hype? Decidedly not. T-Grains proved to be the way out of a rigid speed-grain box that had defined black-and-white photography for decades.