HC-110, introduced by Kodak in 1962, is a liquid-concentrate black-and-white film developer whose nomenclature confuses more often than it clarifies. The concentrate is a thick, honey-like syrup, and Kodak’s published development times are keyed to a series of single-letter dilutions: A, B, C, D, E, and F. Two further letters, G and H, circulate widely but were never described in any Kodak publication; they were added later by users. Understanding where those letters come from requires separating two different reference points the figures can be measured against: the raw syrup and an intermediate stock solution.

The Stock Solution and the Letters

As originally documented, HC-110 was meant to be handled in two steps. The syrup was first diluted one part concentrate to three parts water to make a stock solution, and the lettered working dilutions were then mixed from that stock. The two-stage scheme exists because the concentrate is too viscous to measure accurately in the small volumes a single roll requires; a stock solution of more manageable consistency made repeatable measurement easier.

The letters themselves denote final working strength relative to the raw syrup. Kodak’s Publication J-24, the developer’s technical datasheet, defines dilution A as 1+15 and dilution B as 1+31, with the more dilute lettered strengths running through dilution F at 1+79; the unofficial dilution H is conventionally taken as 1+63, half the strength of B. Several of the Kodak letters were chosen to reproduce the activity of other Kodak developers: dilutions C, D, and E were designed to match the sheet-film development times of DK-50, DK-50 1+1, and DK-50 1+2 respectively, so a darkroom switching from those products could keep its existing time charts.

Why Dilution B Became the Default

Dilution B, 1+31 from the syrup, settled in as the reference strength for general small-tank hand processing. Its activity places typical development times in a window long enough to be controlled by hand but short enough to remain practical. Times much under five minutes are difficult to manage with consistency, because pour-in and pour-out intervals become a large fraction of the total and agitation errors are amplified; dilution B keeps most common films comfortably above that threshold.

It also carries a distinction the more dilute letters do not. Kodak’s datasheet specifies dilution B as the only working strength suitable for replenishment, treating the weaker dilutions as one-shot developers to be discarded after a single use. That combination of a workable time range and a published replenishment regime made B the conventional baseline against which the others are described.

How Working Strength Shapes Activity

Because all the letters draw on the same syrup, the difference between them is purely the quantity of active developing agent per unit volume. A more concentrated working solution carries more reducing capacity and develops faster, while a weaker one slows the reaction. Beyond raw speed, greater dilution lengthens development and tends to moderate highlight density, since the smaller reserve of developer in contact with dense, heavily exposed areas can become locally exhausted before the thinner shadow areas are fully developed.

This is why diluting further is treated as a contrast-control tool rather than a mere convenience. The trade is finite developer capacity: a one-shot working solution at high dilution holds only enough agent for a limited film area, which is also why Kodak reserves replenishment for the more concentrated dilution B. The lettered system, read against the syrup it descends from, is ultimately a shorthand for choosing how much developing agent reaches the film.