Governance of Temperature in the Darkroom

This guide illustrates the principle upon which the time-temperature table is founded, teaching the operator how correctly to compensate for thermal variations in the developing bath without compromising the result.

Among all the elements that contribute to the success of a photographic development, the temperature of the bath is that which most escapes the control of the inexperienced operator, and for this very reason it merits treatment apart. Whosoever approaches the darkroom for the first time tends to consider the developing time as a fixed, immutable datum; whereas chemistry reasons not in absolutes, but in relations. The time indicated in the recipe holds true only for the temperature for which it was calculated: this being altered, the time necessarily alters as well.

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The Physical Principle:

Every chemical reaction (that of photographic development making no exception) proceeds at a velocity proportional to the temperature of the bath in which it unfolds. A warmer bath accelerates the reduction of the exposed silver halide, so that the same image density is obtained in a lesser time; a colder bath retards it, whence a greater time is required to achieve the same result. This relation is not, however, linear, but follows a course which our laboratory tests have permitted us to tabulate with precision for the interval comprised between 18 and 28 °C.

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How to Read the Time-Temperature Table:

The table is to be read by horizontal rows: each row represents one same developing result, obtainable at different temperatures by employing different times. Having identified the base time in the 20 °C column (the standard reference temperature for all our powder developers), the operator may scan the same row leftward or rightward to learn the corresponding time for a different working temperature.

By way of example:

a base time of 9:00 minutes at 20 °C corresponds to 10:50 minutes should the bath stand at 18 °C, or to but 6:50 minutes should the bath stand at 25 °C.

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Why Temperature Escapes Control:

In current practice, the developing bath rarely maintains a constant temperature from beginning to end of the process: the ambient temperature of the darkroom, the season, even the warmth of the operator's hands holding the tank, all conspire to make it vary. It is good practice, therefore, to measure the temperature of the bath immediately before beginning development, and not to rely upon approximate estimates.

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The Lower Limit:

There exists a threshold below which compensation through the lengthening of times no longer suffices to guarantee a controllable result: this threshold is fixed, for our developers, at 18 °C. Below this temperature, many reagents slow their activity in a non-linear and unpredictable manner, wherefore we strongly discourage proceeding with development in colder baths, even should one attempt to lengthen the times further still.

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A Practical Counsel:

For whosoever lacks a thermally stable environment, we suggest the employment of a water bath: immerse the developing tank (or the jugs containing the solutions) in a larger vessel filled with water at the desired temperature, so that the chemical bath maintains its stability for the entire duration of the process. A laboratory thermometer, of the immersion type, is an instrument as indispensable as it is inexpensive, and its absence is often the first cause of inconsistent results from one developing session to another.

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We invite whosoever approaches this practice to note always, alongside the developing time employed, the effective temperature of the bath as well: this is the sole way to build, development after development, a personal and reliable knowledge of one's own process.