I suppose we all know that the chemicals for processing B & W Photographic prints should be used at 20 ° C. The manufacturers of the various papers and chemicals say so. On a cold winter night, with the darkroom in a tin shed and the outside temperature down to -2 ° C This is not so easy to achieve.
Cold developer works slower and hot developer works faster. I've been told that incorrect developer temperature affects the image contrast but have not found this to be the case with resin covered, multigrade paper. Depending on the brand, the paper fogs in the developer at around 35 ° C to 40 ° C. So anywhere in the range 25 ° C to 35 ° C will be fine for the developer, stop bath and fixer.
Fan Heater Not Suitable for Darkroom Heating
In times gone by I used a fan heater which was fine for drying the finished prints hanging on the "clothes line" but created a dust problem with negatives needing to be repeatedly dusted off. Sometimes I'd get hairy strands of dust sticking to the emulsion of negatives while hanging in the darkroom to dry. Sometimes I abandoned this method of darkroom heating, the hassle to keep warm being just too much.
Warming the Developer with Hot Water
To warm the developer to a quick acting temperature, I have for some time used a second, larger tray of hot water with the developer tray sitting in it. Whenever the water feels cool to my fingers it's time to add another jug of hot water, until after a few top-ups, the developer tray starts to float, at which time I tip out the water and start again with more hot water. This method has served me well for 15 years or so and is a good way to go in a low budget darkroom.
Electric Bar Heater for Darkroom Heating
Over recent times I've used a bar heater on the darkroom wall. With some care, given the color and low level of light, the paper has been unaffected by this darkroom heating.
But today I just could not stand the fiddling with jugs of hot water any longer. I unscrewed the bar heater from the wall and placed it on the darkroom floor, facing upwards, under the wet bench, to heat not only the darkroom space but also the chemicals.
Before long the underside of the bench was quite warm and the developer, at a constant 25 ° C, required no fiddling. Although the darkroom is poorly insulated, the space soon warmed up to a comfortable temperature. So this is now my method of darkroom heating.
Should you decide to go this way also, just be sure you do not set fire to the place.