Why 68F/20C?

[Rayt]

Here in the Summer water out of the tap is 78 degrees F. I process my film at 68 because this is the standard temperature listed by manufacturers. I keep ice water handy to lower the temperature but then worry about the difference in temperature during the water stop and fix. I now do my processing at higher temperatures (75F) but is 68F ideal or "better" in any way and will the higher temperature affect the emulsion negatively (sorry!). I mean when all the film makers list dev temp at 68F they must know something?

 

[wpb]

This is one of those conventions rooted in practicality. Your darkroom chemicals are going to wind up at room temperature so just use that as your working temperature unless it is well below 65 or above 80. Once you get outside of this range the developer doesn't seem to work as consistently (being either not active enough or overly active). I've run thousands of RC and FB prints and most emulsions can tolerate a few degrees of difference between any of the chemicals and it has no ill effects on the prints. The key is that you want the results repeatable.

 

[Freakscene]

NIST standards for conditions for experimental measurements to be established to allow comparisons to be made between different sets of data uses 20C/68F, even though the international standard is 25C. More pragmatically, any colder and development times become impractically long, and any hotter and times often become unworkably short, but also grain is often more noticeable even when films are developed to the same CI.

Marty

 

[Roger Hicks]

The standard temperature in the UK used to be 65F, 19C. Source: old BJ Almanacs. In any case, 65F/19C and 78F/25C are, as WPB says, convenient limits for development temperature and they are the range for which most developers are optimized. It is perfectly possible to design developers to be used at 38C/100F -- look at the first B+W developer for E6 -- and I vaguely recall that "photo finish" developers were used at that temperature. There is (or was) a risk of emulsions softening at very high temperatures, though not in the time they were normally immersed. Also, with some old films, there was a risk of reticulation if the variation between the temperatures of the various baths was too great: the last film from a major manufacturer to show this was the version of Tri-X before the current one.

In other words, like most things in photography, it's a mixture of pragmatism and historical accident. I normally process at 24C/75F.

Cheers,

R.

 

[DominikDUK]

During the plate era 18°C was more or less the Standard, look up the old times for Rodinal. It was advised to use it at 18°C. 20°C is a compromise between increase in grain and loss of speed due to lack of developer activity. Also old film emulsions weren't as well hardened as modern one an each degree away from the Gelatine melting Point is one more degree of safety from Emulsion dissolving. Also If you look up old Tropical developers they sometimes had a hardener in the formula or Film. mfg would advise People to use an additional hardening step when developing the film in warm and tropical climates.

20°C was the best compromise at the time.

 

[sevo]

In Germany 20°C used to be the common "room temperature" of legal fiction from the 1950s to 80s. 18°C may have preceded it - nowadays, it is drifting towards 21-22°C. Central heating systems had to be able to heat the warmest (living) room to that point in any weather (short of emergencies), or they were considered broken. At least hereabouts that made it a temperature easy to achieve, maintain and measure. The 38°C of colour film derive from the human body temperature - easy to test with medical thermometers and still a somewhat safe temperature for film (it is wise to specify emulsions to be fit for transport in a inner pocket, so a limit of 38°C ensured that the emulsion did not melt off right when put into the primary hardener of early colour processes).

 

[Rayt]

Thanks everyone for the answers. I am enlightened! After all these years I am still learning. I am processing Tmax 400 with DDX 1:7 at 24 degrees for 8 minutes.

 

[Fotohuis]

It also has to do that above 24C some old school emulsions can peel of the layer when you do not add a hardener to the process. But modern B&W emulsions you can even process on 30C.
Most B&W developers have a regular processing time on 20C. R09/Rodinal is generating above 22C already extra grain hence for this developer best developing temperature is between 18C-20C.

 

[Rayt]

It also has to do that above 24C some old school emulsions can peel of the layer when you do not add a hardener to the process. But modern B&W emulsions you can even process on 30C.
Most B&W developers have a regular processing time on 20C. R09/Rodinal is generating above 22C already extra grain hence for this developer best developing temperature is between 18C-20C.

Thanks. I have done a lot of searches and some people mentioned extra grain and even fogging with certain developers if processed under higher temperatures. I will keep this in mind regarding Rodinal.