"Warm" has nothing to do with it in most normal developers, it's time+temperature. Adjust the time to suit the temperature and results will be so similar as to be indistinguishable. I routinely develop all my black and white at 24C/75F because I use a Jobo CPE2 that will heat, but not cool. In the hottest months of summer sometimes I'm running at 76-77F because of the ambient temperature. I adjust times a bit to suit. Results are fine, no particular grain - and ALL my Delta 3200 is run like this too (albeit in T-Max developer, but it's grainier than D76.)
Originally Posted by Chris Lange
What you have here is a very long development time for the temperature. Ilford specs give only 13.5 minutes at 24C for D76 at EI 12500.
Not that you aren't getting the grainy results the OP is asking for. But you could just as well get it at 20C with a suitably longer time. Ilford publishes a temperature conversion chart that gives equivalent times at different temperatures (this is what I use when I'm a degree or two too hot in the summer - it's close enough for such small differences.) It only goes to 17:15 at 24C, but the equivalent time at 20C is given as 25:00 minutes. Some extrapolation shows the equivalent to 24C/18 minutes would be about 26:30.
Some older films could reticulate if temperature varied too much and developer was warm and then you went into cool stop or the like, but that's a different thing and very unlikely with modern films.