I had a strange experience in the darkroom this evening. I was contact printing a 5x7 negative, which perhaps has lowish contrast but not to an extreme degree. I had a couple of tries at printing it on Ilford MGIV, with a 7.5W bulb pointed at the ceiling, which I've found I have to do if I want exposure times longer than a few seconds.
The results, not to put too fine a point on it, sucked; murky grey things with no blacks or whites in the image area. Increasing the exposure to get the blacks black resulted in a ridiculously dark, no-contrast print; decreasing it to get the whites resulted in a ridiculously light, no-contrast print.
I made some disgusted noises, then flipped the light to point down, swapped in a 15W bulb, and made a straight print on Fomalux (a contact-speed chloride paper). The result was quite nice; I had to do a second print to burn in the sky, but even the initial straight print was quite acceptable apart from that---in particular, it looks like a print with normal contrast.
What's going on here? Both papers supposedly print at grade 2 with unfiltered light, so why would I be seeing a difference in contrast? Is a bounce from an off-white ceiling likely to be changing my colour temperature so much that the Ilford paper is printing at a lower grade than expected? (I've done it this way before and gotten decent prints, though come to think of it, I *did* have to filter to a higher grade than I expected sometimes.)
Is the bounced light my problem, or is there something else I'm not thinking of?