Sounds like it was caused by second hand smoke. I've got an old DeJur (sp) enlarger with condensers and since I have it in a very controlled environment I have never had to clean them. Except when I first got the enlarger from my dad. He was a heavy smoker and guess what color fog (scuz) was all over the condensers? You guessed it, yellow.
I don't use it very much any more as I have found the hassles involved with dust and spotting far out weight any supposed benefits from a condenser system over a diffusion enlarger.
I hope I haven't open a can of worms here.