I was bowing along fairly happily tonight when I felt some little clunk in an otherwise-fairly-smooth bow stroke.  I had applied rosin a little earlier and thought it might be from little chunks of rosin clinging to the hair.  I felt it with some strokes and not others, but it kept returning.

I took a closer look and found that one or two hairs had a layer of rosin caked on, similar to the way rosin cakes on a string.  Earlier I had noticed a dark-greenish discoloration along one edge of the hair.  The layer of rosin caked onto the hair had some sections missing, of length say 2 mm, where the layer had broken off.  Those gaps were producing the clunk.  I suppose the layer was about once or twice the thickness of a bow hair.

Presumably, this is related to me usually tilting the stick scrollward when bowing, that edge of the hair getting more contact with the strings.  My rosin is mostly Super-Sensitive Dark.  Now I'm sad to report that I don't remember which edge (scrollward or bridgeward) had the stain.  I guess it's kind of academic, since I'll know the stain if I see it again.

I cleaned only those few edge hairs that looked fouled, using a patch of tee-shirt material soaked in 91% isopropyl alcohol.  In a minute or two, the layer came off in little shards up to 7 mm long.

Anyway, now the clunk is gone, and the bowing is good, and the tone is good.

On my bow, the stain associated with the rosin build up was darkish green, but that color may have to do with my strings or my rosin.  My recommendation is to use a magnifier now and then to take a close look at your bow hair.