My back brush as accumulated a layer of black grime at the base of the bristles. Googling suggests using various concoctions, e.g., vinegar, bleach, clorox, baking soda, hot water, etc. I want to use vinegar. But I want to get serious, and not dilute it 4:1. And I want it soak it overnight. Is this too harsh? I don't think it will harm the wood handle or the bristles, but I don't know whether the bristles are glued to the handle. If it is, I don't know if the glue will become compromised.
Natural bristles in a wood brush are generally held with a hot-melting glue (either hide/gelatin glue or modern plastic hot glue, depending where it was made). Neither of those will be compromised by the 5% strength of food grade vinegar. Bleach, on the other hand, will damage the bristle and might compromise the glue as well.
Leaving the brush in the sunshine for a day might do as much good as a vinegar soak...
After soaking the wooden brush for weeks, it came out looking like it was in a fire. Still the black grime persists (though probably less). I since bought a plastic brush where the grid of bristle bunches are further apart at the base. So much more drying happens. No problems with the plastic brush so far. So that is the solution. Look for a plastic brush where the grid of of bristle bunches are far apart. Forget this rustic wooden look, with animal hair bristles.