This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
Put the brush in a strong, warm solution of borax and water, and then wash in clean, warm water. If the bristles have become hard, allow the brush to remain in the solution until soft, keeping the solution warm in the meantime; then wash it out in warm water. - Contributed by N. J. Shat-tuck, Woburn, Mass.