This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
To soften a hard paint brush, stand the brush overnight in a pot of soft soap and clean in warm water. Afterwards clean in benzine. If the brush is wrapped with a string do not let the string touch the soap.
Paint brushes which have dried up as hard as stone can be cleaned in the following manner: Dissolve 1 part soda in 3 parts water; pour the solution in a cylinder glass, and suspend in it the brushes to be cleaned, so that they are about 2 inches from the bottom of the vessel. Let it remain undisturbed at a temperature of 140° to 158° F., 12 to 24 hours, after which the most indurated brushes will have become soft, so that they can be readily cleaned with soap. It is essential, however, to observe the temperature, as bristle brushes will be injured and spoiled if the heat is greater.