If by some misfortune you have not cleaned your brush after use, it is not necessary to throw it out. It can most likely be saved.
Apply Sterling Brown Soap onto the brush and wrap it in an airtight plastic bag. Leave for 3-4 days, then rinse thoroughly with water.
The same procedure can be used if a brush has been used for oil paint. However, the brush only needs to lie wrapped in a plastic bag for 1-2 days before it can be cleaned.