It is not necessary to wash Pure Boar Bristles, because they don't take dirt like regular Synthetic Bristles.
Just remove the Hair from the Bristles and the Hairbrush is ready to use again.
Cleaning Boar Bristle Hairbrushes - After Brushing, please remove the Hair from your Hairbrush by hand or with a comb. Then tap the Brush few times on your flat hand (bristles down) and the Hairbrush is ready to use again.
If you feel more comfortable to rinse the Bristles with water after a while, please use a flat Bowl with lukewarm water Boar Bristles down (the water should not be higher than the Length of the Bristles, you can add a mild detergent). Move the Brush through the water for a little while, then rinse the Hairbrush with water - Bristles always down!
If you wash your Hairbrush with the Boar Bristles turned up, the water goes in the Bristle Holes, the wood will swell and crack.