Too much and the hairs will splay. Too little and they won't fit snugly. The brushmaker feeds the hair into a small brass cylinder, taps it to bring each strand into line, then lifts and twists it out of the cannon with a diaphragm. This action pulls the outside hairs down, producing the candle-shaped knot. Brush heads to a size 9 are secured with a dab of glue.
Larger sizes are hand-tied with cotton thread.
The knot of hair is then fed into its ferrule, measured precisely for length, and glued in place with a touch of epoxy resin. The ferrule is heated briefly over a Bunsen burner before the birch wood handle is slipped into place. As the metal cools, it grips tight. To make absolutely sure it's securely in place, the brush is fed by hand into a small vice, the only machine in the room. This hand-operated device crimps the metal and creates the distinctive decorative border at the top of the ferrule, called the knurl.