Any good pipe smoker knows that controlling the carbon build up is essential in getting the most out of your carefully selected briar pipe. A well reamed pipe will smoke cooler, sweeter and last a lot longer. Opinions vary on the optimum depth of Carbon you should allow to build up in the bowl but generally the recommended is around 2mm and should be even around the bowl to prevent hot spots. This can not be done effectively with a pipe knife and is only achievable to a certain level with a 2 prong self adjusting pipe reamer or the ever popular hedgehog reamers.