The result proved that short clay blowpipes of about 30 -- 60 cm ( 12 -- 24 in ) facilitate free - blowing .
They are simple to handle and to manipulate and can be re-used several times .
The result proved that short clay blowpipes of about 30 -- 60 cm ( 12 -- 24 in ) facilitate free - blowing because they are simple to handle and to manipulate and can be re-used several times .