Bongs - Conserving herb when using a bong

[Ben12345]

SWIM is low on money, and has some very good, potent herb. He wants to conserve the herb, and smoking through a bong is the best way to do this. However, when he puts only a little amount of the herb in the metal socket (SWIM has made a homemade bong using a 2 litter bottle, hose from an aquarium filter, and metal socket), the weed often goes through the tube and into the water. If SWIM puts a large solid "ball" of weed in the metal socket, and starts smoking, if SWIM covers the top of the socket after he's had enough, with maybe a metal sheet or something, cutting off the oxygen, will the herb stop burning? Will SWIM be able to effectively reuse the rest of the semi burned weed? It will probably be black on the top of the ball, but will it still be ok to smoke again?