Sale! Add to cart 1 Pair Of Glass Knifes $39.99 $29.99 One pair of glass ‘knives’ on card. The user heats each knife over a flame then places his favourite herb between the heated knives and presses the knives together while inhaling the smoke given off. When knives are heated they should not be placed directly on a cold surface or it may cause the glass to crack. Overall length of each knife is approx 180mm.