Charcoal production from wood and cellulose: implications to radiocarbon dates and accelerator target production

Abstract

Radiocarbon dating with accelerators requires the manufacture of suitable carbon targets. Carbon yield and¹³C fractionation were investigated for the simple and direct pyrolysis of wood and cellulose to charcoal. Under continuous vacuum removal of evolved volatiles, carbon yields of 35 to 40% and δ¹³C fractionation of −2.5‰ were observed in the pyrolysis of wood to charcoal, whereas yields of 30% and fractionation of −0.8‰ were obtained in the pyrolysis of cellulose to charcoal. Yield and fractionation leveled off at temperatures above 300°C. Yields and fractionations were also measured for pyrolysis of wood and cellulose in a continously-flowing argon atmosphere. Yields were higher and fractionations smaller than for the corresponding vacuum cases. For cellulose sealed in evacuated glass tubes and pyrolized at 550 to 600°C, carbon yields greater than 60% and fractionation of about −0.5‰ were observed. Yields increased and fractionation tended to decrease as the ratio of tube volume/mass of cellulose decreased,ie, as the pressure increased. Reheating of this charcoal under continuous vacuum pumping revealed no loss of mass and no alteration of carbon isotopic composition. Fractionation measurements were additionally performed on wood and the charcoal produced from burning in a fireplace, conditions approximating the “natural“ production of charcoal. Despite the large potential fractionation suggested in the wood pyrolysis experiments, charcoal produced in the fireplace showed very small or no fractionation.