Graphite in various forms has become the standard target for accelerator 14C dating. Graphite has been made by catalytic graphitization of charcoals (Lowe,1984). Thin films of graphite have also been produced by thermal cracking (Beukens & Lee, 1981), electric discharge (Andree et al, 1984;Wand, Gillespie & Hedges, 1984). Vogel et al (1984) pointed out the ease of graphite formation on iron from CO2 and H2 mixtures at ca 600°C. The deposition reactions of carbon from the CO, H2, and CO2 equilibria are well known (Wagman et al, 1945) and well studied. Formation of graphite from CO2 was discussed extensively by Boudouard (1902) and Schenck and Zimmerman (1903), and was known to chemists in France in 1851. We have used a related method, where graphite forms away from the iron, by using a higher temperature, and reduction of CO2 to CO over Zn in the presence of H2 (,Dull et al, 1986) as an alternative to the use of Fe alone. The object of this paper is to point out an even simpler graphite preparation system, which eliminates hydrogen. The decomposition reaction of CO (Boudouard, 1902) takes place according to reaction (1). 2C0CO2 + C (gr) (1) If carbon is introduced as C02, and is reduced to CO by hot Zn and any C02 formed due to reaction (1) is reduced to CO by the hot Zn, then the reaction proceeds in one direction. This method uses a small-volume system, which can be of any size but is typically 5 to 10cc. Using a 5cc system, reduction of CO samples of as small as 0.7cc has been achieved. The apparatus is shown in Figure 1. As mentioned, two reactions occur in the system. CO2 is reduced to CO over -30mg Zn at ca 500°C, and the CO disproportionates to graphite over the iron (1 mg) mg) at typically 700°C. Deposition occurs at temperatures as low as 450°C. The graphite is separate from the iron, suggesting a vapor-phase intermediate.The gas pressure is monitored, and 100% reaction is easily observable, as we wait until the gas pressure is zero. This takes from 2 to 10 hours, depending on several factors. It is clear that the reaction is pressure-and temperature-dependent; however, variations in reaction rate appear to be complex, and apparently dependent on the catalyst. The amount of Fe catalyst used can be very small (< 1 mg) without measurable effect on the reac-

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The Tehuacán region in Central Mexico is thought to be the locale of origin of Zea mays, or maize, a cultivated plant pivotal in the development of agriculture in the Americas (MacNeish, 1981, 1985). The age of the earliest maize, and its rate of dispersal are thus important components of cultural development in the New World. We have secured permission from the Federal Government of Mexico to date critical specimens from Tehuacán, which represent what are probably some of the earliest known stages of maize evolution. Twelve Zea mays samples have been dated, six from Cueva San Marcos and six from Cueva Coxcatlán. These were selected as having the best stratigraphic control and correlation with previously dated charcoal samples, and to represent the most ancient maize. Corn from Cueva San Marcos is oldest: four of the six specimens from this cave were within statistics of 4700 BP (uncalibrated). The oldest known domesticated corn is thus no older than 3600 cal BC (dendro-calibrated in calendric years).