The dating of rock art has been a major archaeological problem because, until recently, the
art could not be directly dated, and clear associations with dateable archaeological contexts are
rare. The introduction of accelerator mass spectrometry (AMS) for radiocarbon analysis greatly reduced the amount of carbon necessary for an age estimate so that even small amounts of charcoal taken from rock, paintings could be dated. Van der Merweet al. (1987) and Hedges et al. (1987) were the first to directly radiocarbon date charcoal pigments from rock paintings (from South Africa). Since 1987, a number of laboratories have radiocarbon-dated rock
paintings, with the largest number having been produced by French researchers (Clottes 1994, 1996,1998, 1999a, 1999b; Clottes et al. 1992a, 1992b,1992c, 1993, 1995, 1997; Fortea 1996; Girard et al.1995; Lorblanchet 1994a, 1994b; Lorblanchet et al.1995; Moure Romanillo et al. 1997; Ripoll Lopez1994; Valladas et al. 1990, 1992).
Many of the other charcoal pigments have been dated by Texas A&M University group and their colleagues (Armitage et al. 1997, 1998, 2000; Chaffee et al. 1994; David et al. 1999; Diaz-Granad6s et al. this issue; Hyman et al. 1999; Ilger et al. 1994, 1995; Prous 1999) with isolated dates from others (David 1992; Farrell and Burton 1992; Geib and Fairley 1992; Hedges et al. 1987; Moure Romanillo et al. Over a decade ago, the archaeological chemistry laboratory at Texas A&M University developed a plasma-chemical technique for extracting sub-milligram amounts of organic carbon, permitting the radiocarbon dating of prehistoric rock paintings with AMS (Russ et al. 1990).
The technique has the distinct advantage that it permits radiocarbon dates to be obtained by extracting any organic material that the original painters added to their paints. In dating archaeological charcoal from rock painting samples, an alkali (6 M NaOH) wash is used prior to plasma treatment for removing possible humic acids. An aa-a (acid-alkali-acid) treatment is commonly employed at other laboratories to remove carbonates and humic acids. However, Hedges et al. (1998:36) have shown that “the routine acid treatment had not dissolved all the oxalate present” in rock painting samples from Argentina that had oxalate-rich charcoal pigments. That means that extreme care must be taken in techniques that utilize high-temperature.
This content was downloaded from 126.96.36.199 on Mon, 23 Jun 2014 04:35:33 AM All use is subject to JSTOR Terms and Conditions REPORTS 473 combustion after a-a-a pretreatments for dating rock paintings-at least for high-oxalate samples. We have found oxalate commonly associated with rock paintings on limestone. With the plasma-chemical technique (e.g., Hyman and Rowe 1997 and references therein) the acid treatment used elsewhere is unnecessary. An advantage of the plasma-chemical technique is that the presence of inorganic carbon in the form of carbonates and oxalates does not affect radiocarbon results because the plasmas leave them undisturbed. More will be said about this later in the discussion of the Naj Tunich dates, as this observation has been confirmed. The plasma-chemical technique was used here for five of the six dates reported.
Radio-frequency generated low-temperature (-150? C), low-pressure (-I torr) oxygen plasmas were used to remove organic matter from the paint leaving the substrate rock and carbonate/oxalate accretions intact. Similar plasma conditions were used for all samples studied at TAMU, whether dating organic binder/vehicle(s) in inorganic pigments or charcoal pigments.
The early TAMU studies established the necessity for cleaning the plasma extraction system with oxygen plasmas before sample insertion to rid surfaces of possible contamination. After loading a sample into the plasma chamber, the chamber was evacuated, and then ultra-high-purity argon was leaked into a pressure of -.2 torr on the thermocouple gauge. Low-power argon plasmas were run on each sample to remove adsorbed carbon dioxide by collisions of the unreactive, but high-energy, argon atoms and ions. This process was repeated until the amount of carbon desorbed by the plasma was negligible. Recent blanks from the TAMU laboratory run at the Australian Nuclear Science and Technology Organisation (ANSTO) accelerator mass spectrometry produced results that introduced “a negligible amount of modern carbon compared to “our graphitization process [that]introduces about .0009 mg modern carbon” (E. Lawson, personal communication, 1998). Samples were then oxidized in a low-temperature plasma. The gaseous carbon dioxide produced by the plasma oxidation of organic material in the paints, whether charcoal or an unidentified organic binder/vehicle, was
collected and its pressure was measured after the H20 produced in the plasma reaction was removed by freezing. The carbon dioxide was then frozen atI 940C, sealed into a glass finger, and sent to an AMS facility for 14C analysis.
The process has successfully (apparently at least, within the broad range of ages given by the archaeological inferences for the samples studied) dated iron- and manganese-pigmented pictograph samples around the world, as well as many charcoal-pigmented paintings (summarized in Hyman and Rowe 1997). We subject the technique to a more rigorous test in the present work.
With the permission of Dr. Juan Antonio Valdes, Director General del Patrimonio Cultural y Natural, Guatemala, samples were taken from three inscriptions that had been vandalized in 1989. Even though the paintings had been previously damaged, every attempt was made to minimize further damage through sampling. Tiny samples of pigment (and accompanying basal limestone and accretion minerals) were removed while wearing surgical latex gloves and using new surgical scalpel blades for each sample. The samples were collected and wrapped in aluminum foil and stored in sealable plastic bags. These were kept in a desiccator before and after the alkali pretreatment (see below). Each sample was examined under an optical microscope to ensure that no extraneous microscopically visible organic material was included with the analyzed sample.
Because of the limited sample size, no attempt was made to characterize the Naj Tunich pigments. A multi-spectral imaging project found that most of the Naj Tunich pigments (including that of Drawing 29) show a reflectance pattern characteristic of charcoal (Ware and Brady 1999). The black color in charcoal pigments disappears during an oxygen plasma reaction, while the color of manganese oxide/hydroxide and other inorganic black pigments encountered in rock paintings does not change. The color in the Naj Tunich samples disappeared during extraction, and in all previous cases at the TAMU laboratory in which the black pigment was characterized as charcoal, the disappearing black color was observed. There is, therefore, no reason to suspect any other pigment here. Pigment samples from the three Naj Tunich glyph texts were brought to the TAMU laboratory to be processed as described above prior to submission to the Lawrence Livermore National Laboratory Center for Accelerator Mass Spectrometry (hereafter called CAMS) laboratory for radiocarbon analysis.
The samples were from paintings DI 9, D29, and D82 (Stone 1995:111, 161-167,169,170,179-183, 196-197, 203-204, 217). Figures 3-5 show photographs of the three texts analyzed here.