known deceased teacher, for instance— can produce very precise dates. These sometimes specify the date that a painting was made, but that is rare. The dates of paintings of class B and stronger examples of class C are usually reliable within two generations, that is, plus or minus twenty-five or thirty years. The methods of dating depend upon certain suppositions, such as that an abbot identified in the inscription as patron probably commissioned more works toward the end of his life and particularly during his abbatial tenure. In the analysis of lineages, it is reasonable to presume that the patron was the disciple of the last lineage master depicted, and that he commissioned the painting in the last two decades of that master’s life or in the first two decades after the master’s death. The accuracy of secondary comparison-based dating depends on a continuous series of paintings—drawn from the relevant artistic and religious traditions—that have been reliably dated through internal evidence. Ideally, two or three firmly dated paintings of each main subject should be available for comparison from each generation. When styles change quickly, as they do in certain periods, comparison dating is more accurate. But with slower rates of stylistic change, there is greater room for chronological error. Assuming that a continuous series of internal-evidence-based datings has been made, based mainly on paintings of A and B classes, I believe dating by comparing styles can approach accuracy within one or two generations, that is, plus or minus twenty-five to fifty years, at best. In the worst case, the accuracy of stylistic comparison can achieve that of some of the best connoisseurs today, that is, plus or minus seventy-five to one hundred years. Higher degrees of probability— nearing or reaching 100 percent certainty—can also be achieved for certain
chronological judgments. For example, a thangka must have been painted in or after the lifetime of any historical figure portrayed. But the earlier the figure, the longer the period encompassed by the anterior limit; hence the value of complete lineages, which provide not only anterior but also posterior limits. Greater certainty can be gained at the cost of diminished accuracy. One can assume, for instance, that most paintings with both a complete lineage and a depicted patron were painted in the last thirty years of the last lineal teacher’s life or in the thirty years following his death. The probability that such a painting was made in the last forty years of the last master’s life or in the forty years after his death is, of course, still higher. Such a thangka was almost certainly painted within the last seventy years of its last master’s life or in the seventy years after his death.
example, deposits from the smoke of butter lamps, dust or material from recent attempts of conservation may lead to a too recent dating. (iv) An inherent source of uncertainty is the irregular production rate of 14C in nature. It can cause grave ambiguities in the dating. This unfortunate fact of nature may be appreciated by the calibration curve [in the illustration, for the years 1000 to 2000]. The dating is hampered particularly in the ranges 1000–1150, 1260–1420 and 1460–1620. In these periods, the dating uncertainty is grave. For paintings [that date] after about 1660, dating by 14C analysis is virtually impossible. Often, it is merely possible to distinguish material produced before and after the nuclear bomb testing of the 1950s. 647
Carbon 14 Dating
Pigment Analysis
For dating surviving works of art, one should also take into account radiocarbon (carbon-14 or 14C) analysis results, if they are available. The possibilities and limitations of this method have been summed up by Richard Ernst:
Another possible source of chronological clues is the paint material of the thangka. Through most of their history, Tibetan artists used a limited, fixed palette of pigments and dyes derived from minerals and plants or insects.648 Yet a rare pigment can give clues for dating and provenance. Some test cases investigated by Richard Ernst produced interesting results; the examination of a small number of paintings revealed the presence of such unexpected pigments as smalt (a blue cobalt glass) and Prussian blue.649 Gilles Béguin presented pigments found in the sixteenth-century Ngorpa Lamdre teacher and abbot series, “sMra ba’i seng ge” (=Lha mchog seng ge), and some other Beri (Bal bris) paintings; the pigments that differ significantly are those for the colors white, green, blue, and orange.650
C analysis is the only truly quantitative method available for paintings. But it has numerous inherent limitations: (i) The dating refers to the support material, which could be older than the painting itself. This is of particular relevance for wooden objects where the 14C date reflects the date of growth of the part of the tree stem that has been used. (ii) The precision of the 14C content measured by accelerator mass spectroscopy limits the accuracy usually to plus or minus 50 years, depending on the period to be dated. (iii) Contamination by more recent insoluble material, for 14
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