5 DISCUSSION AND CONCLUSIONS

THE LINEAR RELATIONSHIP between fading rate and oxygen concentration observed for colorants #1 through #4 is in accord with the prediction of Giles8 and the observations reported by Lasareff.9 However, a nonlinear relationship between kr and P/Pa was found to be the most common pattern of behavior. The down-turn in rate at lower oxygen concentrations, though less pronounced than expected, is nonetheless in general accord with the prediction of Thomson.7 Furthermore, the square root of (P/Pa) was found to correlate linearly with the fading rate of colorants #5 through #11. This behavior suggests the occurrence of free-radical autoxidation reactions.15,16

The linear and square root relationships indicate that the beneficial influence of an oxygen-free environment can be obtained without the need to reduce the oxygen concentration to the parts-per-million level suggested by Thomson.7 As much as 0.2% oxygen (equivalent to P/Pa = 0.01) would appear to give at least 90% of the effectiveness of a completely oxygen-free environment. Furthermore, a 0.2% oxygen concentration might be achieved easily in a practical situation at relatively little expense.17

From the data in Table II, one can estimate the practical benefit of an oxygen-free environment. The inverse of the relative rate at P/Pa = 0, (1/i in Table II) is the factor by which the life expectancy of the colorant would be multiplied if displayed in the absence of oxygen. In general, a significant enhancement in life expectancy (1/i > 10) is not observed. Furthermore, colorants #16 and #17, as well as many others reported in the literature12,13,14 fade more rapidly in the absence of oxygen. Thus, although an inexpensive display case might be constructed that is capable of achieving better than 90% of the efficiency of a truly oxygen-free environment, such a display case would be useful only if the colorants in an object have been identified, are known to be particularly fugitive, and are known to benefit significantly from an oxygen-free environment.

ACKNOWLEDGEMENTS

THIS INVESTIGATION was made possible by grants from the Andrew W. Mellon Foundation and the National Endowment for the Arts. The authors also express their thanks to Dr. R. L. Feller, Director of the Center on the Materials of the Artist and Conservator, for invaluable consultation and discussion.