4 CONCLUSIONS

FOURIER TRANSFORM INFRARED ANALYSIS can identify five natural resins (shellac, sandarac, mastic, copal and rosin) individually and as components in a mixture based on a set of band positions presented in this paper. The precision of the band positions was demonstrated to be within �2.6 cm−1 for: 1) the instrument; 2) sample preparation used in this study; 3) variations due to resin supplier or degree of refinement. Changes in band position due to varnish preparation and photochemical aging were shown not to diminish the effectiveness of the band identification scheme.

For resin mixtures of two components, with a total sample mass of approximately 1 mg, a component with a concentration of 50 w/w% or greater may be identified by the presence of the band positions listed in Table 2. Minor components in a resin mixture may be identified by computerized deconvolution of the spectra to resolve any overlapping bands. With deconvolution, a component with a concentration as low as 6 w/w% may be readily recognized and components with concentrations as low as 2 w/w% may be tentatively recognized. Additionally, the computer may be used to spectrally subtract most of the major component from a mixture, thus, in effect increasing the relative proportion of the minor components. Using computer subtraction followed by deconvolution, components in concentrations as low as 1 w/w% may be identified.

ACKNOWLEDGEMENTS

I WOULD LIKE TO THANK Barbara Roberts and Gillian Wilson from the Getty Museum for allowing samples to be taken from museum objects and for Barbara's help and encouragement. I would also like to thank Marjorie Cohn and Eugene Farrell for their permission for, and Johann Peron for her help in, sampling the Gettens and Stout collection at the Harvard Art museums. I appreciate Robert Mussey for generously sharing his translation of Brachert's article. I would like to thank Eric Hansen for his advice on the accelerated aging of resin films. I am grateful to Frank Preusser, Neville Agnew, Jack Gromek, John Perkins, George Rogers, David Scott and Dusan Stulik for their very helpful comments and contributions.