Train Chemical technicians On the In Ac
MODERN CHEMICAL TECHNOLOGY is a unique, hands-on, practice-oriented program developed and newly updated by the American Chemical Society. It's a concise, seven-volume practical approach to chemistry for both students and on-the-job, degreed technicians and a valuable set of reference manuals as well.
egg tempera (9). Investigations of this nature, however, must take into ac count the possibility of pollutants from later restorations. The painting could have been treated many times with a variety of materials. Until re cently, few records were kept on the treatments applied to paintings, so conjecture about modifications to a specific work is based on the practices common to specific historical periods. For example, it would have been com mon practice during the 19th century to remove the varnish and apply new layers of varnish or oil, either singly or in combination. In addition, hide glues or gelatin may have been used to con solidate flaking paint. It is also rea sonable to assume that surface dam age may have been retouched or areas reworked in varnish, oil, or temperatype paint. A sample of red media from the sur face of the painting's lower left corner
gave a spectrum (Figure 2) similar to shellac. This material was in common use in the 15th century and is men tioned in treatises of the time (10). The "core" sample (Figure 1) from the upper right quadrant included both the ground or priming layer and the paint layers. The white ground layer was applied to the wood panel to make the surface smooth and to pro vide a proper surface for the paint layers. It also provided a white base layer to reflect light back through the various paint layers. The priming nor mally would be composed either of shell white (calcium carbonate) or, more typically for an Italian painting, gesso (calcium sulfate) with an animal skin glue as a binding medium (11). The spectrum in Figure 3a clearly shows the presence of calcium sulfate dihydrate, but the identity of the binder is uncertain. The paint layer that lies immediate-
4400
3000
2000 1500 Wavenumber (cm-1)
1000
400
4400
3000
2000 1500 Wavenumber (cm-1)
1000
400
Students and technicians focus on a wide range of practical applications of theoretical concepts...and cover sampling, analytical, and instrumentation techniques. Cost effective. Thorough. Unique. For complete information, write or call collect: AMERICAN CHEMICAL SOCIETY Division of Education 1155 Sixteenth St.,N.W, Washington, D.C. 20036 PHONE: 202/872-4588
Figure 3. Spectra of core sample (a) White layer—aperture: 120 μπ\\ scans: 512. (b) Blue layer—aperture: 150 μπ\\ scans: 512. (c) Red layer—aperture: 120 μπ\; scans: 512. (d) Gold layer—aperture: 80 μιχν, scans: 1536
878 A · ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
