Structural Changes from Heating Amber and Copal as Observed by Nuclear Magnetic Resonance Spectroscopy.

Structural changes caused by heating of fossilized (amber) and semifossilized (copal) resins have been examined by nuclear magnetic resonance spectroscopy. A set of 28 samples was constituted to include different geographical sources, degrees of maturation, colors, and structural groupings. The onset of structural alterations was determined by observation of the lowest temperature at which spectral changes occurred. Both proton spectra in solution and carbon-13? spectra in the solid state then were recorded of cooled samples after heating for 12 hr at temperature increments, until liquification of the sample began. The spectra of both nuclides exhibit loss of a few peaks, broadening of most peaks, and enhancement of the unsaturated or aromatic region at the expense of saturated resonances. Such changes are irreversible and lead to a harder and less soluble material on cooling. The changes parallel those that occur with maturation of fossil resins or materials that lead to coal.