Senior Project Specialist Jim Druzik discusses with GCI visiting scholar Isabel Kanan the results produced by thermogravimetric analysis. The equipment is to their right. Photo: Jeffrey Levin.

Thermal methods of analysis measure chemical and physical changes that a material undergoes as it is heated. The changes measured include weight gain or loss, change in dimension or strength, and release or absorption of energy. The temperature at which these changes take place is characteristic of the material and its thermal history.

	In thermogravimetric analysis, weight changes in the sample material—due to loss or gain of gases—are measured during temperature transitions. Photo: Jeffrey Levin.

By measuring the full course of reactions over several heating rates, reaction kinetics can be studied that inform important decisions on properties of materials under normal room conditions. This information can, in turn, influence the use of stable materials near works of art in storage and on display. By combining thermal methods with analytical techniques, such as infrared analysis or mass spectrometry, information about the chemical constituents of thermal decomposition can be derived.

There are several different thermal techniques:

• Differential Scanning Calorimetry (DSC) measures comparative changes in heat capacity.
• Thermogravimetry (TG) measures weight changes during temperature transitions due to loss or gain of gases in the sample material.
• Thermomechanical Analysis (TMA) measures the penetration, expansion, contraction, and extension of materials as a function of temperature.
• Although not strictly a thermoanalytical method, light element analysis—a modified pyrolytic GC technique measuring carbon, hydrogen, nitrogen, and oxygen—is a useful adjunct to TG-MS.

Applications:

TMA has been used to determine the firing temperature of ceramics, which can, in turn, reveal something about the technology used by potters. It is an excellent tool to measure properties like the linear coefficient of thermal expansion, glass transition, and melting temperatures on extremely small samples applied to contemporary paints and other modern polymeric materials.

TGA has been used to investigate pollution adsorbents and to determine under what conditions they would re-release gases they have adsorbed, posing a risk to collections. Since desorption of pollutants generally re-emit a limited number of small molecules, linking TG to mass spectrometry sheds valuable information on the relative importance of the physical versus the chemical adsorption of the adsorbent in question. TGA lends itself well to the study of inorganic materials and we have used it extensively to probe the carbonation reaction of lime mortars and to explore mineral compositions of Roman cements. TGA has also been recently used in our labs to confirm the location of indigo dye on the surface of palygorskite clay particles. This dye/clay association is the basis of an important Meso-American pigment: Maya Blue.

DSC has been used to assess the effects of solvent cleaning on plastic conservation materials.