Calibration and use of the heating microscope for indirect evaluation of the viscosity and meltability of archeological glasses

The heating microscope is a widely used instrument in many industrial applications, especially in the ceramic field, to obtain information on the sintering behavior and, indirectly, on the viscosity curve of glass. It is therefore important for the study of glass technology. Usually, heating microscopes work by taking images of the sample outline, during a preset thermal cycle, which define five characteristic temperatures corresponding to particular shapes of the specimen. The first two characteristic temperatures can also be extracted from the sintering curves provided by the instrument. In agreement with three published models, the five temperatures are associated with five viscosity values, producing the two best-fitting plots (Log₁₀(η) - T). In this work, the heating microscope was calibrated using a glass with known properties, to determine the most reliable model, so that it can be applied successfully in the field of archeological science. The selected methodologies were used to analyze two opacified Roman mosaic glass tesserae excavated in Pompeii, previously characterized chemically. A Pompeiian transparent glass, sampled from a large block of base (raw) glass, and its experimental reproduction were also studied. Finally, the relationship between the opacifiers and the thermal behavior of base glass was investigated.