Page 549 - Handbook of Thermal Analysis of Construction Materials

518 Chapter 12 - Clay-Based Construction Products

3.9 Archaeological Investigations

The thermal analysis of different archaeological samples of bricks,
terra-cotta, and local clays provides useful information on the chemical
composition of historical samples and can provide confirmation of specific
sites of origin. [30] The thermal characterization of local clays is valuable to
assist the restoration of historical pottery materials.
The temperature at which ancient ceramics, terra-cotta, and pottery
were fired varies from 500–1300°C and depends on the type of clay. The
clay minerals are the main materials for the production of bricks and terra-
cotta figures. The clays contain kaolinite, feldspar, quartz, aluminite,
gehlenite, hydromagnesite, calcium, magnesium and iron carbonates and
hydroxides, hygroscopic and bound water, and organic substances. TG and
DTG thermograms for Leharu clay are presented in Fig. 21. The weight-loss
peak around 100°C is due to the removal of adsorbed water. Peaks at 200 to
350°C are attributed to bound water. The dehydration of hydromagnesite
(250–280°C) and brucite (350–420°C) are followed by the decomposition of
calcium hydroxide (400–520°C). Magnesium carbonate decomposes in the
450–520°C range, and calcium carbonate in the 700–900°C range. The
dehydroxylation of clay minerals is, thus, considered to occur over the wide
temperature range 125–555°C.

Figure 21. TG-DTG analysis of Leharu clay in air and nitrogen atmosphere: (A) TG curve
in air; (B) DTG curve in air; (C) TG curve in nitrogen; (D) DTG curve in nitrogen. [30]