A new method of measuring the water evolved on thermal dehydration of a variety of clay minerals and related hydrated compounds is described. By placing the substance in a tubular furnace and passing over it a dry inert gas, water released on heating is carried to a sensitive electrolytic hygrometer for measurement. The heating is programmed to give a uniform rate of temperature rise, so that continuous monitoring of the evolved water permits discrimination between successive dehydration processes occurring at different temperatures. The procedure is appropriately termed thermohygrometric analysis (THA).
The applications described include investigations of the thermal decomposition behaviours of hydrated aluminas and hydrated iron oxides, and of clay minerals such as kaolin, montmorillonite, chrysotile and micas. The relations between the results given by thermohygrometric analysis and those obtained by differential thermal analysis are discussed. As with DTA, the record trace from a THA examination gives a graphical representation of the dehydration process, but THA has an advantage in providing a quantitative measurement of the water evolved.