296 15·No Ceramics Age? such a comparison is not possible,the thermoluminescence tech- nique is utilized.This method makes use of the fact that many clays and soils contain minute amounts of radioactive elements such as uranium,thorium,or potassium.Their emitted a-,B-,or y-radiations excite under certain circumstances some electrons of the clay into higher energy states where they might be trapped in impurity states(see Chapter 13).If thermal energy is supplied to this substance,the electrons may be forced to leave their metastable positions and revert to a lower energy state by con- comitantly emitting light.In other words,thermoluminescence unlocks the stored energy that has been radiation-induced over time by slightly heating the object under investigation.For this it is essential that the clay has been fired at some time in order that "the time clock is reset to zero."The age is then assigned to an object by measuring the amount of emitted light and by know- ing its and the surrounding soil's radioactive content as well as by taking into consideration how susceptible the material is to radiation damage.Another dating technique that is,however,still in its developmental state involves electron spin resonance. At this point the reader probably wants to know about the chemistry and physical properties of ceramics.Specifically,why is clay pliable and what is the composition of clay?This will be explained in the next section. 15.4.The Science Behind Pottery The principal purpose of this section is to provide some under- standing of why clay is such a remarkable material which is duc- tile when wet and hard after firing.First of all,"clay"is not just one substance but a whole family of minerals whose common characteristic is that they have a sheetlike crystal structure,as we shall see momentarily,which allows the platelets (that are <1 um in diameter)to slide easily past one another even when only little force is applied.Further,clays are so-called hydrous alu- minum or magnesium silicates,which are distinguished by the property that they lose physically adsorbed or structural water when heated.Clays have formed as a result of marine sediments or from hydrothermal activities during all ages.Clays vary in composition and additional constituents depending on the envi- ronment in which they formed and the hydrological or climatic conditions.They are found in mudstones,shales,and soils al- most everywhere on the earth. As was just indicated,clays are composed of silica(SiO2),alu- mina (Al203),possibly magnesia (Mgo),and water,along with impurities of iron,alkalies,or alkaline earths.They are some-such a comparison is not possible, the thermoluminescence tech￾nique is utilized. This method makes use of the fact that many clays and soils contain minute amounts of radioactive elements such as uranium, thorium, or potassium. Their emitted -, -, or -radiations excite under certain circumstances some electrons of the clay into higher energy states where they might be trapped in impurity states (see Chapter 13). If thermal energy is supplied to this substance, the electrons may be forced to leave their metastable positions and revert to a lower energy state by con￾comitantly emitting light. In other words, thermoluminescence unlocks the stored energy that has been radiation-induced over time by slightly heating the object under investigation. For this it is essential that the clay has been fired at some time in order that “the time clock is reset to zero.” The age is then assigned to an object by measuring the amount of emitted light and by know￾ing its and the surrounding soil’s radioactive content as well as by taking into consideration how susceptible the material is to radiation damage. Another dating technique that is, however, still in its developmental state involves electron spin resonance. At this point the reader probably wants to know about the chemistry and physical properties of ceramics. Specifically, why is clay pliable and what is the composition of clay? This will be explained in the next section. The principal purpose of this section is to provide some under￾standing of why clay is such a remarkable material which is duc￾tile when wet and hard after firing. First of all, “clay” is not just one substance but a whole family of minerals whose common characteristic is that they have a sheetlike crystal structure, as we shall see momentarily, which allows the platelets (that are 1 m in diameter) to slide easily past one another even when only little force is applied. Further, clays are so-called hydrous alu￾minum or magnesium silicates, which are distinguished by the property that they lose physically adsorbed or structural water when heated. Clays have formed as a result of marine sediments or from hydrothermal activities during all ages. Clays vary in composition and additional constituents depending on the envi￾ronment in which they formed and the hydrological or climatic conditions. They are found in mudstones, shales, and soils al￾most everywhere on the earth. As was just indicated, clays are composed of silica (SiO2), alu￾mina (Al2O3), possibly magnesia (MgO), and water, along with impurities of iron, alkalies, or alkaline earths. They are some- 296 15 • No Ceramics Age? 15.4 • The Science Behind Pottery