The growth of cracks in glass arising from thermal shock on contact with a second material is an important problem in the volume production of glass articles. The thermal properties of the contacting material are controlling factors in producing cracking and a simple model of crack extension from thermal shock was developed to investigate the process. Given a pre-existing microscopic crack at the surface of the glass, it is shown that one dimensionless parameter, termed the thermal index of the materials combination, controls the process of crack extension and crack arrest. If the index is very much less than unity, the probability of crack extension can be made very small. The consequence for practical applications of new materials in handling hot glass is discussed.