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Why do hard enamels last longer than soft enamels?

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Samson killing Philistines — Enamelwork Standing dish depicting Samson crushing the Philistines with the jawbone of an ass, enamel on copper by Pierre Courteys, c. 1580; in the Taft Museum of Art, Cincinnati, Ohio. Height 9.8 cm, diameter 22.9 cm.

Hard enamels last longer than soft enamels because of their superior resistance to environmental factors and physical wear.

In the context of enamelwork, the hardness of the enamel is determined by the proportions of its components, which include flint or sand, red lead, and soda or potash. Hard enamels require very high temperatures to fuse, and this increased hardness allows them to better withstand atmospheric agencies. In contrast, soft enamels are more susceptible to surface decomposition caused by these environmental factors, which can eventually lead to the total breakup of the enamel. Furthermore, while soft enamels are more convenient to use because they require less heat to fire, they do not wear as well as hard enamels, particularly when subjected to friction.1

Please note that the term "enamel" also refers to the hard tissue covering teeth in anatomy, which is a distinct biological substance unrelated to the glass-based enamelwork used in art and jewelry.2