Physics tells us that, when a container is sealed, if you apply heat, pressure will build within the container. Conversely, if you apply pressure, whatever is contained within that container will grow hot.
Potters are artists. More appreciated possibly in less developed parts of the world where their wares provide for the transportation and storage of produce and the preparation of food. Pot making is not a simple task, it requires great skill not only in molding the clay but also in its baking – if the fire is too hot, the pot will crack, making it useless for cooking.
The unscrupulous pot maker learned to place wax in these cracks and cover this over with potting material in order to make the pot appear to be whole. The unfortunate buyers of these pots would take them home and, only when they were employed over the fire, containing the liquids required in preparing their food, would the wax melt, the food would be ruined.
Neither that pot maker nor his pots were “sincere” which in Latin means “without wax”.
Under pressures, it’s going to get hot.