The most difficult ceramic riddles

Diffusion is a physical process where molecules of a material move from an area of high concentration (where there are many molecules) to an area of low concentration (where there are fewer molecules). It happens in every state of matter, even in solid. In 17th century, Robert Boyle shown the process of diffusion in solids, with the example of zinc and a copper coin. Another known example is blurring of the colours of the glass or ceramic.

Glass-ceramic materials share many properties with both non-crystalline glass and crystalline ceramics. The negative thermal expansion of the crystalline ceramic phase can be balanced with the positive expansion of the glassy phase. At a certain point (~70% crystalline) the glass-ceramic has a net expansion near zero and can sustain repeated and quick temperature changes up to 1000 °C.

A biscuit is a flour-based baked food product. This article covers the type of biscuit found in Africa, Asia and Europe, which is typically hard, flat and unleavened. In North America, a biscuit is typically a soft, leavened quick bread, and is covered in the article Biscuit (bread).