Before Pilkingtons invented plate glass in the mid-19th century, flat panes could not be made. Old windows are uneven.
Some once thought this was because glass is a liquid that flows down slowly over the centuries. That’s wrong. But although glass does not move on timescales as “short” as centuries, the true nature of glass, whether it is truly solid or a very viscous liquid is something we do not know.
Solving the challenge, determining whether glass can become truly solid, requires identifying a clear transformation, the equivalent of liquid water freezing to ice as temperature changes. Of course glass can be found in a liquid state, but does it become a true solid once it cools down?
The problem with glass-forming materials – which include plastics, alloys and ceramics in addition to everyday “glass” – is that there is no obvious transformation. On cooling we cannot definitively say yet that glass has become a solid.