The shift in temperature with timescale is not very large however as indicated by the transition of polypropylene glycol of 72 °C and 71 °C over different timescales 13 To observe window glass flowing as liquid at room temperature we would have to wait a much longer time than the universe exists Therefore it is safe to consider a glass a solid far enough below its transition temperature: Cathedral glass does not flow because its glass transition temperature is many hundreds of degrees above room temperature Close to this temperature there are interesting time dependent properties One of these is known as aging Many polymers that we use in daily life such as rubber, polystyrene and polypropylene are in a glassy state but they are not too far below their glass transition temperature Their mechanical properties may well change over time and this is serious concern when applying these materials in construction edit Physical properties The following table lists some physical properties of common glasses Unless otherwise stated, the technical glass compositions and many experimentally determined properties are taken from one large study 22 Unless stated otherwise, the properties of fused silica quartz glass and germania glass are derived from the SciGlass glass database by forming the arithmetic mean of all the experimental values from different authors in general more than 10 bilglass interi-rglass sikkerhetsglass