Tin pest related problems such as tin whiskers. At 13.2 °C (about 56 °F) and below, pure tin transforms from the silvery, ductile metallic allotrope of β-form "white tin" to the brittle, nonmetallic, α-form "grey tin" with a diamond cubic structure. The transformation is slow to initiate due to a high activation energy but the presence of germanium (or crystal structures of similar form and size) or very low temperatures of roughly −30 °C aids the initiation. There is also a large volume increase of about 27% associated with the phase change to the nonmetallic low temperature allotrope. This frequently makes tin
Wedding customs by country were considered a personal event and inviting people to the wedding who are not known to at least one member of the couple well enough to be able to choose an appropriate gift was considered inappropriate, and registries should therefore be unnecessary. Whether considered appropriate or not, others believe that weddings are opportunities to extract funds or specific gifts from as many people as possible, and that even an invitation carries an expectation of monetary reward rather than merely congratulations. Letters of thanks for any gift are traditionally sent promptly after the gift's receipt. Tradition allows wedding gifts to be