Growing Bismuth Crystals and Bismuth Oxide Films

Some students asked if they could try growing hopper crystals of bismuth from molten bismuth, so I tried it first by myself today.  It was conceptually simple to do (melt bismuth in a ceramic crucible and carefully pick crystals out from under the cooling surface layer), but I think to do it well (like much other crystal growing work) requires a bit of practice and maybe even artistry.  What I also thought was fun was watching the hot liquid bismuth oxidize in contact with the air.  When the skin of oxide was pushed away from the liquid bismuth, the freshly exposed liquid surface turned from silvery to yellow to red to blue as the oxide layer thickens (see pictures below).  This reminds me of a paper I was involved with a few years ago - see: D. J. Campbell, M. S. Baliss, J. J. Hinman, J. W. Ziegenhorn, M. J. Andrews, K. J. Stevenson, “Simple Methods for Production of Nanoscale Metal Oxide Films from Household Sources” J. Chem. Educ., 2013, 90, 629-632.  It would be interesting to calculate and directly measure the visible spectrum of that oxide layer and connect it to the layer thickness.