I can tell you the story of this one.
Mina Colvin loved arachnoideum 'Tomentosum' and wanted to see what variations it might produce from seed. She grew a large number and sent a slide of the group of seedlings in our round robin that eventually became SFAN. I saw two that looked distinctive in the slide and said "pull these two out and grow them on". Those two turned out to be 'Raspberry Ice' and 'Silver Thaw', both great plants although frustrating as parents because they produce few seeds. Both grow great for me although my colony of 'Silver Thaw' looks a bit chimeric, as some of the rosettes are differently colored than the rest. Generally I would blame seedlings, but not in this case.
Kevin
