We recommend using Zephyr, Aether, Effetre 004, or Vetrofond 004 for encasing. Here are a few links that refer to some reported problems using certain clears.
(copy and paste the urls into the address bar in your browser)
http://lampworketc.com/forums/showthread.php?t=189306&highlight=aether
http://lampworketc.com/forums/showthread.php?t=143321&highlight=lauscha+crack
http://lampworketc.com/forums/showthread.php?t=174635&highlight=lauscha+crack
