devitrification

A white, frosty or matte surface on glass that can appear after fusing.  Often described as "scum" and referred to as "devit".

Glass molecules are normally arranged randomly.  Devitrification (literally "to become unglass-like") is the organization - or crystalization - of glass molecules.

Devitrification usually requires three things:

  • Air:  Devitrification almost always occurs on the  air-side (as opposed to the shelf-side) of the glass.
     
  • The correct temperature:  Although devitrification can occur at any temperature above the melting point, most glass intended for fusing is most likely to devitrify in the upper 1300s.
     
  • A "nucleus" on which the crystals can grow.  Oil used in glass cutters or from one's hands is a frequent contributor to devitrification.  To minimize devitrification always clean your glass well.

Cleaning you glass well and avoiding temperatures between 1350° and 1400° F (for most glass intended for fusing) can help minimize devitrification.