Simultaneous Fusing and Slumping

“I sometimes slump at the same time as I do a tack fuse. Is slumping at this higher heat bad for the mould? “

Image credit: Creative Glass

Mould

 It is possibly not bad for the mould, but it does depend on your temperature and heat work.  Ceramic moulds are typically fired to 1200° or 1300°C so higher kilnforming temperatures are unlikely to affect the moulds.  The speed at which the target temperature is reached is of concern though.  Ceramics have what is called quartz inversions.

 Two of the constituents of ceramics – cristobalite and quartz – have significantly large expansions at 226°C and 570°C / 440°F and 1060°F.  Rapid rises through these two temperatures risks breaking the ceramic mould.  This is not the case with steel moulds, of course.

Glass

 There may also be effects on the glass.  Slumping typically ranges between 620°C to 677°C (1150°F to 1250°F).  Tack fusing typically is done in the 740°C to 790°C (1365°F to 1455°F)range.  This is a significant difference even at the higher end of the slumping range and the lower end of the tack fusing range. 

 Some of the effects are:

·        The marking of the slumped glass will be greater at tack fusing. 
·        The glass will slip down the mould more. 
·        Any pieces applied to the base are likely to slide during the slumping process.
·        There is a risk of creating an uprising or bubble at the bottom as the glass slips down the side of the mould. 
·        There is more risk of creating needle points at the edges.

 Performing two processes at the same time risks difficulties.  Inevitably, compromises will need to be made between slumping and tack fusing.  Eventually, it will come to a time when the two process won't work together.

  

A slump taken to tack fusing temperatures is at risk from uprisings at the bottom, needling at the edges, excessive marking on the back, slipping down the mould and thickening