Break Diagnosis in Slumping

The usual advice in looking at the reasons for breaks in your pieces must be considered in relation to the process being used.  Breaks during slumping need to be considered differently to those occurring during fusing.  

The advice normally is that if the edges are sharp, the break occurred on the way down in temperature. Therefore, the glass must have an annealing fracture or a compatibility break.  It continues on to say if the edges are rounded it occurred on the heat up, as it broke while brittle and then rounded with the additional heat.

This is true, but only on rounded tack and fused pieces.

When the process is a slump, there is not enough heat to round the edges.  So, the edges will be sharp whether the break was on the heat up or the cool down.

How can you tell in a slump process when the break occurred?

The first, but not obvious, way to tell if the break is on the heat up is to peek at about 260C/500F as most heat up breaks occur around that temperature, and again at the strain point, about 540C or 1000F.  These are the two critical heat up temperatures that will give the knowledge of when the break occurred.  If at either of these peeks, the glass has broken, the firing can be abandoned.  If the break is at the higher temperature, it needs to be annealed though.

 

The other way to tell if the break occurred on the cool down is that if you can put the pieces of the slump back together and they fit perfectly, the break was on the cool down, as the piece was already fully formed at the time of the break.

If the pieces do not fit together perfectly, the break was on the heat up.  This is because the break occurred, and then the two (or more) pieces slumped independently, thus leaving slightly different shapes at the break line.

There is a special case here, of course.  Sometimes the break is only a split in the bottom, that does not come all the way to the top of the piece. This split (or splits) occur when the heat up is too fast.  The theory goes that the top became plastic while the bottom was still brittle/stiff.  The weight of the hotter, more pliable glass overcomes the strength of the cooler and heat stressed bottom, causing it to split.  

Another theory is that the layers of glass were not fully fused together, so forming a thermal break between the upper and lower pieces, again allowing the top to heat more quickly, but causing thermal shock to the lower part.

More information is given in the blog post Diagnosis of Breaks, and a full discussion in the ebook Kilnforming Principles and Practices available from Bullseye.

There is also extensive information on diagnosis of breaks in this blog entry on slumping cracks.