Bowl Split Analysis

The visual evidence relating to this enquiry is a sharp-edged break through the middle of the slumped piece. The two parts have slumped separately, and seem attached at the rim, leaving the middle opened.  A moderate slumping temperature was used to fire the piece at the bottom of a stacked kiln load.

This is used as an example of the kind of thinking required when investigating breaks in slumping.

The split occurred before the slump was complete. We know this because the pieces no longer fit exactly together.  This means the crack opened as the slump continued.  There is other evidence.

The opening of the crack cannot have happened at or after the annealing. It would have already formed to the mould in a whole state. It would break completely across, because it would be in a brittle state.  And the pieces would fit exactly together.  But they do not.

This piece was at the bottom of a stack of shelves in a deep kiln.  At the bottom, there is no radiant heat, only side heat.  This could be a major cause the kind of break described.

It is possible that the split was not across the whole piece.  At the bottom of the kiln, the glass is not receiving any radiant heat from the top.  It is getting radiant heat only from the sides of the kiln.  That means the edges were considerably hotter than the centre.  The edge may be in a plastic state while the centre is still in the brittle state.  The contrasts in expansions are often great enough to break a piece.

From the evidence we have, it can only be said the ramp rate was too fast for the conditions. 

This little exercise shows that a lot of information about layup, schedule, place in the kiln, and any other relevant variation on the usual, must be detailed when asking why something has not turned out as expected. 

 

 

Diagnosis of Slump Breaks

pot melt broken during slumping

The diagnosis of breaks in slumps is a little more difficult than in full fuses. In full fuses sharp edges to the break indicate the fracture occurred on the way down in temperature and rounded edges indicate the break happened on the way up.  The rationale is that an early break will have the edges rounded by the heating during the way to top temperature, while the break at anneal or lower temperatures will have sharp edges as they won't be subject to high enough temperatures to round them.

The temperatures in slumping are not high enough to round the edges no matter when the break occurred.  If the break occurred on the way up, it will remain sharp, just as it would if it broke on the way down.  There needs to be another way to diagnose when it broke.

The key is to try to fit the pieces back together. 

If the break occurred on the way up, the pieces will separate to some degree – depending on the force of the break.  After the break, the glass will slump into the mould according to their separate places.  When cooled and you attempt to put them together they will not fit perfectly.  This may be difficult to determine if the separation due to the break was small, or the mould shallow and regular.

If the break occurred on the way down, the whole piece will have slumped as one whole piece.  So when the broken pieces fit together, it indicates the slump broke apart during the cooling.