Uneven slumps

Credit: Lara Duncan

Uneven slumps – where the glass does not slide down the sides evenly, leaving one side higher than the other – are common in moulds with steep sides.  Another common cause is uneven weight on the blank – where there are more layers on one side than another.  Yet another common cause of uneven slumping is the blank having large areas of glass with different viscosities.

Things I can think of to avoid the problem.

While the glass is firing

Use a moderate rate of advance to the target temperature. Once that is reached, peek every 10 minutes to observe how the slump is proceeding.  When the slump begins to go off centre, reach in with protective gear and adjust it back to even.  The kind of protective gear you need is shown in this post.

An alternative to moving the glass is to tip the mould.  If the mould is relatively shallow with a flat bottom,  there is not much you can achieve by this action.  On deeper moulds, you can elevate one side of the mould.  This puts the elevated side closer to the top and so into a hotter part of the kiln.  This means that you elevate the side that is not slumping as quickly.  You do this because the slowly slumping glass needs more heat in relation to the faster slumping side.  It seems counter intuitive, until you realise you are putting the slow side into greater heat.  You will need to continue observing at intervals to know when the glass is slumping evenly. At that point you can return it to level.

I admit that moving the glass is my choice almost all the time.  It works well on moderately deep moulds.  Elevating one side of the mould while firing requires more time in the kiln that I want to give.  Tipping the mould works best on very deep moulds and so I view it as a special case.

Before the slumping begins

Most of the time we make our blanks the same diameter/dimension or slightly larger than the mould. This allows the glass to rest on the rim and be certain it is as level as the rim of the mould is.  However, this also creates an edge which the glass needs to slide over as it slumps.  Especially with steep sided moulds.

A fix for this is to make the blank fit just inside the rim.  Then it does not have to slide over the rim, and avoids the risk of hanging up on one part rather than another.  You will need to ensure the glass is level within the mould as well as the mould itself, in this case.  If you take this approach of internal placing and you want a piece with a particular final dimension, you should buy a mould larger than the final size needed.

You can combine this placing of the glass internally with another preventative for uneven slumps in deep moulds.  You can grind a small bevel on the underside of the edge to help the glass have greater contact with the mould, so resisting uneven movement.  This can be done separately from fitting the glass inside a steep sided mould, but is most likely to be successful if performed on a blank smaller than the mould dimensions.

Uneven slumps in kilnforming can be corrected during the firing or by preparation of the blank in relation to the mould before the firing.

Further information is available in the ebook: Low Temperature Kiln Forming.