Removing Shelves for Slumping

There are those who advocate removing the kiln shelf(s) before slumping.  The advantages claimed include:

Better heat distribution around mould.  The shelf acts as a heat sink. During the firing the shelf absorbs heat and during the cooling the heat is released, so slowing the cool down. 

Additional height. For kilns with little head room, greater height is provided by this practice.

Observations

My observations on this practice lead me to some questions about the necessity, desirability and in some cases the practicality of it.

Elevation of mould above the shelf

This is a widely recommended practice.  I haven’t found the need, but many people do.  One of the points of this is to allow increased air circulation around the mould and under the bottom.  Another is to let air out from under the bottom of the mould to avoid creating air pockets between the mould and the glass.

If the elevation of the mould allows air circulation, what is the necessity to remove the shelf?  There is air circulation around the bottom of the shelf and of the mould. If the mould is placed on the floor of the kiln, the mould will still need to be raised from the bed of the kiln to allow air circulation under the mould. Of course, if the kiln does not have enough space for the height of the mould, it will be necessary to remove the shelf, but not for circulation purposes.

There is also the fact that the floor of the kiln is most often made of refractory bricks even if the walls and top are of refractory fibre.  This also is a heat sink.  I don’t see the advantage of removing the shelf to avoid a heat sink when the base of the kiln works in holding heat in the same way as the shelf.

Difficulty of removing shelves from some kilns

It is difficult to remove shelves from many kilns.  This can be avoidance of damage to the thermocouple; difficulty of getting fingers around the shelf; weight; size; or even depth of the kiln.  It is impractical to remove the shelves from kilns of this nature.  It is still possible to get a good slump in these kilns.

Uneven cooling of the glass

Research shows long soaks lead to a cooler bottom of the glass than top during the anneal – sometimes greater than the +/- 5°C for adequate annealing.  This is a consequence of the fact that the hot air above the glass is not balanced by the same amount of heat below the glass.  So, there may be good arguments for retaining that heat sink of a shelf under the mould to more evenly balance the cooling of the upper and lower surfaces of the glass during the anneal soak and cool.

Height

I don’t have any argument that when extra height is needed, as removing the shelf will provide some.

Some consideration needs to be given on whether to remove the kiln shelf when slumping.  Research implies that increased cooling of the bottom of the glass may go outside the parameters for the even cooling of the glass.