Drop Rings

Mould

It is possible to purchase drop rings of various sizes. It is also easy to construct one from vermiculite board or ceramic fibre board. Merely cut a circle of the desired radius from the board. Leave at least 50mm of board outside the circle ( more for thinner boards).

Kiln wash the top and inner sides of the drop ring

Glass

The glass should be larger than the hole in the ring. This will vary by radius of the hole. The glass will need to be from 50mm larger diameter than the hole for smaller holes to 100mm larger diameter for holes over 300mm.

Glass should be at least 6mm thick for the first 100mm of drop and an additional 3mm for each 50mm more. So, a drop of 200mm would require glass of 12mm thick.  A more accurate method of determining the thickness of glass in relation to hole diameter and length of drop is given by Frank van den Ham.


Temperatures

The temperature rise should be no more than 150C per hour to about 675C for 6mm glass and less for thicker glass. Remember the glass is much closer to the elements than normal and it is easy to thermal shock the glass.

With close inspection you can see that the edge of the glass rises from the mould as it sinks in the middle.

The outside edges of the glass rise from the mould as the centre begins to drop in the centre.  As the glass gets hotter, this raised edge settles back on to the mould.  If the glass is really near the elements, there is a small risk the glass will touch the elements.  No harm will be done to the kiln, but the glass edge may have some needles.

The rate and amount of slumping is controlled by temperature, span (the width of unsupported glass on the mould) and time. The higher the temperature the faster a piece will slump and the thinner the walls will be. However you can slump at lower temperatures by holding the temperature for a longer time to reduce the thinning of the sides.

Also note that the wider the span, the faster the glass slumps.

If you slump at high temperatures with a drop ring the sides of the bowl tend to be straight and steep. The strain is limited to the region immediately inside the rim. Therefore the glass tends to thin next to the rim and the colours are diluted. If you slump at a lower temperature for a longer period of time the strain is distributed over the entire unsupported area. This results in a more rounded shape for the bowl and even thickness of the glass across the bottom of the bowl.


Experiment

Finding the right combination of time and temperature requires a bit of experience and guess work. If you want a rounded bottom, heat the glass to the point that it starts to bend on the mould and wait for 30 minutes. If it has slumped about 1 inch in that time wait another 30 minutes. You are looking for a slumping rate that is acceptable. If it hasn't moved very much then increase the temperature 15C and check again in 15 minutes. Keep moving temp up and waiting for 15 minutes until the piece has completely slumped. This might take several hours.

If you want straight sides keep heating the piece rapidly.

Stopping
When the piece has slumped to the desired shape, flash cool the kiln to about 30C above the annealing point to stop movement in the glass. Extend the annealing soak and increase the length of the annealing cool time (reduce the rate of temperature fall) over normal slump firings of the same thickness.

Glass falls through drop rings in relation to the size of the glass on the drop ring, the size of the opening, the temperature rise rate and to some extent the colours and amount of opalescent glass used. 

There is an introduction to aperture drops here, that also links to many other elements of the subject.

Revised 5.1.25

Aperture Drops Blank Sizes

As the glass drops through the aperture, it stretches, but the whole substance of the piece is drawn toward the hole. If there is not enough spare glass around the hole, the whole piece will be drawn through the aperture.

There is a minimum size of the glass in relation to the size of the drop out hole. Up to some maximum size, the greater the diameter of the hole the greater the amount of spare glass there needs to be.

Also relevant is the depth of the drop. A shallow drop needs only a few centimetres larger than the hole. While a deeper drop needs a greater amount of glass surrounding the hole.

I have found that for a 300mm diameter hole, with a 150mm drop the glass needs to be 35mm larger all around. Thus an aperture of 300mm needs to be at least of 370mm diameter for this 150mm drop. I have done drops with 550mm diameters with only a 650mm diameter blank. This indicates to me that there is an amount of spare glass that will be sufficient even for larger diameter drops, but I have not found it yet. For a large drop with an aperture of 500mm and a depth of 350mm, I used a 100mm margin, giving a disc of 700mm which successfully dropped with the rim moving only about 20mm.    An 80cm/31.5” diameter aperture with a drop of 35cm/ca.13.8” needs a rim of 10cm making a blank a diameter of 100cm./39.4”.

One element that can reduce the size of the blank is to make an inclined collar around the aperture of the drop mould.  This idea is based on the observation that as the glass begins to fall through the aperture, the outer edges of the glass rise from the mould surface so the glass is resting only on the inner edge of the drop out mould. 

This inclined drop out mould will be like a shallow bowl rim, but without a bottom. The glass blank then rests with only its outer edge on the collar.  When the temperature increases to the point that the glass begins to slump, the glass will conform to the slope and so create enough friction to restrict the glass from falling through the aperture, although it is with a smaller than normal rim.  The actual size of the rim for each size and depth will need to be determined by experience. 

More information can be found here

Revised 22.11.24