When glass drops through a ring, you need to check on some things relating to the placement and firing.
When thinking about the relationship between the size of the flat glass and the size of the aperture, you need to remember how the glass behaves as it heats up toward the drop temperature.
The glass begins to sag at the middle of the aperture, however the glass is still relatively stiff. The weight of the rim is not enough to keep it from rising from the ring. The rim of the disc maintains the angle from the centre of the drop to the edge, until it gets hot enough for the weight of the rim to allow the edge of the disc to settle back down onto the ring. This is the source of a lot of the stretch marks at the shoulder of drops.
To avoid the glass dropping through, you need to have an adequately sized rim. The width of the rim sitting on the ring, needs to be related to the size of the hole.
I have found that for apertures up to 300mm diameter there needs to be at least 35mm on the rim. The consequence of this is that your blank diameter needs to be 70mm more than the hole diameter. For larger apertures – up to 500mm – you need 50mm, or 100mm added to the diameter of the hole. I do not have the experience to say how much more is required for larger diameter drop rings. There is more discussion on blank sizes here.
|The consequence of an inadequate rim|
The rate at which you heat the glass and the top temperature both have effects on the possible drop through.
High temperatures. The higher temperature you perform the drop out, the more likely you will need larger rims or other devices to reduce the drop through possibilities. It also promotes excessive thinning below the shoulder.
Fast rates. The surface will become hotter than the bottom, but at different rates. The glass over the hole is heating from both top and (to a lesser extent) bottom. The rim is sitting on the ring and so heats only from the top. The differential in heat may cause a break.
Weight. The thickness of the glass effects when the drop will begin. The heavier the glass and larger the hole, the effective weight will be greater. In these cases, you can use a lower temperature for the drop.
Additional methods. You can use other methods to reduce the chance of a drop through. Two of them are:
Weights. You can put kiln furniture on the glass rim to keep it from rising during the initial stages of the drop. These must be placed symmetrically. Four or six pieces of kiln washed props or small dams would be sufficient up to 300mm diameter. More would be required for larger apertures. Of course, these will mark the rim, meaning that it must be cut off.
Inclined rings. Another possibility is to use an inclined ring, with the glass resting on the upward incline, so the glass is held above the aperture and is heating evenly until the drop begins.