Needle Points

How to avoid prickles when damming glass?

In using dams sharp needle points on the finished piece are common. This is a time when CoE has a practical effect.  The glass is expanding all the way to the top temperature. During this expansion upwards (it’s linear expansion is constricted by the dams), it takes up all the texture of the things it touches. When the glass begins to cool, it contracts and some glass is stuck in the rougher parts of the fibre paper. As the glass continues to contract, the stuck part becomes long, thin, and needle sharp as it separates from the dam materials. The length of these points indicate how much the glass expands and subsequently contracts.

Prevention

Changing the top temperature, the soak time, the bubble squeeze, etc. will not change the fundamentals of expansion and contraction. The physics of expansion and contraction are immutable.

There are several strategies to reduce the amount of needling which can be combined or used singly.

• Use clear iridised glass, iridised side to the fibre paper that is separating the dams from the glass. This should be no higher than the expected final height of the completed piece.

• Use shelf paper between the glass and the fibre paper covered dam.

• Use boron nitride (Zyp or similar) on the fibre paper.

• Use 3mm fibre paper which is 3mm shorter than the expected finish height of the glass.  This allows the glass to form a bull-nose effect without the edges touching the damming materials.

• Pile the glass in the centre of the dammed area and let it flow to the edges. But if the fibre paper is textured, the glass will still form needle points as it cools. So, one or more of the above methods also need to be used in combination.

The whole effort in preventing needling in casting is to reduce the effects of the hot glass grabbing the separating material as it cools and contracts. Provide the smoothest of surfaces or engineer the separators so that the expanding glass does not touch the damming material

Prickles in thin glass

Firing pieces less than 6mm, most often creates needle points as the glass contracts trying to reach the 6-7mm natural height at normal fusing temperatures. The cause is the same as the glass contracting in dammed glass. The glass expands when hot and conforms to any imperfections in the surface, and forms prickles upon contracting. Providing smoother or powdered surfaces minimises the needling effect.

Sometimes even 6mm thick glass forms short rough points during the firing.  The glass does not maintain its 6mm thickness throughout the firing.   As the temperature rises toward contour and full fuse, the glass spreads and slightly thins due to the reduced viscosity. On cooling the glass contracts, “beading up” toward 7mm. This sometimes leaves a rough edge at the contact points between the glass edges and the separator. It is simple to clean up these pieces by removing the rough edge with diamond hand pads, which is much quicker than grinding and fire polishing, and it has less risk and cost.