Edge formation

What about edges

With many kilnformed objects a most important feature will be the shape and finish of the edges. This is particularly the case with platters and tableware items which will be handled frequently. With fired pieces which are to be mounted in a door window or other frame the shape of the edge may not be as important as the visible texture or design.

Glass can be shaped at temperatures anywhere between about 600°C (1110°F) and above 820°C (1510°F).Over this range there can be enormous difference in the way the edges of the glass will develop.

At around 600°C (1110°F) to 650°C (1200°F), during glass bending, a cut edge will barely lose its sharpness. If it has been lightly ground, the frosted appearance will have barely disappeared.

If doing a slump at .....

For these, a nicely rounded edge free of needlepoints and other irregularities is desired.

Needlepoints

These are fine needle-like points which can develop randomly along the bottom outer edges of fired glass. Sometimes they can be so fine that they feel like a fine toothed saw blade.They can develop when surface tension is pulling thin glass inwards to make it thicker.

Minor variations in viscosity, surface tension and adhesion to the supporting surface can cause unevenness in the rate at which the glass pulls back from the outer edge. This will occur only when the glass is below the ‘magic thickness’ (equilibrium thickness).They are usually a problem only with glass up to 6mm thick.

Should the glass be 8mm or more thick it will roll outwards to get thinner, so the points won’t develop.They can occur during both slumping or fusing processes and a common belief is that they are indicative of ‘over firing’.It is true that they are more likely to develop when firing to a higher temperature; but the firing pattern may have been dictated by the task at hand and would thus not be ‘over-fired.

However, firing temperature is not the only cause of this phenomenon. In a particular instance when they were presenting a production problem, changing the grinding head to a finer grit when dressing the edges of 4mm glass, prevented them developing; without any change in firing temperature. Extremely fine needlepoints can sometimes appear when firing 6mm glass, or two layers of 3mm. They will be fine, because the glass won’t pull in far; much less than with 3mm or 4mm where the needlepoints can be more prominent.

While they are undesirable and to be avoided if possible, there are workarounds. They can be removed by rubbing with a very fine grade abrasive stone. The dull spots can be camouflaged  by applying a touch of butter or vegetable oil to the spots; or maybe some clear nail varnish.

This is a work in progress. Look for a notice in NEWS when some more is done.