Aperture Drops – Length of Drop

The height of the drop is related to the thickness of the glass.  The glass moving at the edge of the hole becomes thinner than the rim, so the deeper the drop, the thicker the glass required.

The general rule of thumb is to have 6mm for the first 50mm drop. For each additional 50mm an additional 3mm of glass is required. So, by this method a 20cm drop will require glass at least 15mm thick.

A more accurate method is described by Frank van den Ham in his book – Kilnforming Glass, a Master’s Approach.  This is based on obtaining an approximately 4mm thick rim and relies on measuring the amount of glass needed to provide an average wall thickness of 4mm.  The method is:

• Double the drop length, and add the diameter
• Divide the result by the diameter
• Multiply that result by 0.4cm (the average thickness to have a robust result)
• This gives the resulting thickness of glass required in centimetres.
• Divide centimetres by 2.54 to get the decimal part of an inch.

This method relates the diameter (or other dimensions of the opening) to the length of the drop. 

By this method a 20cm drop through a 20cm aperture would require a 1.2cm/0.5” thick blank.  If it were to be a 30cm drop, a 1.6cm/0.625” thick blank would be required, but by the rule of thumb, a 2.1cm/0.825” blank would be needed.

The thinning effect of the stretching can be influenced by both the temperature and material of the supporting material, so this method cannot be infallible.

Revised 22.11.24