When creating a casting, pot melt or other object from glass cullet or billet, you need to be sure you have a large enough volume of glass to fill the area. You can do it by measuring the volume or by calculating the weight. This note is about calculating the weight.
Filling a damed area with enough pieces of glass provides an illustration of volume control. To help make sure you have enough glass to fill the space, measure in centimetres to determine the area. For a rectangle, measure length by width in centimetres. For a circle multiply the radius by itself (radius squared) times 3.14 (pi) to get the area.
To determine the minimum volume required, multiply the area by 0.6 cm. This is the approximate thickness that glass takes up at full fuse. As the amount of time and heat that we normally give to the process is insufficient to allow the glass to fully flow, the glass will tend to be thicker in the middle when using pieces of glass rather than sheets. So you may wish to multiply the area by 0.7 cm (to make sure you have enough).
To get the weight of glass required for the space, multiply the calculated volume by 2.5 (specific gravity) to get the weight in grams. Divide by 1000 to get kilograms. If you must use pounds, multiply the kilos by 2.2, the number of pounds in a kilo.