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Determining the Amount of Glass

Needed for Kiln Casting

This is part three of a multi-part tip on ways to determine the amount of glass needed when kiln casting.  Click here to go to the first section of the tip.

The specific gravity method

Specific gravity is a measure of the density of a particular material.  More specifically, it's the ratio of the density of a material to the density of water.  Water has a density of approximately 1 gram per cubic centimeter, and is said to have a specific gravity of 1.  Materials that are less dense than water will float, while materials that are more dense than water will sink.

The specific gravity of most soda lime glasses (such as Bullseye, Spectrum, and Uroboros) is around 2.5.  This means that the glass is two and a half times as dense as water.  Technically, each color and type of glass has a slightly different specific gravity, but 2.5 is a good average to use for most purposes.

To use specific gravity to calculate how much glass to use for a casting, follow these steps.

1.   Determine the volume of the water required to fill the mold.  For regular shapes, this is a simple height times width times length calculation. Slightly irregular shapes can be estimated, but highly irregular shapes need a more precise way of measuring.  The simplest is to fill a dry mold with rice, then pour the rice into a box and measure the dimensions to determine the cubic measurement.

2.  Once you know the volume required to fill the mold, the next step is to calculate the volume in metric units (cubic centimeters).  This is simple if you did your original measurement in metric units; if you didn't, then just multiply your cubic inch measurement by 16.39 -- that will give you the volume in cubic centimeters.

3.  The final step is easy (well, it's easy if you have a calculator).  Just multiply the volume from step two times the specific gravity of 2.5.  That will give you how many grams of glass you need to fill the mold.  If you want grams converted to ounces, just multiply by .035.

Click here for information on how to determine specific gravity for glasses where the specific gravity is unknown.

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Thanks to Paul Tarlow for assistance with this tip.  For a great calculator to help with the math required for the specific gravity approach, check out  http://www.glassfacts.info/glass/castvol.asp

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