Determining the Amount of Glass
Needed for Kiln Casting
This
is part three of a multipart 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 to read about the water displacement method of calculating how
much glass your mold will require.
Click here for information on how to
determine specific gravity for glasses where the specific gravity is
unknown.

Copyright 2006 Brad Walker.
All rights reserved.
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
