Specific Gravity of Unknown Glass

(warning: lots of arithmetic)

Knowing the specific gravity of a glass can be useful in calculating the required amount of glass needed, e.g., for casting, and screen and pot melts, where a specific volume needs to be filled.

Most soda lime glass – the stuff kilnformers normally use – is known to have a specific gravity of approximately 2.5.  That is, one cubic centimetre of glass weighs 2.5 grams. 

If you have glass that is of unknown composition for your casting, you will need to calculate it.

Calculating the specific gravity of unknown glass.

Specific gravity is defined as the ratio of the weight of a substance to (in the simple case) the weight of water.  This means first weighing the item in grams.  Then you need to find the volume.

Calculating the specific gravity of regularly shaped items

For regularly shaped item this is a matter of measuring length, width and depth in centimetres and multiplying them together. This gives you the volume in cubic centimetres (cc).

As one cubic centimetre of water weighs one gram, these measurements give you equivalence of measurements creating the opportunity to directly calculate weight from volume.

To calculate the specific gravity, divide the weight in grams by the volume in cubic centimetres.

An example:

To find the specific gravity of a piece of glass 30cm square and 6mm thick, multiply 30 x 30 x 0.6 = 540cc.  Next weigh the piece of glass. Say it is 1355 grams, so divide 1355gm by 540cc = s.g. of 2.509, but 2.5 is close enough.

Calculating specific gravity for irregularly shaped objects.

The unknown glass is not always regular in dimensions, so another method is required to find the volume.  You still need to weigh the object in grams.

Then put enough water in a measuring vessel, that is marked in cubic centimetres, to cover the object.  Record the volume of water before putting the glass in.  Place the object into the water and record the new volume.  The difference between the two measurements is the volume of the suibmerged object.  Proceed to divide the weight by the volume as for regularly shaped objects.

Credit: study.com

Application of specific gravity to casting and melts.

To find the amount of glass needed to fill a regularly shaped area to a pre-determined depth, you reverse the formula.  Instead of volume/weight=specific gravity, you multiply the calculated volume of the space by the specific gravity.

The formulas are:

v/w=sg to determine the specific gravity of the glass;

v*sg=w to determine the weight required to fill a volume with the glass.

Where v = volume; w = weight; sg= specific gravity;

You determine the volume or regular shapes by deciding how thick you want the glass to be (in cm) and multiply that by the volume (in cc). 

For rectangles

volume = thickness * length * depth (all in cm)

For circles

Volume = radius * radius *3.14 (ϖ)* thickness (all in cm)

For ovals

Volume = major radius * minor radius * 3.14 (ϖ)* thickness (all in cm)

Once you have the volume you multiply by the specific gravity to get the weight of glass to be added.

Calculating weight for irregularly shaped moulds.

If the volume to be filled is irregular, you need to find another way to determine the volume.  If your mould will hold water without absorbing it, you can fill the mould using the following method.

Wet fill

Fill the measuring vessel marked in cc to a determined level.  Record that measurement.  Then carefully pour water into the mould until it is full.  Record the resulting amount of water. Subtract the new amount from the starting amount and you have the volume in cubic centimetres which can then be plugged into the formula.

Dry fill

If the mould absorbs water or simply won’t contain it, then you need something that is dry.  Using fine glass frit will give an approximation of the volume.  Fill the mould to the height you want it to be.  Carefully pour, or in some other way move the frit, to a finely graduated measuring vessel that gives cc measurements.  Note the volume and multiply by the specific gravity.  Using the weight of the frit will not give you an accurate measurement of the weight required because of all the air between the particles.

An alternative is to use your powdered kiln wash and proceed in the same way as with frit.  Scrape any excess powder off the mould.  Do not compact the powder.  In this case, you must be careful to avoid compacting the powder as you pour it into the measuring vessel.  If you compact it, it will not have the same volume as when it was in the mould.  It will be less, and so you will underestimate the volume and therefore the weight of glass required.

Irregular mould frames

If you have an irregular mould frame such as those used for pot and screen melts that you do not want to completely fill, you need to do an additional calculation.  First measure the height of the frame and record it.  Fill and level the frame with kiln wash or fine frit.  Do not compact it.  Carefully transfer the material to the measuring vessel and record the volume in cc.

Calculate the weight in grams required to fill the mould to the top using the specific gravity.  Determine what thickness you want the glass to be.  Divide that by the total height of the mould frame (all in cm) to give the proportion of the frame you want to fill.  Multiply that fraction times the weight required to fill the whole frame to the top.

E.g. The filled frame would require 2500 gms of glass.  The frame is 2 cm high, but you want the glass to be 0.6 high.  Divide 0.6 by 2 to get 0.3.  Multiply that by 2500 to get 750 grams required.

Regular mould frames

For a regular shaped mould, you can do the whole process by calculations.  Find the volume, multiply by specific gravity to get the weight for a full mould.  Measure the height (in cm) of the mould frame and use that to divide into the desired level of fill (in cm).

E.g. The weight required is volume * specific gravity * final height/ height of the mould.

The maths required is simple once you have the formulae in mind.  All measured in centimetres and cubic centimetres

Essential formulae for calculating the weight of glass required to fill moulds (all measurements in cm.):

Volume of a rectangle = thickness*length*width

Volume of a circle = radius squared (radius*radius) * ϖ (3.14) * thickness

Volume of an oval = long radius * short radius * ϖ (3.14) * thickness

Specific gravity = volume/ weight