# calculating the weight of glass for pot melts.

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• #10091
JanneR
Participant

I am trying to make pot melts and I am using the following formula (all metric):
volume x 2.5 = weight. Example: 12 in circle, 6mm thick:
12 in = 30.48 cm (area = 15.24 squared x pi (3.14) = 752.44 sq. cm) x .6 = 437.57 cu. cm.
437.57 x 2.5 (specific gravity of bullseye glass) = 1,094 gms.

I put slightly more in the pots due to left over and followed the Fused Glass.org pot melt schedule (top process temp 1700 degrees) and I got about 2/3 or less of the 12 ring filled.

What did I do wrong? I am planning to use a lot of scrap glass for pot melts and I don’t want to waste it.

#13247
Jeanice
Participant

1094g is correct.  Maybe the error happened when you weighed the glass?  Is there a way you can calculate how much glass was left over in your pot?  Think I read somewhere to add 5% to 10% extra glass to account for the glass that remains in the pot.

Hope this helps.  Hopefully someone else with more pot melt experience will come along.

#13248
betnca
Participant

I don’t have an answer but you are not alone.  I have done three pot melts and had the same results that you did.  I double- checked everything including my scales. I thought I might need new scales but that wasn’t the problem.

#13249
Anonymous
Inactive

How thick is the pot melt in the middle? It takes a while for glass to fully spread to a full 12 inches. If the glass is even a little thicker than 6mm that would explain it not being 12 inches across.

When I make pot melts I include a 30 minute hold at 1500° F on the way down.

Paul
FusedGlass.Org
Helios Kiln Glass Studio
PaulTarlow.com

#13250
JanneR
Participant

I’ll try that.

#13251
JanneR
Participant

I tried again with similar non-acceptable results. I put about 25% more glass into the pot, fired at 1700 for 90 minutes, 1500 for 60 minutes, 900 for 3 hours and continued to cool slowly. I measured enough glass to have between 8 – 9cm. I also got tons and tons of bubbles. I don’t want to give up but I just don’t know what else to do.

#13252
Stephen Richard
Participant

I don’t know what happened to the maths but the correct weight is 1766 gms.

Stephen Richard

#13253
betnca
Participant

I have had the same problem on 3 melts so haven’t done any more. What math formula did you use? I thought maybe my scales were off. Do you have a particular way of arranging the glass to weigh it?

#13254
Stephen Richard
Participant

30cm dia = 15 squared = 225 times 3.14 =706.5 times 2.5 (SG) = 1766.25g.  You may also want to add an additional 100g for the pot

Stephen Richard

#13255
JanneR
Participant

I tried again with french vanilla, white, medium amber, and clear and weighed just about 1700 gms. I had a “stopper” in the dam so that I would have a hole in the glass (3.5 inches in a 12 inch dam) which I later filled with a piece made with murrini. 500-1700-2hrs, full-1500-1 hr, full-900-3hrs, 100-800-0, 200-700-0, 300-100-stop. I have a beautiful piece of glass although the stainless steel dam was warped. I re-fired at a full fuse with the extra piece, and although the edges need a lot of coldworking, the end result is more than I hoped for.

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