Volume in, weight out
A 20 inch pot filled 11 inches deep — 14 to the rim with 3 inches of working level below it — is 3,456 cubic inches. At 2.3 grams per cubic centimetre hot, that is 287 lb of glass. Nobody weighs a pot, so the only way to that figure is through the volume, and the only way from that figure to a scale is through the yields.
At 40 percent cullet, 82 parts of glass per 100 of batch and 98 per 100 of cullet, it takes 210 lb of batch and 117 lb of cullet. That is 327 lb of raw material for 287 lb of glass — 1.14 lb in for every pound out.
Why it takes all day to charge
The loose volume is the number that catches people. Batch at 1.25 g per cubic centimetre in the bucket and cullet at 1.4 come to 6,970 cubic inches between them, which is 2.02 times the volume of the pot they have to end up inside. Powder has air in it and a fair share of the weight leaves as gas, so a pot can never be filled in one pass. At 35 percent of the pot per charge, that is six charges, and the last one goes in about three quarters full.
The bulk densities are measurements, not lookups. Weigh a level bucket of your own mixed batch, weigh the same bucket of your own cullet, and the charge count stops being a guess.
Cold density is the wrong number
Glass expands as it heats, so molten glass is lighter than the same glass on the bench. Putting a cold density in the box overstates what the pot holds and then overstates the batch, and the error runs through everything downstream. The field asks for the density at working temperature for that reason, and the honest source is the supplier or the pot itself — melt a known volume, and you have measured it.
The difference is not small. Using 2.5 instead of 2.3 on the default pot puts 25 extra pounds of glass in the answer and 29 extra pounds of raw material against it, on a pot that only holds 287 to begin with.
The cullet fraction moves two things
The block near the bottom runs 0, 25, 50, 75 and 100 percent cullet on the same pot. Cullet is cheaper per pound and it also loses less on the way in, so both effects push the same way — on the defaults the material cost for a pot goes from 297.65 dollars at all batch to 131.85 at all cullet, and the charge count drops from seven to five because there is less raw material to get in.
What the page will not do is tell you what fraction to run. Cullet brings its own history with it, colour and all, and what that does to the glass you are about to blow is a question for your supplier and for what came out of the pot last time. Arithmetic has nothing to say about it.
Questions people ask
How much batch does it take to fill a glass furnace pot?
On the defaults — a 20 inch pot filled 11 inches deep, holding 287 lb of glass at a hot density of 2.3 — it takes 210 lb of batch and 117 lb of cullet at a 40 percent cullet share. The yields are the reason the raw weight exceeds the glass weight: batch loses a share of itself as gas during melting, and how much is a property of the recipe rather than of glass in general.
Why does raw material take up more room than the pot?
Because loose batch and cullet are mostly air between grains, and because some of the weight leaves as gas rather than becoming glass. On the default figures the loose volume is 2.02 times the volume of the pot. That is why a pot is charged in stages with melting time between them rather than filled once.
What density should I use for molten glass?
The density at your working temperature, which is lower than the cold density of the same glass. The figure comes from your supplier, or from melting a known volume and weighing what you get. Using the cold figure will overstate both the glass in the pot and the batch you need, and the error carries through every line below it.
How many charges does it take?
Loose volume divided by whatever share of the pot you can get in before waiting for it to melt down. On the defaults, 6,970 cubic inches of loose material at 35 percent of a 3,456 cubic inch pot is six charges. Both of those are your own numbers: the bulk densities come from weighing a bucket, and the charge fraction comes from your furnace and your practice.
Should I run more cullet?
The page prices it but takes no view. Cullet is cheaper per pound and loses less on melting, so the cost falls both ways as the fraction rises. It also carries whatever was in the glass it came from, including colour and whatever the previous melt picked up, and that is a decision about the work rather than about the invoice. Ask your supplier and look at what came out last time.