Blown Glass Material Calculator

This calculator works backward from a target amount of finished soda-lime glass to the raw batch a glassblower or hot shop needs to melt. It splits the total glass weight between recycled cullet (which replaces raw batch 1:1 by weight, since it's already fully reacted glass) and virgin raw materials, then breaks the raw-material portion down by the standard soda-lime oxide targets — roughly 72.5% silica, 13.5% soda, 10.5% lime, 2% magnesia, 1.5% alumina by weight of the finished glass. Because most raw ingredients aren't pure oxides, each oxide percentage is scaled up by a stoichiometric conversion factor that accounts for mass lost as CO2 during melting (loss on ignition): soda ash (Na2CO3) needs about 1.71x its Na2O contribution, limestone (CaCO3) about 1.786x its CaO contribution, and dolomite roughly 4.6x for its magnesia share, while sand and feldspar convert close to 1:1.

That's why the total raw batch always weighs noticeably more than the glass it produces — carbonates release gas as they decompose in the furnace. Separately, the calculator estimates the "gather" — the blob of molten glass gathered on a blowpipe for one piece — by modeling the finished piece as a hollow sphere shell (outer diameter minus wall thickness gives the inner radius) and converting that shell volume to weight at soda-lime density. Melting energy is a rough industry rule of thumb of about 5 MJ per kilogram of glass, useful for furnace-fuel budgeting but not a substitute for a specific furnace's measured efficiency.