The bill is mostly the empty hours
Run the defaults. A furnace holding at 0.42 units an hour around the clock is 70.6 units a week before anybody picks up a pipe. Add ten hours of charging at 0.55 on top, three sessions with six hours of glory hole at 0.7, and twenty annealer hours a session at 0.12, and the week comes to 95.9 units — 129.41 dollars at 1.35 apiece.
Of that, the furnace holding through the 150 hours nobody is in the building is 63 units, 85.05 dollars, 66 percent of the bill. Three sessions of six hours is 18 worked hours out of 168.
Average and marginal are different numbers and both are true
Forty-two pieces a week over 129.41 dollars is 3.08 a piece. But a fourth session adds only the glory hole and the annealer, because the furnace is already lit — 6.6 units, 8.91 dollars, spread over 14 more pieces at 64 cents each. The average piece carries nearly five times the energy the extra piece does.
Neither figure is the right one for everything. Costing a year of production uses the average. Quoting one more commission on a week you are already firing uses the marginal. Studios that price everything at the average turn down work that would have paid, and studios that price everything at the marginal never cover the standby.
What a session count does to the per-piece figure
The block near the bottom runs one through six sessions on the same furnace. At one session a week the energy in a piece is 7.97 dollars; at six it is 1.86. Nothing about how the piece is made has changed. That is the whole economics of a hot shop in one column, and it is why shared-access studios exist at all — a furnace is a fixed cost looking for hours to spread itself over.
The same block shows the limit of it. The curve flattens: going from one session to two saves 3.67 a piece, going from five to six saves 24 cents. Past a point the standby is already thin and you are just working more.
Meter it, do not read the plate
The single biggest error in this arithmetic is putting the burner rating in the holding box. A burner rated for the climb runs at that rate with the valve open, and a furnace sitting at temperature spends most of the hour with it shut. The gap between the two is often a factor of two or three and it lands on the biggest line on the page. A week of meter readings that includes a quiet stretch and a working day separates them properly, and after that the page is describing your shop rather than a generic one.
One caution that is easy to trip over: the page adds every line together in whatever unit you picked. A gas furnace and an electric annealer are two bills, not one, and putting kilowatt hours in the annealer box while the furnace is in therms produces a number that means nothing.
Questions people ask
How much does it cost to run a glass furnace?
On the figures in the form — 0.42 units an hour holding, around the clock, at 1.35 a unit — the furnace alone is 95 dollars a week before anything else burns, and the whole shop comes to 129.41. Your own number depends entirely on the furnace, the insulation and the fuel price, which is why every one of those is a field rather than a constant here.
Why is the cost of one more session so much lower than the average?
Because the furnace is already lit. An extra session on a shop that holds glass around the clock only adds glory hole hours and annealer hours, which on the defaults is 8.91 dollars, or 64 cents a piece, against an average of 3.08 across 42 pieces. The standby is already paid for by the week whether you use it or not.
Should I use the average or the marginal figure when I price work?
Both, for different questions. Work out a year of production on the average, because over a year the standby has to come out of something. Price an additional job on a week you are already running on the marginal, because that is the money that actually leaves the account. The page prints them side by side so the difference is visible rather than assumed.
Can I mix gas and electricity in this?
No, and it will not warn you. Everything is added in the single unit you picked at the top. If the furnace burns gas and the annealer is electric, run the page once for each fuel and add the two answers. Mixing them produces a total that is arithmetically valid and physically meaningless.
What rate should I put in for the furnace holding?
A metered one. The data plate gives what the burner does wide open, which is the climb rather than the hold, and putting that figure in the holding box can double the biggest line on the page. Read the meter over a week that includes both idle hours and a working day and the holding rate falls out of the difference.