Melting takes most of the energy used to make glass, so the furnace matters. This article compares fuel-fired, oxy-fuel, boosted and all-electric furnaces, explains why electric furnaces can be more efficient and cleaner at the point of use, and lists the questions to ask a glassmaker when you specify a bottle.
Melting the raw materials makes up 75 to 85% of the total energy used in glass manufacture.1 The furnace runs at between 1,200 and 1,700°C,2 so the way it is heated shapes the footprint of every bottle that leaves it. Furnace choice rarely appears on a specification sheet.
Most glass is still melted in furnaces fired with natural gas or oil. Burners sit above the melt and heat it from the surface, and the hot exhaust leaves through the flue.
Regenerative furnaces recover some of that heat by passing the exhaust through brick chambers that preheat the incoming combustion air. Recuperative furnaces use a heat exchanger for the same job. Even with heat recovery, the thermal efficiency of a gas-fired furnace peaks at around 45%, with the remainder lost through the furnace structure and the waste gases.3
Oxy-fuel firing burns fuel with pure oxygen instead of air. With the nitrogen removed, the exhaust volume falls by about two-thirds and the formation of thermal NOx drops sharply.1
Electric boosting adds electrodes to a fuel-fired furnace. It typically raises output by 20 to 50%,4 and hybrid designs add more precise temperature control.5
An all-electric furnace has no burners. Electrodes sit in the molten glass and current passes through the glass itself. Molten glass conducts electricity, so the current generates the heat the batch needs by the Joule effect.6 The glass heats from within instead of from above.
Many designs feed batch from the top and keep a layer of it on the surface. That layer insulates the melt and limits heat loss. A well-designed all-electric melter of this type can reach a thermal efficiency of 85%, close to double that of the most efficient fuel-fired furnaces.7
The build is simpler as well. There are no regenerators or burner skids to construct and maintain,8 and rebuild costs are lower than for fuel-fired furnaces.1 Control is more flexible,8 and direct emissions of CO2, thermal NOx and SOx at the furnace are very low.3
Electric melting has been used for decades in many sectors of the glass industry.4 Higher-volume production has used it less so far, and hybrid furnaces offer a step in that direction by combining fuel firing with electric heating. Some hybrid designs can run on up to 80% renewable electricity.5
The Cullen bottle is manufactured using an electric furnace, in 15ml, 30ml, 50ml and 100ml capacities.
Explore the Cullen bottleAn electric furnace produces no combustion emissions on site, so its footprint follows the footprint of its electricity. On a low-carbon supply, electric melting has the advantage over burning fuel. On a grid that still relies heavily on fossil generation, burning fuel directly in the furnace can come out lower. As renewables take a larger share of the grid, that changes, and the furnace needs no modification to benefit.3
The furnace is one lever on melt energy and carbon. The batch is the other. Soda ash and limestone release CO2 as they react in the melt.2 Recycled glass has already been through that reaction, so each tonne in the batch replaces raw material that would otherwise release it. It also melts at a lower temperature than raw batch, so the furnace needs less energy for every tonne.
The Bastia bottle, from our Signature Collections, contains over 50% post-consumer recycled glass and still looks completely clear. The lower melt temperature results in an energy saving of around 10% in making it. The glass is also melted in an electric furnace.
Ask us at the Define stage of your project and we will confirm the answers for the range you are considering.
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