Does the Intergovernmental Panel on Climate Change want a glass plant to record cullet made in-house separately from cullet it buys?
Yes, it says this is useful. The Intergovernmental Panel on Climate Change (IPCC) says it is useful to report whether cullet was made at the plant or bought as post-consumer cullet, because it may matter when checking recycling savings. The cullet ratio is the cullet share of the furnace charge.
The ratio and its range
The guide defines the cullet ratio as "the fraction of the furnace charge represented by cullet", and puts it at 0.4 to 0.6 for container applications. Insulation fibre glass makers "will generally fall below this level, but will use as much cullet as they can purchase."
Two sources
Cullet comes from in-house return of glassware broken in process or other glass spillage, and from foreign cullet from recycling programmes or brokers. The guide says foreign cullet is important in developed economies but less so in developing countries, where recovery of glassware is not popular.
What the guide asks you to report
"It is also useful to report whether cullet was internally generated at the facility or is post-consumer cullet (i.e., purchased cullet). This information may be important for identifying, or verifying, mitigation activities that are based on recycling." Elsewhere it calls the cullet ratio one of the largest sources of uncertainty for Tiers 1 and 2, varying between plants and over time at one plant.
A log that does it
- Raw batch tonnes charged each day.
- In-house cullet tonnes charged.
- Bought cullet tonnes charged, with the supplier slip.
- Monthly cullet ratio: total cullet divided by total charge.
Why bought cullet is the change a buyer can check (my reading)
In-house returns arise whether or not you recycle them, so a ratio that rises because you bought more cullet is the change a buyer can check. This is my reading of why the guide asks for the split.
One month, example tonnes
- Raw batch 540 tonnes, in-house cullet 150, bought cullet 60. Charge 750 tonnes.
- Cullet ratio: (150 + 60) / 750 = 28%, of which in-house 20% and bought 8%
- Container factor: 0.21 x (1 minus 0.28) = 0.1512 tonnes of carbon dioxide (CO2) per tonne of glass
- Buy 60 tonnes more cullet and charge 60 less raw batch: ratio 270 / 750 = 36%, factor 0.21 x 0.64 = 0.1344
- That is 11.1% less batch CO2 per tonne of glass
Tonnes are example numbers; 0.21 is the guide's container glass default. Furnace energy changes with cullet too, and is counted separately.
What Senseible does here
Senseible reads fuel invoices and electricity bills into monthly records, so the fuel and power lines of a furnace or kiln are on file. In the self-serve product it does not read batch or cullet records, so the ratio is worked from your own charge log.
- How cullet cuts glass CO2
- IPCC glass factors and cullet ratios
- Glass furnace gas per tonne benchmark
- Glass counted in pieces to tonnes
- Glass container furnace emissions per tonne
Sources: IPCC, 2006 Guidelines, Volume 3, Chapter 2: Mineral Industry Emissions
Last reviewed 2026-11-06. Quotes and figures are from the 2006 IPCC Guidelines, Volume 3, Chapter 2. The 2019 Refinement was not checked for this page; check it if a buyer names it.