In glass production, ‘furnace pull’ refers to the total tonnage of molten glass that exits the furnace. However, not all of this molten glass is converted into usable products; a significant portion becomes waste due to various factors. The losses in the float glass loop can be attributed to the following:

• Approximately 10-15 cm of the glass ribbon’s right and left edges often have an undesirable thickness gradient, differing from the main body of the ribbon, resulting in approximately 8-10% waste.
• Defects in the molten glass body or surface, along with mechanical issues—whether directly or indirectly—can lead to breakage of the ribbon in the annealing lehr and cutting line, or result in the cut glass being rejected and sent to the crusher, either online or offline. Glass defects can lead to 6-8% of waste. 
• Sometimes, even high-quality molten glass is rejected due to thickness variations or when the thickness falls outside the acceptable range. This issue can be particularly pronounced during job changes or when adjusting the thickness. Inappropriate production planning and improper setting of process parameters in the tin bath—especially for top rollers, coolers, and flags—can lead to hours of reduced production yield.
• During stacking on racks and transportation to the warehouse, breakage can account for an additional 1-4% waste. 
• Further handling in the warehouse, such as transferring and loading the glass onto trucks for sales, may result in an extra 1-4% loss.
• Transportation and unloading at wholesalers’ warehouses or shops can cause a 1-5% breakage rate, depending on the quality of the glass, the type of trucks used, the skill of the driver, the proficiency of the loading and unloading workers, and the setup of the warehouse and racks.
• The most significant loss occurs during the processing of the glass—cutting, edging, tempering, laminating, and IGU (Insulating Glass Unit) manufacturing—where waste can average around 10-20%. Mistakes in meeting the exact specifications of customer demands are common at this stage. Reworking and recutting may occur repeatedly in some sensitive cases or with particularly picky customers.
• Handling and transferring the ready-to-use glass to the end user, up to the point where it is glazed into window frames, can generate an additional 1-2% waste.

Taking all these factors into account, under typical conditions—where the purity and grain size of raw materials, as well as process parameters in the batch plant, furnace, tin bath, annealing lehr, and cutting line, are managed using traditional methods—there is an estimated 25-30% loss within the float glass loop, from the point where the molten glass exits the furnace to its final application as a framed window.

Those working in glass manufacturing know that conditions are not always perfect. There are many instances where everything goes awry, leading to non-production periods and a dramatic drop in production yield. In the worst scenarios, especially as the furnace approaches the end of its campaign life, a human error can result in production halting entirely for days or even weeks. Now the big question for glass experts is what are the impacts of this massive loss, and what can be done to ensure a smooth, end-to-end glass production process?