Why is cullet usually made from glass with the same composition as the new glass batch?

In industrial glassmaking, cullet—which consists of crushed rejected glass—is typically used in proportions ranging from 25 to 60 percent of a new glass batch. It is essential that this cullet matches the composition of the new mineral mixture to ensure consistency and quality in the final product.1

The primary reason for including cullet is to facilitate and accelerate the melting process. Because cullet has already been processed, its early melting in the furnace helps bring the raw mineral particles together, which promotes faster reactions. Using glass of the same composition prevents the introduction of foreign chemical constituents that could disrupt the desired properties of the glass or interfere with the homogenization process, which is the rate-limiting step in glass production. Maintaining a uniform composition is critical, as variations in the melt can lead to defects or issues with the glass's chemical and physical stability.123