Some drinking glasses, especially those made of crystal and lead-containing glass, are very harmful to the recycling process. They may also contain heat-resistant glass, milk glass or flat glass. These types of glass have a different chemical composition from regular glass, requiring much higher temperatures to melt and contaminating the recycling process. That is why they may not be mixed with the glass of bottles, flasks and jars.
The melting temperature of dishes made from porcelain, ceramics, earthenware and terracotta is also much higher than the melting temperature of regular glass. As a result, unmelted shards might end up in the recycled glass, causing new products to be not uniformly solid and thus break easier.