Glass production starts in the second half of the sixteenth century BCE. Glass was produced from the combination of quartzite pebbles with a plant ash flux, usually with the addition of copper, cobalt, antimony or manganese colorants, and opacifiers. The earliest surviving glassmaking workshop is a subject of debate since archaeological evidence for glass production is rare and often equivocal. No glassmaking factories have yet been found in Mesopotamia or Northern Syria, but several candidates are known from ancient Egypt, including the sites of Malqata, Amarna, and Qantir. This is still very much a topic of current research, both through archaeological investigation and scientific analysis.