Metal prills in primary glass: A puzzling aspect of the production process

•Characterization of metal prills embedded in Late Roman-Early Byzantine raw glass chunks from Umm ez-Zinat.•Three prills from Bet Eli’ezer, dated to the seventh-early-eight centuries CE, were also analysed.•The prills from Umm ez-Zinat are mainly lead, with minor contribution of copper, tin, antimony and silver.•The prills from Bet Eli’ezer are copper lead alloys of varying composition.•Potential origins of the metal prills in the glass are discussed.