The black-colored pottery slips produced in Athens from the 6th to 4th centuries B.C., had a consistent composition achieved through processing and refinement of raw clay. Little direct evidence has been established as to what were these refinement methods. To better understand how the slip material was prepared, the major and trace elemental compositions of 19 slips from different ceramic vessels and their corresponding bodies of Athenian red-figure and black-figure vases were determined using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Notably higher Zn concentrations were found in the slips (271-1959ppm) than in their corresponding body ceramics (<361ppm). The Zn concentrations in the slips were also found to be above the natural background for typical clay (between 10 and 300ppm) suggesting an unintentional anthropogenic enrichment of this metal. Based on the abnormally high Zn content of the slip, it is speculated that the clay was treated using vitriol (concentrated acid mine runoff which is rich in Zn), to induce flocculation and remove carbonate mineral phases from the raw material that, if present, would prevent the slip from vitrifying. This same signature of elevated levels of Zn with a corresponding Ce anomaly is also observed for black glosses produced in Corinthian and Etrurian (Italy) workshops indicating that these trace element signatures were imparted to the material by means of shared methods of manufacturing instead of being indicative of a single unique source for this material.