What was the purpose of the thick clay coatings on large Harappan jars found at Omani sites?

To prevent the percolation of liquids stored in the jars

Recent archaeological discoveries indicate that copper was likely obtained from Oman, located on the southeastern tip of the Arabian peninsula. Through chemical analysis, traces of nickel have been found in both Omani copper and Harappan artefacts, suggesting a shared origin. Additional evidence of contact between the two regions includes the presence of a distinct type of vessel known as a large Harappan jar with a thick layer of black clay coating. These coatings were designed to prevent liquid percolation, although the specific contents of these vessels remain unknown. It is plausible that the Harappans engaged in trade, potentially exchanging the contents of these vessels for Omani copper.