Full text can be accessed at http://link.springer.com/article/10.1007/s10437-015-9196-5This paper investigates the purpose of lime mortar-coated potsherds found along the East African coast. Recent sites investigated are in areas of Kaole, Kiswere, Rushungi, Sudi, and Mikindani in Tanzania. Desktop research revealed similar potsherds from Manda in the Lamu Archipelago of Kenya and Kilwa Kisiwani in Tanzania. From the late first millennium AD, asphalt has been recorded on pottery at Manda to make it waterproof. From around the same period, mortar was found on pottery at Kaole and on other artefacts in the midden deposit such as ‘bead’ grinders and bone deposits. This suggests natural cementation from lime introduced to the midden deposit. A thin layer of plaster on pots dating to the late twelfth to late thirteenth centuries at Kilwa Kisiwani, and eleventh to fourteenth centuries at Sudi, has been interpreted as deliberate to make the vessel more watertight. Later evidence indicates that the tradition of coating pots with lime mortar probably for the purposes of storing liquids continued up to the seventeenth and eighteenth centuries at Mikindani. However, vessels and deposits dating from the late twelfth and thirteenth centuries at Kiswere and Bwembweni, near Kaole, contain layers of mortar too thick for the purpose of waterproofing the vessel, and were probably used for mixing and then coating a building. The coastal and estuarine settings of the find spots indicate the importance of water transport for this lime mortar industry. The storage and transport of lime along the coast and inland would have been a significant part of local East African trade for its use in iron making and building.