Source: Period sources
Kemalau, also known as gutta-percha, is a natural thermoplastic latex made from the sap of the Palaquium gutta tree.1
Among its many uses, it was used to join the blade of the mandau sword of the Dayak of Borneo to its hilt.
A Dayak mandau hilt, made of carved deerhorn.
The black substance between rattan braid and blade is kemalau.
1. S.W. Tromp; Mededeelingen omtrent mandau's. Den Haag, 1887. Published in Internationales Archiv für Ethnographie, International Gesellschaft für Ethnographie; Rijksmuseum van Oudheden te Leiden, 1888. Volume 1, page 24.