In this research, leather tanning using gambir processing liquid waste has been done. The research was done by varying the percentage of adding alum as mordant, respectively 0%, 2%, 4%, and 6% into the waste of gambier processing. The addition of alum crystal was done in step II of the tanning process. The observation was done by investigating several parameters, such as the skin color, physical and chemical properties of the tanned leather-based on SNI 06-0463. 1989 A. The results show that the use of alum crystal as a mordant made the skin color became yellowish, lower solubility of tannin in the water, smaller raw leather content, higher bonded tannin to the leather, higher tanning degree, higher tensile strength, and higher flexural strength. The alum crystal concentration affected the physical properties, i.e., tensile strength and flexural strength. Increasing the alum crystal concentration increases the tensile strength but decreases the tanned leather's flexural strength.
gambier processing waste; leather tanning; alum crystal; combination tanning