The use of perlite as a filter aid has been studied for the separation of tincal insolubles from borax solutions during the dissolution of tincal concentrate. Precoating and body-feeding techniques were applied with perlite/insolubles ratios ranging between 0-3 with filtration pressures ranging between 2 and 4 bar. Lower specific cake resistances of the order 1 x 10(10) m kg-1 were obtained for the body-feeding technique. Incompressible cakes were obtained with compressiblity coefficients of c. 0.016. The specific cake resistance was found to increase with the bulk density of the perlite grade.