Ball mill charge size distributions originally specified for Bond grindability tests to determine the Bond Work index (BWi) are not commercially available. Those proposed to date do not match all of Bond's (1961) original specifications of total ball mass, number and surface area. An alternative mill ball charge is proposed that closely approximates Bond's original total ball mass, number of balls and ball surface area. Results of 30 Bond Work index tests of six pure materials (calcite, magnesite, labradorite (feldspar), quartz, andalusite and glass) using closing screen apertures (P-1) values of 500, 250, 125, 90 and 63 pm are analysed. The samples were selected on the basis of having distinct hardness's (Mohs hardness 3 to 7.5), being relatively free of crystallographic defects and having distinct cleavage properties The 80 percentile size of the fines produced (P-80) concur with those of published values. The trend based of P-80 values for P-1 values of 150 to 44 pm recommended by Bond poorly fit with published P1 values > 250 mu m. It is demonstrated that the BWi test P-1 values of mono-mineralogical or mono-material samples determines the P-80 value obtained. Based on test results of this investigation a polynomial exponential equation relating P-80 to P-1 values with an R-2 correlation factor of 0.9977 is presented. This relation is independent of the tested material hardness and crystallographic defects and planes of weakness. (C) 2017 Elsevier B.V. All rights reserved.