There is growing evidence that, in general, aluminium castings under-perform by a large margin. This is not only because of shrinkage or gas porosity, but particularly because of the existence of extremely thin but detrimental defects called bifilms. Bifilms are the initiators of porosity and failures. The opening of a bifilm to make a pore or a crack involves negligible energy, being so easy that it can be assumed that this process will be overwhelmingly favoured. The discriminating use of the Reduced Pressure Test (RPT) clearly reveals the existence of bifilms. Until now, the unpredictable behaviour of bifilms has made the RPT hard to quantify. In the present study, a new fundamental parameter is proposed to assess melt quality called the Bifilm index which is defined as the total length of bifilms estimated on a polished RPT section. Typical total bifilm lengths vary from 3 to 300 mm. The effect of the bifilm index on the mechanical properties was studied using several common Al-Si alloys cast under different conditions. The total bifilm length plus perhaps the total bifilm number seems a rational basis for the quantification of the quality of liquid Al alloys. For the first time, evidence has been uncovered that high oxide content not only reduces ductility, but, acting as a composite material, can simultaneously increase strength.