AbstractAqueous suspensions of Milling Yellow dye have been found to lie useful for engineering flaw studies by employing established birefringent techniques. In the present work, the basic birefringent and rheologic properties of Milling Yellow suspensions wore measured for a range of deformation rates important to the engineering applications. A concentric‐cylinder polariscope was used to measure the amount of birefringence and the angle of extinction for ten Milling Yellow suspensions at 25°C. varying in concentration from 1.25 to 1.46% dye by weight. The optical properties exhibited by these suspensions are similar to those of a number of colloidal suspensions as reported by past investigators. Rheologic data for Milling Yellow suspensions in the same concentration range were measured at 25°C. for deformation rates of 50–10 sec.−1by means of a capillary viscometer. Over the complete range of deformation rates the rheologic data are accurately described with the Powell‐Eyring relationship for pseudo‐plasti