When a new product is in development or an existing product is being improved, unforeseen complications may arise in terms its rheological performance. The objective of the work was to determine the cause of an unwanted rheological phenomenon that will be referred to as 'viscosity drift.' The work involved experimental design, viscosity as a function of time at a constant shear rate, and pH as a function of time. Organically modified hormitic clay was the sole influence on the viscosity drift exhibited by the plaster pastes. It was determined that setting was not the mechanism by which the clay caused the viscosity to drift. It was, however, established that a structure was being developed within the plaster paste, specifically a clay network. (C) 2003 Kluwer Academic Publishers.