This paper presents the results of a Time-Temperature-Transformation (TTT) study of the effect of different heat treatments on the final form and durability of the wasteform produced by the vitrification of Hanford High Level Waste (HLW). A borosilicate glass formulation containing 60 weight % of a Hanford Tank AZ 101 simulant approximated the radioactive waste glass. The TTT diagram was generated by analyzing thirty-six wasteforms produced by melting the frit-waste mixture at 1450°C, and then heat-treating the melts at specified temperatures for a predetermined length of time. The temperatures studied ranged from 500°C to 1200°C, and the length of the heat treatments ranged from 0.75 hour to 768 hours (32 days). The wasteforms produced were analyzed by Toxicity Characteristic Leaching Procedure (TCLP), X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM) - Energy Dispersive Spectrometry (EDS). Rietveld analysis was performed on some of the XRD results. Crystalline forms detected include transition metal (predominantly Fe) spinel, zircon, zirconia, and iron oxide. These crystalline forms are characteristic to specific heat treatment conditions. The types or concentrations of crystals have little to no impact on the Product Consistency Test (PCT) performance of the wasteform. The results show a strong relationship between heat treatment conditions (time and temperature) and TCLP test response for cadmium only. Samples heat-treated at higher temperatures or longer periods of time show a considerably higher cadmium response (by TCLP) than samples heat-treated at lower temperatures or for shorter times.