Time–temperature–transformation (TTT) diagrams have been determined for high-lime (10.2%) and moderate-lime (5.2%) bulk raw materials from the Peterborough Member of the Oxford Clay Formation of King’s Dyke [TL 525 297] and Orton [TL 517 293] brickworks. Study of the chemistry and mineralogy of the brickclays provides the starting point for 79 firing experiments on each raw material, at temperatures between 800 and 1100°C, and for firing times at maximum temperature from 15 minutes to 336 hours. Quantified X-ray diffraction of each fired briquette enables individual TTT diagrams to be constructed for each phase. Finally, a summary diagram shows the distribution of mineral phase assemblages in time–temperature space. The original mineralogy (quartz, illite, kaolinite, chlorite, calcite, aragonite, K-feldspar, albite, pyrite, gypsum, anatase and apatite) changes on heating to a mixture of calc-silicates (anorthite, melilite and pyroxene) with quartz, hematite, anhydrite and glass in commercial bricks and also wollastonite and cordierite when longer firing times and temperatures are used. The mineralogical changes can be followed in quantified equations using the actual compositions and quantities of the phases. The calc-silicates are produced by lime (from calcite) reacting with the clay minerals or their breakdown products such as metakaolinite. The glass has granitic affinities. The data provide a case for considering shorter commercial firing times.