The <8 mm fraction of aged incinerator bottom ash from a commercial incinerator (energy from waste) plant has been collected at regular intervals, characterised and processed to form ceramic materials. Ashes were sieved, wet ball milled, dried, compacted and sintered at temperatures between 1080 and 1115 °C. Variations in the chemical composition and mineralogy of the milled ash, and the mineralogy, physical properties and leaching of sintered products have been assessed. Milling produces a raw material with consistent chemical and mineralogical composition with quartz (SiO2), calcite (CaCO3), gehlenite (Ca2Al(AlSi)O7) and hematite (Fe2O3) being the major crystalline phases present. Different batches also milled to give consistent particle size distributions. Sintering milled incinerator bottom ash at 1110 °C produced ceramics with densities between 2.43 and 2.64 g/cm−3 and major crystalline phases of wollastonite (CaSiO3) and diopside (CaMgSi2O6). The sintered ceramics had reduced acid neutralisation capacity compared to the as-received ash and exhibited reduced leaching of Ca, Mg, Na and K under all pH conditions. The leaching of heavy metals was also significantly reduced due to encapsulation and incorporation into glassy and crystalline phases, with Cu and Al showing greatly reduced leaching under alkali conditions.