Reducing the energy consumption in ceramic kilns because of lower firing temperatures is very relevant in terms of both fuel cost and carbon dioxide (CO2) emissions. In this sense, this work evaluates changes in the sintering starting temperature when ashes from the combustion of agro-industrial wastes (exhausted olive cake and almond shell) are added (0–20 wt%) to two types of clays showing different plasticity and refractory behaviour. Both, reductions and increases in this temperature were measured by means of a Hot Stage Microscopy (HSM) which registers changes in the height of the sample subjected to a specific temperature profile. A selection of the more adequate clay-ash mixtures attending to their sintering behaviour were performed. Fired pieces of the selected mixtures were subjected to further characterization (tensile strength, linear shrinkage, firing loss, bulk density, porosity and water absorption) to evaluate mechanical performance when compared to the original clay. The results indicate that the sintering temperature of the clay-ash mixture is a compromise between the fluxing oxides content and the pores created by the loss of volatile matter and carbonates in the ash. While the former favours sintering, the latter inhibits it because of the appearance of gaps between mineral particles. Ash contents higher than 10% did not provide benefits in terms of melting behaviour in any case. Reductions in the sintering starting temperature up to 120 °C were reported when ash from exhausted olive cake was used, which could lead to estimated fuel-energy savings around 2 €/t of product (with around 6 kg/t of CO2 emissions avoided). Moreover, percentages above 5% as well as the use of ashes from almond shells could be an attractive way for producing materials with enhanced insulation properties.