In the present study, the waste scrap agate, obtained as a by-product of rock agate beneficiation as decorating ends and media for wet grinding was used as inert filler in a traditional triaxial red stoneware tiles composition. The effects on the technological properties have been investigated. The study has been carried out using the mixtures experiments method for the experimental design. Characterization results were used to calculate statistically significant and valid regression equations, relating dried and fired body properties with clay, feldspar and scrap agate contents in the unfired mixture. The regression models were then used simultaneously to delimit the combinations of those three raw materials most adequate to produce a ceramic body with specified properties. The use of mathematical optimisation shown that, for the particular raw materials under consideration, there is a rather forgiving composition range of clay (20-68 wt.%), feldspar (17-50 wt.%) and scrap agate (15-45 wt.%) contents within which it is possible to simultaneously specify the technological requirements for triaxial red stoneware tiles bodies, not only of the fired products but also of the intermediate materials at important stages of the processing.