Sustainable architecture is a design approach that minimizes negative environmental impact through the efficient use of resources and a conscious focus on energy and ecological conservation of the built environment. Within the ceramic sector, porcelain tiles are considered eco-friendly due to their low environmental impact throughout their life cycle, from the extraction of raw materials, energy-efficient industrial manufacturing process, and full recyclability at the end of their life. The environmental impact of such products can be further decreased by the modification and optimization of the conventional formulations used to prepare porcelain stoneware tiles. For instance, modern floor and wall tiles are composed of four layers, namely, the ceramic support, the engobe, the decoration (ink-jet printing) and the glaze (top-coat). The preparation of engobes and glazes involves the use of frits in their formulation. As frits are granulated glasses melted at high temperatures and then quenched in water, their production requires high quantities of energy, increasing the environmental impact of engobes and glazes.
 
The study explores different combinations of two different natural raw materials (feldspar and nepheline) and one recycled raw material (recycled granite powder) in the formulation of ceramic engobes and glazes without frits. Seven formulations were statistically selected using the Design of Experiments (DoE) approach, in particular, a simplex-centroid mixture design, with the input variables as varying compositions (0 to 100%) of the three raw materials. The engobes and glazes were prepared using conventional industrial procedures and were deposited over porcelain stoneware tiles and fired at 1205°C. The characteristics of the modified engobes and glazes were investigated by optical microscopy, hot stage microscopy, Thermogravimetric analysis (TGA) Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), and by measuring their roughness, CIELAB colour, and gloss. The role of each raw material (or combination of them) in the development of engobes or topcoats was investigated. The solar reflectance (SR) was also measured to determine whether the engobes and glazes prepared using secondary raw materials without frits can serve as a base material for the development of solar-reflective ceramic products. The ceramic industry is one of the major consumers of non-renewable natural resources, and this study highlights the potential of using effective combinations of primary and secondary raw materials in the development of energy-efficient sustainable ceramic products.