Bone ash as alternative fluxing agent in the ceramic sanitary-ware production

. The partial substitution of feldspar fluxing agent with bone ash into the traditional ceramic sanitary-ware formulation was investigated. Technological properties (rheology, water absorption, fired shrinkage, thermal expansion and flexural strength) as well as crystallographic insights and microstructural details were evaluated. The study was divided into two series of body formulations. The first revealed that the substitution of feldspar (2.5 and 5 wt.%) with bone ash is not recommended, due to the higher level of fusibility achieved by the bone ash-containing formulations, especially at the typical process temperatures (i.e. higher than 1200°C), where pore development and sagging were observed in the fired body. Conversely, the second series demonstrated that, if the initial body composition is tuned to become more refractory by replacing an additional amount of feldspar (∼ 10 wt.% total replacement) with quartz, good technological results can be reached with a small amount of bone ash (i.e. 2.5 wt.%) in the formulation. Calcium from bone ash was involved in the formation of a newly formed Ca-rich plagioclase while phosphorus from bone ash promoted the formation of β-tricalcium phosphate before being fully dissolved in the aluminosilicate melt. These results are of considerable importance in identifying alternative fluxing minerals to feldspar, which was recently included in the European Union list of critical raw materials.