The invention provides a powder comprising at least 95% by number of fused grains, with the following chemical composition, as a percentage by weight based on the oxides, for a total of 100%: 0≦BaO≦40.8%; 0≦SrO≦31.8%; 27.2%≦Al2O3≦31.3%; 32%≦SiO2≦36.9%; other species ≦1%; the quantity of at least one of the oxides BaO and SrO being more than 0.3%, the size of said grains being such that D5≧5 μm and D95≦150 μm.