I'm asking this for a friend who is not on the computer. She makes her own terra sig out of Red Art and her pottery is made using Highwater Clay's Stans Red clay. The sig buffs up beautifully if applied straight to the Stans Red on bone dry pieces. She recently started covering her leather hard pieces with a white slip (pieces are majolica decorated), then, after bisquing, glazes with her regular base glaze before applying the majolica colors.
In her last session of making pieces, she wanted the bases of her mugs and vases to have a black terra sig instead of the red sig so she added black stain to it. It was applied on the bottoms of bone dry pieces that also had had slip applied. The color came out a beautiful black, but no matter how many layers, or how hard or long it was buffed, it would not form a sheen. On the final fired pieces the black terra sig is a very matt black. Is this because it is being applied over slip? Aside from adding the black stain, the addition of the slip is the only difference.