Blue decoration on a white GROUND was the most common tin-glaze combination, partly because the blue-and-white export porcelains were so popular, but also as a matter of practicality. Tin-glazed earthenware colors were limited by the firing techniques of the time. Artisans could use only those colors that would stand up under the high temperatures (1000 degrees Fahrenheit) needed to set white tin oxide glaze. Blue UNDERGLAZE was the easiest to handle, but red, purple, yellow, and green could also be used. More OVERGLAZE colors could be added in a second, lower-temperature KILN firing, but this made the pottery more expensive.
Tin-glaze is composed of fine sand (silica), calcined lead, and tin oxide (ash). It is the tin oxides that make the glaze OPAQUE and give it its soft, glowing quality. Since tin glazes do not become highly fluid when fired in the kiln, decoration can be applied without running or smudging. The vessel was first dipped in, or covered with, the tin glaze and allowed to dry. Then blue decoration was painted onto the dry, almost powdery surface of the tin-glaze. Because the dry glaze was very absorbent, this required a quick but sure hand, which resulted in the fresh, spirited style seen in this barber's bowl.