Spectrum 500 Underglaze
These underglaze pens have a frit base rather than a slip (clay) base for Spectrum's underglazes. The advantages to the user are that these underglazes can be applied on greenware or bisque and they can be used with or without a clear glaze covering. When used without a clear glaze cover they have a matte velvet like surface. However, when used without a clear glaze cover they are not considered dinnerware safe because the rough surface can be difficult to clean and can encourage bacteria growth.