It could have been so simple...but it wasn't! Until recent times (about 1800), permanent enamels had to be fired on glass vessels by an amazingly laborious process, shown here. Today, the process really is as easy as it looks.
A vitreous substance made of finely powdered glass colored with metallic oxide and suspended in an oily medium for ease of application with a brush. The medium burns away during firing in a low-temperature muffle kiln (about 965°-1300°F or 500°-700°C). Sometimes, several firings are required to fuse the different colors of an elaborately enameled object.