Porcelain signs are thin sheets of metal coated on all sides with a durable glass, bonded at a high temperature. These signs were then coated with enamel to further protect them. They are still popular and collectible today. Modern signs, and most signs manufactured after the 1950s, are made of stamped metal that have been painted. Some stamped signs can be very good reproductions of the porcelain signs, but there are some ways to determine if the sign is porcelain enamel-coated or painted metal.
Inspect the thickness of the sign. Porcelain signs have been bonded to sheet metal, so signs with porcelain enamel are typically thicker than painted metal signs.
Look closely at the sign, especially the edges. Some porcelain signs show chips and other wear in the porcelain enamel, and the metal may be exposed. If you see chipping on the sign, it is most likely porcelain.
Check the sign for rust. All over rust is a sign that the advertisement is most likely painted metal. Porcelain signs may show rust in areas where the porcelain is chipped or broken away.
Flex the sign by lifting it and bending it slightly. Porcelain signs seem rather inflexable for their shape and thickness. Pressed metal signs will flex and bow.
Rap gently against the sign with a small metal object. Compare the sound to the sound of a known metal or porcelain sign. Which does it sound more like? Metal signs tend to ring more than porcelain.