Enamel is a decorative technique in which a glass "paste" is applied to the surface of a metal--normally bronze, copper or gold. This glass composition adheres to the metal through fusion under very high temperatures.
Why are some enamels translucent & others opaque?
The color of the enamel and its degree of transparency depend on the metal oxides that exist in the glass and the temperature at which the glass melts and coheres to the surface:
"Harder"=fused at higher temperatures=more durable, more translucent
"Softer"=fused at lower temperatures=more fragile, more opaque
When & where did enamel technique originate?
The precursor to true enamel dates all the way back to the 15th century BCE with the ancient Egyptians, who, to ornament objects, used pieces of cut-glass in decorative patterns embedded in gold; however these substances were not fused together through the use of heat.
The first true enamel, using molten glass, can be seen on Greek gold jewelry that dates from the 4th century BCE. Since then, enamel has been used throughout history, by cultures all over the world, because of its colorful and lustrous qualities.