Egyptian faience (also known as Egyptian paste) is the oldest known glazed ceramic. It was first developed more than 6000 years ago in Mesopotamia, Egypt and elsewhere in the ancient world. It is known for its bright colors, especially shades of turquoise, blue and green, and it can vary widely in appearance, from glossy and translucent to matt and opaque. Because it is composed mainly of silica (sand or crushed quartz), along with small amounts of sodium and calcium, faience is considered a non-clay or siliceous ceramic. It is a precursor to glazed clay-based ceramics, such as earthenware and stoneware, and also to glass, which was invented around 2500 BC.
Egyptian faience is a self-glazing ceramic: salts in the wet paste come to the surface as it dries and develop a glaze when it is fired in the kiln. This is called efflorescence glazing. Metal oxides in the paste color the glaze. Two common colorants are copper (turquoise) and cobalt (blue). Faience can also be created by placing small items such as beads in a container full of glazing powder (cementation glazing) or by painting on a glaze (application glazing). More than one glazing method may be used on a single piece. I make my own faience bodies and glazes, and I fire my faience pieces one or more times, usually using the efflorescence and application glazing techniques.
The word faience comes from the Italian town of Faenza which is famous for its pottery. This pottery is a red earthenware covered in a white glaze and decorated with colorful designs. It's not known for certain how the ancient ceramic came to be called Egyptian faience, but there are some leading theories. One is that when Europeans came to Egypt in the 19th century, they thought the brightly colored scarabs and other small faience items looked like the pottery of Faenza and so they called these items faience. Another is that at that time the term faience was sometimes used as a generic word for any kind of glazed pottery. Whatever the origin, the word faience has caused much discussion for more than a century and there still is not full agreement on its use. Because Egyptian faience is very different from earthenware pottery, some potters prefer to call it Egyptian paste. Archaeologists and other related professionals refer to it as Egyptian faience or ancient faience or even just faience.
Here is a small faience dish that has dried in the air but has not yet been fired in the kiln (on the edge of the pot at the bottom of the photo there is a fuzzy white layer visible; this is the effloresced sodium that will become a layer of glaze when the pot is fired):
