Having never been a potter myself, I could never understand why archeologists seemed to assume that some tribe or region made the same kind of pottery over and over. They name prehistoric cultures that way: the Grey Ware culture, the Black Stripe Ware culture, etc. When the pottery is found in a more recent time, in a different place, they assume the people moved (well—they used to assume that, but that’s a different topic). I always wondered why they ruled out that maybe people got tired of making round pots with black stripes and felt like making something different.
Then I learned a bit about the history of pottery. It’s not at all like what I imagined, in which the clay pot is decorated with colored paint and then baked. Making and firing pottery is a chemistry experiment and if it’s not done right, it fails. Worse, what it looks like before it’s fired in a kiln is often very different from how it looks after. The intensity of the kiln’s heat (which is based on the furnace design, another engineering experiment) interacts with minerals in the clay and in whatever glaze and paint have been applied. What’s applied clear may turn blue or red. Kiln chemistry is not at all an obvious art. So it really is plausible that for centuries, potters in one place had no way to innovate without creating useless items.
Every now and then, there’s a breakthrough discovery. In Abbasid Iraq, in the 9th and 10th centuries, they had one of those periods.
Invaders and merchants going to China had brought back kaolin pottery that was pure white. It was painted in mysterious blue and other colors, on top of white. Kaolin’s mineral composition was a Chinese local secret, so they had a monopoly and kept prices high. If potters in Basra and Baghdad could figure out how to make white pottery, they could secure the profits and ruin the Chinese. The only known glaze at the time was made of powdered lead; it was widely used in Roman-settlement lands, and it turned yellow or green.
Now Persian/Arabic potters experimented with glaze made from ground tin. Fired in a medium-hot kiln, it turned white. Further experiments into the 11th century gave them a way to decorate it. When the pottery was unglazed and unfired, it could be painted with cobalt or copper; then when it was tin-glazed and fired, it came out with blue or green designs. It still wasn’t Chinese porcelain, but it was a pretty good substitute.
Baghdad’s methods with tin, cobalt and copper spread to other points in the Muslim empire: Damascus, Alexandria, Tunis and points east like Samarkand. Further experimentation in the medieval Muslim world showed how to fire tin-glazed pottery a second time, this time with silver and copper oxides. It came out with a high gloss and is often called lusterware.
They also found that some mineral-rich clays could be mixed with water (the mud was called slip) and painted onto unglazed pottery, which then received a clear glaze. In firing, the minerals turned red, brown and black. Later, they sometimes used a blue glaze over slip painting.
Although there was a Koranic prohibition against depicting the human form, this was not always enforced. The absolute safest decoration was to paint slip, with a fine brush, in elaborate Koranic inscriptions around the dish. The second safest was to stick with geometric designs, and then 3rd, leaves and flowers. But there’s pottery from medieval Iraq and Egypt with animals and human figures shown in scenes that tell stories. The painting can be crude in an abstract/modern way, but it can also be very detailed.