Final answer:
The red clay used in ancient mortar came from the earth's crust, where natural processes created clay deposits rich in iron oxide, giving the clay its red color. Rivers near civilizations exposed these clay layers, which were then easy to access and use in bricks and pottery.
Step-by-step explanation:
Where did the red clay used as mortar come from? The red clay, commonly used as mortar and in the production of bricks, can be traced to various deposits throughout history. Ranging from Mesopotamia around 7500 BCE to the advanced civilizations that localized their resources like the Babylonians and the Romans. The red clay originated from the earth's crust which over millions of years has been reshaped and modified by natural processes, creating a layer of topsoil rich in minerals, including deposits of rocks and clay.
Rivers would expose these layers, making them accessible for early civilizations. The distinct red color, often seen in ancient mortar and pottery, is a result of iron oxide present in the clay. This red color can also be attributed to hematite, which was used for creating red pigments in ancient murals. More sophisticated brick-making and kiln technology emerged in some cultures, resulting in stronger and more uniform building materials, marked by the presence of kilns and potter's wheels which were a testament to the technological advancements in working with red clay.