The common brick is one of humans' greatest innovation. it is, after all, a man-made stone. "Brick-making transforms low-strength mud into strong materials that can endure for centuries when properly cared for." There are two different types of bricks; clay and the fired brick.
Clay bricks are primary clay, of course. Clay is a group of surface minerals that arise from the weathering of igneous rocks. One it's own, clay isn't particularly useful. But when formed into bricks that are then baked in the sun, they become a wonderful hand-made stone that can be used to building. having a little sand in the clay mixture helps to keep the brick from cracking. "Sun-dried clay is little different than soft shale." Many of the ancient building in the Middle East were made of sun-dried bricks. These buildings generally lasted about a generation before they began crumbling from neglect, earthquakes, and/or weather. "With old buildings melted into piles of clay, the ancient cities were periodically leveled and new cities built on top. Over the centuries these city mounds, called tells, grew to considerable size." When straw of animal dung is added to the clay before the bricks are made, it yields a product that is equivalent to adobe.
Then came along the fired brick. "The ancient Persians and Assyrians made stronger bricks by roasting them in kilns. The process takes several days, raising the temperature above 1000 °C for a day or so, then cooling gradually." As they did with concrete and metallurgy, The Romans advanced the technology of brick-making. The idea of firing the bricks spread to every part of their vast empire.
When the brick was fired, the clay literally became a metamorphic rock. "Clay minerals break down, release chemically bound water, and change into a mixture of two minerals, quartz, and mullite. The quartz crystallizes very little in that time, remaining in a glassy state." The main mineral is mullite, a blended compound of silica and alumina that is rare in nature. Mullite gets its name because it is found most often on the Isle of Mull in Scotland. In addition to being hard and tough, mullite can grow in long thin crystals that function like straw in an adobe mixture, which binds the mix in an interlocking grip. "Iron is a lesser ingredient that oxidizes into hematite, accounting for the red color of most bricks. Other elements including sodium, calcium, and potassium help the silica melt more easily—that is, they act as a flux. All of these are natural parts of many clay deposits."
So are there any natural bricks? The short answer is perhaps. Earlier in the blog, I wrote that natural soft shale is similar to bricks. If an actual fire could bake the right kind of sandy shale, we could come pretty close. In fact, that does happen in coal country. Forest fires can start coal beds burning, and once started these coal-seam fires may go on for centuries. Sure enough, shale overlying coal fires can turn into a red clinkery rock that's close enough to true brick. The big downside though, are the forest fires needed to make the bricks occur naturally. In my humble opinion, is if far easier and way safer to have bricks made in a drying kiln.
I hope you've enjoyed learning a little bit about the humble brick. Think about this the next time you look at a brick building or a brick chimney. What I find interesting is that even the most basic (and boring-appearing) item often has an incredible story to tell.
Please come back tomorrow for another valuable life lesson from my friend The Dharma Frog.