Making bricks from clay involves four core stages: preparing the raw material, shaping it into uniform blocks, drying them slowly, and firing them at high heat to permanently harden them. The process is surprisingly accessible for small-scale projects, though each stage has details that determine whether you end up with durable bricks or a pile of crumbled clay.
Choosing and Testing Your Clay
Not all clay works equally well for bricks. The clay you dig from your yard or a riverbank is a mix of fine mineral particles, sand, silt, and organic matter. The mineral content matters most. Clays rich in certain silicate minerals (the kind that feel slippery and hold shape when wet) bind tightly during firing. Clays with a high proportion of quartz or sand already in them will be grittier and less plastic, meaning they won’t mold as easily but also won’t shrink as much when drying.
To test whether your clay is suitable, grab a handful, wet it, and roll it into a coil about the thickness of a pencil. Wrap the coil around your finger. If it bends without cracking apart, the clay has enough plasticity to work with. If it crumbles immediately, it has too much sand or silt and won’t hold together in a mold. If it bends perfectly with zero cracking, it’s extremely plastic and will likely shrink too much during drying, which means you’ll need to add sand.
Preparing the Mix
Raw clay almost always needs modification before you shape it. The goal is a mix that’s plastic enough to hold its shape but contains enough non-plastic material (sand or grit) to control shrinkage and prevent cracking as the bricks dry and fire.
A good starting ratio is roughly 40% clay to 60% sand by weight. Research on clay and sand mixes finds that a ratio in the range of 0.43 to 0.66 parts clay to sand by weight produces the best balance of compressive strength and minimal shrinkage. If your clay already contains a lot of natural sand (you can feel the grit when you rub it between your fingers), you’ll need less added sand. Pure, smooth pottery-type clay needs more.
For unfired or sun-dried bricks (sometimes called adobe), adding chopped straw or grass fibers helps reinforce the block the same way rebar reinforces concrete. A handful of straw per brick is a rough guide. For bricks you plan to fire in a kiln, skip the straw since it will simply burn away during firing.
Preparation starts by breaking up any dry clay chunks, soaking them in water for a day or two, and then mixing thoroughly with your sand. You want a thick, uniform consistency similar to stiff cookie dough. Remove any stones, roots, or debris. The more consistent the mix, the more uniform your bricks will be.
Shaping the Bricks
The simplest method is hand-molding with a wooden form. Build a bottomless rectangular mold from scrap wood. Standard modular brick dimensions are roughly 92 x 57 x 194 mm (about 3⅝ x 2¼ x 7⅝ inches), but you can size your mold however you like. Make it slightly larger than your target size to account for shrinkage.
Place the mold on a flat surface dusted with sand or fine ash to prevent sticking. Pack the clay mix firmly into the mold, pressing it into all corners and striking off the top with a straight edge to create a flat surface. Lift the mold straight up, leaving the formed brick behind. This is essentially the soft-mud process used by brickmakers for centuries. It produces bricks with a slightly rough, handmade texture. Wetting or sanding the inside of the mold before each use prevents the clay from bonding to the wood.
Work on a level surface, and space your freshly molded bricks a few inches apart so air can circulate. You can produce dozens of bricks per hour once you find a rhythm.
Drying Before Firing
This is where most beginners lose bricks to cracking. Fresh clay bricks contain a significant amount of water, and if that water escapes too quickly or unevenly, the brick tears itself apart from the inside out. Drying happens in two distinct phases: first the brick shrinks as water leaves the spaces between clay particles, then the remaining moisture evaporates without further size change.
Volumetric shrinkage during drying typically falls in the range of 5% to 7.5%, depending on the clay type and how much water was in the original mix. That means a brick that starts at 8 inches long might lose nearly half an inch. This shrinkage is normal, but it must happen slowly and evenly.
Place your bricks in a shaded, sheltered area with gentle airflow. Direct sun or wind on one side will dry that face faster than the rest, almost guaranteeing cracks. After two or three days, turn the bricks on their sides to expose the bottom. Total drying time depends on your climate, brick thickness, and humidity, but plan on one to three weeks for bricks to dry completely at ambient outdoor temperatures. The bricks should feel lighter, sound hollow-ish when tapped, and show no dark damp spots before they’re ready for firing. Rushing this step is the single most common cause of failure.
Firing the Bricks
Firing transforms soft, water-soluble clay into a hard, permanent building material. The heat triggers a series of chemical changes that cannot be reversed, which is why a fired brick doesn’t dissolve in rain the way a dried mud brick eventually will.
Here’s what happens inside the brick as temperature rises:
- 25°C to 200°C: Any remaining moisture, both surface water and water trapped between particles, evaporates. The brick must pass through this stage slowly or steam pressure will crack it.
- 300°C to 400°C: Organic matter in the clay burns off. The brick may darken temporarily during this phase.
- 400°C to 700°C: Clay minerals begin to decompose and release chemically bound water from within their crystal structures. This is a critical transformation where the mineral structure starts breaking down permanently.
- 900°C to 1050°C: Vitrification begins. The minerals partially melt and fuse together, filling pore spaces and creating a dense, glass-like bond. This is what gives a fired brick its strength and water resistance. Higher temperatures within this range produce harder, denser bricks.
For a small-scale project, you can build a simple clamp kiln: stack your dried bricks in a compact formation with small gaps for airflow, leaving channels at the base for fuel (wood or charcoal). Encase the stack with a layer of already-fired bricks or packed mud, leaving vents at the top. Light fires in the base channels and gradually increase heat over 24 to 48 hours. The key word is gradually. Jumping from cold to full heat will crack every brick in the stack.
Hold peak temperature (aim for the 900°C to 1000°C range, where the bricks glow orange to yellow-orange) for several hours, then let the kiln cool completely on its own. Don’t open it or pull bricks out while it’s hot. Cooling takes as long as heating did, sometimes longer. The entire firing cycle from first flame to pulling cool bricks often takes three to five days.
How to Tell if Your Bricks Turned Out
A well-fired brick has a consistent color throughout. If you break one and the center is darker or a different color than the outside, it wasn’t fired long enough or hot enough at peak temperature. Good bricks ring with a clear, metallic sound when tapped together. A dull thud suggests underfiring.
The color of finished bricks depends on the mineral content of your clay, not the firing process alone. Iron-rich clays produce the classic red and orange tones. Clays with less iron and more calcium or aluminum fire to buff, cream, or yellowish colors. You won’t know your clay’s firing color until you test it.
Expect some loss. Even experienced traditional brickmakers lose 10% to 20% of a batch to cracking, underfiring on the outside of the stack, or overfiring closest to the fuel channels. Your first batch will teach you more than any guide can. Start with a small test run of 20 to 50 bricks, adjust your clay mix and firing schedule based on results, and scale up from there.