brick
A brick is the humble red block that has built cities for six thousand years: a lump of shaped clay, dried, and fired hard in a kiln until it becomes a durable stone-like solid you can stack into walls. It is the original mass-produced engineering material — cheap, made from dirt that is almost everywhere, sized to fit one human hand, and so weather-proof that Roman and medieval brickwork still stands. Structural clay products are the same idea scaled up and out: bricks, hollow blocks, and roof tiles, the workhorses of everyday construction.
The making is the whole of ceramics in miniature. Common brick clay is not clean kaolin but a cheap, iron-rich, sandy clay — the iron is why fired brick is red (iron oxide, Fe2O3). The wet clay is extruded as a long bar and wire-cut into blocks, or pressed in moulds, then carefully dried to drive off water without cracking. Firing to roughly 900 to 1100 degrees C does two things: it burns away organics and drives off chemically bound water, and it partially vitrifies the clay so a little glass forms at particle contacts and cements the mass together. The brick does not fully melt or fully densify — it stays somewhat porous on purpose, which keeps it light, gives mortar and plaster something to grip, and lets a wall breathe. Underfire and it is weak and crumbly; overfire and it warps, bloats, and fuses shut.
Because brick is a fired ceramic it inherits ceramic strengths and weaknesses: it is excellent in compression (a wall carries enormous downward load), fire-resistant, and chemically durable for centuries, but weak in tension and bending, which is why brick walls need mortar and, in modern building, steel or reinforced concrete to handle pulling and sideways loads. Its deliberate porosity is a feature, not a defect — it makes the brick a modest thermal insulator and lets it absorb and release moisture. Note that a common structural brick is a very different animal from a refractory brick: ordinary brick would soften and slump long before it reached a furnace's working temperature.
A standard red building brick is extruded from iron-rich clay, wire-cut, dried, and fired near 1000 degrees C; it comes out around 15 to 25 percent porous, red from Fe2O3, and able to carry many tonnes per square metre in compression while a single brick still snaps easily if you try to bend it — the everyday face of strong-in-compression, weak-in-tension.
Shape wet clay, dry, and fire to partial vitrification: a cheap, porous, compression-strong building ceramic — but not a furnace refractory.
Ordinary building brick and refractory brick are not the same product. Common brick uses cheap iron-rich clay and would soften in a furnace; refractory brick uses high-alumina or other refractory raw materials to survive 1400 degrees C and beyond.