What the Kiln Decided

Heat is not even in a pile

A temporary kiln is built out of the green brick it is going to fire. The bricks are stacked with tunnels through the base, the outside of the stack is covered over with clay to hold the heat, and fires are lit and fed for a week or more.

Nothing about that arrangement distributes heat evenly. Brick close to the fire tunnels reaches a much higher temperature than brick at the outer skin of the stack, and the difference is not marginal: it is the difference between a body that has begun to vitrify and one that has barely sintered.

Three products, one burn

• Overburned. Hardest, darkest, sometimes warped, sometimes fused together in lumps. Very strong, very low absorption, awkward to lay because the sizes have gone. Used in footings, in paving, in wet positions, anywhere strength beat appearance.
• Well burned. The intended product. Even in size, sound, moderate absorption. This is the brick a wall is built of.
• Underburned. Pale, pink, salmon colored, soft. Absorbent, weak, and vulnerable to frost. Not waste: it went into backing courses, inner leaves and rear walls, where it is still working a century later.

A yard sorted the burn by hand into those grades, and the sorting was a real skill. It is also why a single yard could offer a range of prices without changing anything about its clay.

What color records

Color in fired clay comes from two things: what is in the clay, and how hot and how oxidizing the fire was.

Iron compounds are the main coloring agent, and in an oxidizing burn they deepen from pale through red to dark plum as the temperature rises. So within one clay, color tracks temperature closely enough to be useful. A salmon brick and a dark brick from the same yard are the same material at different doses of fire.

Across different clays it tracks nothing at all. A pale brick may be a cool burn, or it may be a clay full of lime that fires light no matter how hard it is pushed. That second case has its own entry: cream brick from the lakeshore counties.

So: color compares bricks from one source. It does not compare sources. That is the whole rule, and most confident statements about old brickwork break it.

What the kiln cannot undo

A clay with lime nodules in it will spall in service no matter how well it is fired, because the nodules slake and expand after the brick is in a wall. A clay with too much soluble salt will bloom white on the face. A brick dried too fast before firing will carry cracks that firing sets permanently.

The burn is powerful and it is not a correction. This is why the better yards spent effort on the clay before the kiln: screening, weathering it over a winter, tempering it properly. Those are unglamorous operations and they decide more about a brick's life than the fire does.

Why an even wall means a permanent kiln

A permanent kiln, fired continuously with the chambers drawn in sequence, holds a far steadier heat than a stack. Its product is consistent in hardness, size and color, which is exactly what a facing material has to be.

So a wall of closely matched brick with a thin joint is not primarily evidence of a good bricklayer. It is evidence of a works with a permanent kiln, a freight connection, and a catalogue. The bricklayer's skill is real and it is the second thing you are looking at, not the first.