Heart: The rubble that holds walls together

In everyday speech, the heart is the seat of emotion and courage. A person acts from the heart, or loses heart, or has heart in their work. The metaphor runs deep in English, reaching back through centuries of poetry and prayer. But on a construction site, the word sheds all sentiment. The heart is simply the interior mass of a wall or a breakwater, packed with rubble, shale, or broken stone to bulk out the structure without wasting good ashlar or cut blocks on the middle.

In masonry, a wall is rarely solid from face to face. Instead, builders lay two outer skins of finished stone or brick, carefully selected and fitted, and fill the cavity between them with hearting: rough rubble, offcuts, and whatever stone comes to hand. This saves material and weight while keeping the wall rigid and weather-tight. The hearting must be packed tight, rammed down hard, sometimes wedged with smaller chips. It bears none of the aesthetic burden that the outer faces do. The outer stones are the public face of the building; the heart is hidden and honest.

Heart bond and heartbond are closely related techniques in stone masonry that govern how headers (stones laid perpendicular to the wall face, binding the two skins together) are arranged. In a heart bond, two headers meet in the centre of the wall, their joint running along the middle like a seam, and a third header is laid on top to cover that joint and lock the two halves. This pattern distributes the weight evenly and prevents the inner and outer skins from separating. The term hearting, meanwhile, refers to the loose material that fills the space between those carefully placed headers and outer stones.

The word heart in this context comes from the same root as the everyday sense: the inner part, the core, the centre. But the metaphorical leap stopped there. A mason's heart is not noble or brave; it is inert, structural, and indifferent to what happens outside. The divergence is not one of borrowing from poetry, but rather the meeting point of two uses of the same word: the everyday sense of inner essence, and the builder's need to name the actual interior material of a wall. Rubble is what fills the heart of most old buildings, and the name stuck.