heart

To heart a structure is to fill its interior cavity with rubble, broken stone, or similar coarse material. In masonry walls, this means packing the space between the outer stone or brick faces with whatever loose material is on hand: crushed rock, broken tile, mortar scraps, or clay. The hearted wall thus becomes a composite object: faced on both sides, hollow in the middle, the void made solid and stable by weight and friction alone.

Breakwaters and harbour walls are hearted in the same way. The outer skin is built to specification, carefully laid stone or rubble masonry. The interior is then dumped full of whatever aggregate is cheapest and nearest. The hearting both saves expensive facing material and provides the mass needed to resist water pressure and wave action. A cross-section of such a structure resembles a sandwich: good stone outside, cheap fill within.

Hearting must be done with some care. Material packed too loosely leaves voids where water can pool and freeze, or where it migrates upward by capillary action into the facing stone, causing spalling in winter. Material that is tamped or vibrated too aggressively can shatter to finer pieces and eventually pack into an impermeable mass that traps moisture. The hearting also contributes to the structure's overall weight and stability, so gaps and settling matter to the engineering.

The verb appears in specifications and contracts as a discrete operation: once the facing is up to a certain height or section, the hearting is done before the next course of facing begins. This rhythm of facing, hearting, facing again defined much masonry work before reinforced concrete. The term remains active in restoration and in places where rubble-stone construction is still the norm.

Heart is also used as a noun for the rubble itself, and adjectivally: a hearted wall, hearting stone, the hearting process. It is an economical technique born of scarcity; it persists where labour is cheap and material abundant but unrefined.