browning

Browning is the intermediate coat of mortar applied to a wall or ceiling during plastering, sandwiched between the initial scratch coat and the final finishing coat. It is typically composed of lime mortar, sand, and sometimes cement, mixed to a consistency thicker than finish plaster but workable enough to be troweled smooth. The coat is usually 8 to 12 millimeters thick and serves both structural and preparatory functions in the three-coat plaster system that dominated interior and exterior walls before the widespread adoption of drywall.

The name derives from the color of the base mortar mix, which appears brown or tan compared to the white or near-white finish coat that follows. Browning gets its body from sharp sand, which provides mechanical key for adhesion, whereas finish plaster uses fine sand for a smooth appearance. On masonry substrates, browning bonds to the scratch coat and creates a level base; on lath (wooden strips), it fills the spaces between struts and develops mechanical interlock as mortar squeezes through and around the mesh.

Application and variants

Browning is applied using a wooden float or steel trowel, working it firmly into the scratch coat to eliminate voids and ensure good adhesion. The coat must be firm enough to support the finish plaster, but left slightly rough or keyed (sometimes lightly scratched with a broom or comb before it sets fully) to give the finish coat something to grip. On external walls exposed to weather, browning formulations sometimes included higher cement content for faster set and better water resistance. Internal work typically relied on slower-setting lime-based mixes.

Common problems occur when browning is applied too thickly, dries too quickly, or is laid over a poorly prepared scratch coat with inadequate mechanical key. If the browning shrinks differentially or is too strong (high cement content) relative to softer underlying coats, cracks propagate through the finish. Poor troweling technique leaves the surface too smooth, reducing mechanical grip for finish plaster, or too rough, creating an uneven base that requires excessive finish thickness to conceal.

In modern practice, browning has been largely superseded by plasterboard and skim coating, which compress the work into fewer stages and use materials with more consistent properties. However, browning remains essential in heritage work, lime-plastering, and projects where breathable finishes are required. Understanding browning proportions, drying times, and surface preparation is still necessary for plasterers working on older buildings or specialized sustainable construction methods.