In Scottish usage, harling describes an exterior building-surfacing technique which results in a long-lasting weatherproof shield for a stone building. A pigment can be embedded in the harled material, thus obviating the need for repainting. Harling as a technique provides the surface of many Scottish castles, but it is also used for a variety of common everyday building types. Long-lasting and practical, it well suits structures in the Scottish climate.
Harling as a process covers stonework using a plastering process involving a slurry of small pebbles or fine chips of stone. After a wall is complete and has been pointed and allowed to cure then a base of lime render is applied to the bare stone. While this render is still wet a specially shaped trowel is used to throw the pebbles onto the lime surface, which are then lightly pressed into it. Harl, being mostly lime render, cures chemically rather than simply drying. After this setting process, the harl is sometimes lime washed in various colours using traditional techniques.
It is not recommendable to replace more than around 20% of the lime content with cement. Cement-based render is very stiff and prone to crack or detach from the wall when subjected to stress induced by expansion due to solar radiation and moisture. It is also less permeable to moisture and water vapour. Water entering through fine cracks in the surface does not easily diffuse and can penetrate into the softer stone, thus causing the deterioration which harling aims to prevent. For similar reasons modern barrier paints should not be used in place of traditional lime washes.
The technique of harling features in a large number of famous Scottish buildings including: