Harl
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Harling is a Scottish term describing an exterior building surfacing technique. The theory of harling is to produce a long lasting weatherproof shield for a stone building. The pigment is embedded in the harled material, thus obviating the need for repainting. Harling is a technique used to surface many of the Scottish castles, but it is also used for a variety of common building types. The long term life cycle economics of harling are compelling to the resourceful Scots.
Harling is a process of covering stonework, using a plastering process involding a slurry of small pebbles or fine chips of stone. Initially a base of lime render is applied to the bare stone wall (which should first be correctly pointed between joints), and whilst still wet a specially shaped trowel is used to throw the pebbles onto the lime surface, then lightly pressed into it. After setting, the harl may be lime washed in various colours using traditional paints (not modern barrier paints). Cement base renders are also incorrect to proper harling, as these form an impermeable barrier to moisture - when the inevitable cracking occurs during cold weather, water is trapped behind the surface and penetrates inside the softer stone, thus causing rapid deterioration.
The technique of harling is utilized in a large number of famous Scottish buildings including Crathes Castle, Craigievar Castle, Muchalls Castle and Myers Castle
