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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.
Cement-base render cannot replace the lime, as it forms an impermeable barrier to moisture. (The traditional harling process allows water to escape.) This means that if cracking occurs the water becomes trapped behind the surface and often penetrates inside the softer stone, thus causing the rapid 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:
- Crathes Castle in Aberdeenshire (16th century)
- Craigievar Castle in Aberdeenshire (17th century)
- Muchalls Castle in Aberdeenshire
- Myers Castle in Fife (1530 onwards)
- Stirling Castle in Stirling