What is trachyte?
Trachyte is an extrusive volcanic rock: lava that solidified at or just below the earth’s surface. It consists mainly of alkali feldspar (especially sanidine) and contains little or no quartz. Typical are the large, tabular feldspar crystals that show as glinting specks in a fine-grained, light grey to yellowish grey groundmass. The name comes from the Greek trachys, “rough”, because the surface feels rough.
Origin and use
For building history in the Rhineland and the Low Countries, the most important source is the Siebengebirge near Bonn in Germany, with the Drachenfels as the best-known quarry. In the Middle Ages the stone was shipped along the Rhine and used for churches and other monumental buildings. The medieval part of Cologne Cathedral is largely built of Drachenfels trachyte. Trachyte is also found in medieval churches in the Netherlands, often next to tuff and other Rhenish stones.
Trachyte was used for:
- Facing and masonry of churches and towers.
- Architectural sculpture, cornices and ornament.
- Columns, plinths and steps.
Properties
- Relatively light and workable for an igneous rock.
- Porous, and therefore sensitive to moisture, frost and air pollution.
- When it weathers, the large crystals can work loose, leaving small pits and a rough, crumbling surface.
Maintenance and restoration advice
- Always have damage to trachyte assessed by a stone conservator. For listed buildings, consult the heritage authority.
- Replacement stone must match in colour, porosity and hardness. A stone that is too hard or dense next to old trachyte can make the surrounding stone weather faster.
- Use a mortar that is softer than the stone (such as a lime mortar), so that any cracking happens in the joint and not in the stone.
- Clean gently: no pressure washing or aggressive acids, which damage the surface.
Related terms
- Basalt
- Porphyry
- Natural stone maintenance
- Natural stone replacement
- Limestone
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