What is porphyry?
Porphyry is an igneous rock recognised by its porphyritic texture: relatively large crystals, visible to the naked eye, set in a much finer groundmass. That texture arises because the magma first cools slowly — allowing the large crystals to grow — and then solidifies quickly in a dyke or close to the surface, leaving the remainder fine-grained.
The name reaches us through the French porphyre from the Greek porphyros, purple: classical Egyptian porphyry was purple-red. In north-west Europe most people know porphyry as the dark grey to purplish stone from the Belgian quarries around Quenast and Lessines; Italian quartz porphyry from Trentino is the classic material for garden paving.
Properties
- Hardness comparable to granite
- Very resistant to wear and impact, and to compression and abrasion
- Frost resistant and well able to withstand chemical attack
- Barely polishable — a working stone, not a material for mirror finishes
- Rough surface, and therefore naturally slip resistant
Applications
- Setts and paving. The classic Belgian sett is porphyry. Because it is both grippy and wear resistant, a porphyry pavement lasts for decades.
- Crushed stone and aggregate. Porphyry chippings are used in high-strength concrete and in asphalt precisely because they do not crush easily.
- Surface courses. In heavily trafficked asphalt, porphyry chippings provide skid resistance that lasts.
- Railway ballast. A ballast bed needs sharp-edged, hard material that will not grind down to dust.
- Armourstone and rip-rap. For embankment protection and bank revetment.
- Garden and terrace. Porphyry slabs and small setts for paths and driveways.
Practical points
- Working it takes time. Porphyry is too hard to snap with an ordinary tile cutter; use a wet saw with a diamond blade.
- Colour variation is part of the deal. Natural stone varies between batches and between beds in the quarry. Mix from several packs while laying, or you will get patches in the finished surface.
- The sub-base decides the outcome. Setts and small paviors need a well-compacted, free-draining sub-base; settlement almost always comes from below, not from the stone.
- Rough stays rough — and holds dirt. The textured surface retains moss and green growth in shaded spots.
Related terms
- Natural stone
- Igneous rock
- Granite
- Sett
- Aggregate
- Basalt
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