Why It Pays To Know Your Stone

Walk through any old English village and you can usually tell where the stone came from within a few miles of the church tower. That is the first lesson of historic stonemasonry: for most of building history, stone was quarried as close to the site as a cart and a team of horses could reasonably manage. Before you repoint, clean or replace anything in a period wall or cottage, you need to know what you are actually looking at. Limestone, sandstone and granite behave in completely different ways, and each demands a different approach to mortar, cleaning and repair. Get it wrong and you can do more damage in a weekend than two centuries of weather has managed.

Limestone: Fine Grained, Often Fossiliferous, Always Fizzy

Limestone is the classic stone of the Cotswolds, the Chilterns, Lincolnshire, Derbyshire and the Jurassic belt running from Dorset up to Yorkshire. Colours range from creamy white and pale honey through to grey, buff and the deep blue-grey "hearting" found inside many blocks. The texture is usually fine and even, though you will often see fragments of shell, crinoid stems or tiny fossils scattered through the face, particularly in the older Jurassic and Carboniferous beds.

The single most useful test is acid. A few drops of dilute hydrochloric acid, or even household vinegar on a warm day, will fizz enthusiastically on limestone. Do this in an inconspicuous spot, then rinse with clean water. Related stones such as the magnesian limestone of South and West Yorkshire are dolomitic, with a distinctive yellow-cream colour and a much weaker reaction, but they still fizz enough to tell them apart from sandstone.

Limestone weathers in a distinctive way. Exposed faces dissolve slowly, rounding off sharp arrises and eventually producing the "sugaring" you see on Victorian churches where the surface grains loosen. In towns, soot and gypsum crusts form a hard black skin that traps moisture underneath, which is why gentle poulticing beats aggressive blasting every time.

Sandstone: Gritty, Bedded, Wonderfully Variable

Sandstone is the great regional stone of Britain. The red and pink New Red Sandstone of Devon, Shropshire, Cumbria and Dumfries, the buff and grey-brown millstone grit of the Pennines, the pale Yorkshire and Derbyshire freestones, and the grey Carboniferous sandstones of Northumberland and the Scottish Borders all belong to this broad family.

Run a thumb across a broken face and it feels like coarse sandpaper. With a hand lens you will see individual quartz grains, sometimes sparkling, sometimes cemented with silica, sometimes with calcite or iron oxide. Because it is a sedimentary rock laid down in beds, you will often spot cross-bedding, ripple marks or thin clay seams running through the block. Those bedding planes are also its weakness: sandstone splits along them, and water migrating along a clay-rich seam will eventually blow the face off.

Weathering patterns vary enormously. Hard siliceous sandstones develop a bleached crust and stand proud, while softer calcareous layers erode back, leaving a wall with visible ribs and hollows. If the stone fizzes weakly, it is calcareous; if not, it is siliceous. Both are common, sometimes within the same quarry.

Granite, Whinstone And Slate

Granite turns up in old walls far less often as walling and far more often as kerbs, gate piers, lintels, plinths, steps and boundary markers. It is an igneous rock, crystalline and hard, and it sparkles in sunlight because you are looking at visible quartz, feldspar and mica. Silver-grey Cornish granite, pink Shap granite and the grey granites of Aberdeen and Leicestershire are the usual suspects. It does not fizz. It barely weathers, which means when a granite plinth looks shabby, the problem is almost always the mortar around it rather than the stone itself.

Whinstone or basalt is darker and finer grained, with no visible crystals, and appears in the drystone walls and rubble work of northern England and Scotland. Slate is finer again and splits into thin sheets, used for copings, sills and hanging. Both are hard and unforgiving to tool, but excellent in exposed positions.

Practical Field Tests And Clues In The Wall

• Fizz test: dilute acid or vinegar. Strong reaction, limestone. Weak or patchy, calcareous sandstone or dolomitic limestone. No reaction, sandstone, granite, whinstone or slate.
• Grain and feel: smooth and fine, think limestone or slate. Gritty and abrasive, sandstone. Crystalline and sparkling, granite.
• Tooling marks: diagonal chisel or drag marks on a smooth face usually mean limestone; broad punch or boaster marks suit sandstone.
• Location: a wall within ten miles of a known quarry is very likely built from it, whatever the geology map says.
• Mortar clues: lime putty or natural hydraulic lime with a gritty local sand points to pre-1900 work. Hard grey cement is a twentieth-century intrusion and should be removed with care.

Matching Repairs To The Original

Once identified, the rules are fairly consistent. Never use cement-based mortar on limestone, sandstone or old whinstone rubble; it is harder, less breathable and traps moisture, forcing decay into the stone. Use a lime mortar matched to the original in strength, colour and aggregate size, and point it to the same profile, whether flush, struck or recessed by three to five millimetres.

For replacement stone, aim for a close match in porosity and hardness rather than colour alone. A new block that is much harder than its neighbours will push the load onto softer stones and cause spalling. Newly quarried stone also weathers to a different tone, so a slightly paler or greener match will usually settle in better than one chosen on the day.

Take a small sample to a stone merchant or your local geological society if you are unsure. It is a five-minute job that can save you a great deal of heartache.