Cleaning Each Stone Type: Granite, Marble, Limestone, Slate & York Stone

In short: There is no single right way to clean a headstone — the safe method depends entirely on the stone. Hard stones like granite and slate tolerate careful brushing and water, while soft, porous stones like marble, limestone and sandstone need gentle, acid-free biological cleaning only. Acid and pressure washing must never be used on soft stone: they cause permanent, irreversible damage.

A memorial is one of the most personal things a family ever leaves behind, and the impulse to restore it to its former dignity is a loving one. But the kindest thing you can do for the stone is to understand what it is made of before you touch it. The same method that brightens a granite kerb can quietly destroy a Victorian marble angel. This guide walks through each common memorial stone found across London’s cemeteries, how it weathers in our damp, polluted air, and the correct, safe way to care for it.

How do I identify my stone?

Before cleaning, take a moment to work out what you are looking at. A few simple checks usually settle it.

• Granite feels cold, hard and glassy. Polished granite has a mirror-like shine and shows tiny flecks of black, grey, pink or speckled crystals. It is very heavy and difficult to scratch.
• Marble is smooth and slightly waxy or sugary to the touch, usually white, cream or pale grey, often with faint veining. It is far softer than granite and you may see rounded, worn edges on older pieces.
• Limestone and sandstone look matt and grainy. Sandstone often shows visible sand grains and warm buff, yellow or pinkish tones; limestone is paler and finer. Both feel comparatively soft and may shed grains if rubbed.
• Slate is dark grey to near-black, smooth, fine-grained and splits into flat layers. Lettering is typically crisply incised, sometimes painted or gilded.
• York stone is a hard, durable buff-grey sandstone, common on traditional and older memorials.

If you are genuinely unsure, stop and ask. Treating a soft stone as if it were granite is the single most common cause of accidental damage. The full step-by-step approach is set out in our guide on how to clean a headstone safely.

Granite

Granite is the most common modern memorial stone, and for good reason: it is hard, dense, igneous rock that resists weathering and rarely stains deeply. It comes in two main finishes, and they behave a little differently.

Polished granite has a sealed, glassy face that sheds water and dirt. In London’s air it tends to collect surface grime, traffic film and a thin biological haze rather than deep staining. A soft brush, clean water and patience restore most polished granite beautifully. Work gently across the polished face so you do not leave fine scratches in the shine.

Rough or honed granite — flame-textured, pitched or sandblasted surfaces — has more nooks for algae and dirt to settle into, so it benefits from a soft-bristled brush worked into the texture, again with plain water and, where needed, a stone-safe biocidal treatment. Granite tolerates careful cleaning well, but it still deserves respect: never reach for wire brushes, abrasive pads or harsh chemicals, which can dull the surface or damage lettering and gilding.

Marble

Marble is soft, porous and quietly beautiful — and the most easily ruined of all memorial stones. It is metamorphosed limestone, made largely of calcium carbonate, which means it reacts chemically with acids. In London’s damp, sulphur-laden air, white marble slowly yellows and greys, grows lichen and algae, and develops a dulled, weathered skin.

The single most important rule is this: marble must never be cleaned with acid or with pressure washing. Acidic cleaners — including many off-the-shelf “stone” and patio products, and even vinegar or lemon — etch and dissolve the surface, leaving it pitted and permanently dull. Pressure washing blasts away the soft outer layer and erodes fine carved detail. Both forms of damage are irreversible. The only safe route is gentle, acid-free biological cleaning with soft brushes, plenty of clean water and time, allowing a stone-safe treatment to do the slow work rather than force.

This matters especially for Greek Orthodox memorials, which are typically carved from white marble and central to family remembrance. Their bright white surfaces show grime and biological growth keenly, which tempts people towards aggressive cleaning — exactly the thing that harms them most.

Limestone & sandstone

Limestone and sandstone are sedimentary stones, porous and absorbent, and they need the gentlest handling of all. Across London’s historic cemeteries — places like Highgate, Abney Park and Nunhead — you will find them on countless Victorian memorials, and many show the two classic forms of decay.

The first is “sugaring”: the surface crumbles into a loose, sugary, granular layer as the stone breaks down. The second is black crust, a hard, dark, sooty skin that forms where pollution reacts with the stone, particularly on sheltered faces. It is tempting to scrub black crust off, but on a sugaring or fragile stone, scrubbing simply takes the carved surface — and the inscription — away with it.

Soft sedimentary stone often needs conservation-minded care rather than ordinary cleaning. Sometimes the right and most respectful decision is to clean very lightly, or not at all, and to consolidate or stabilise the stone instead. If a limestone or sandstone memorial is flaking, sugaring or losing its lettering, gentle assessment by someone experienced is far safer than well-meant effort with a brush.

Slate

Slate is hard-wearing, elegant and one of the most weather-resistant memorial stones. Its dense, fine grain shrugs off the damp and resists biological growth better than the soft stones, which is why old slate headstones often survive in remarkably good condition.

The vulnerability lies not in the body of the stone but in its lettering. Slate inscriptions are finely incised and frequently painted or gilded, and that paint and gilding are delicate. Over-enthusiastic brushing, or any abrasive, can lift the lettering and rob the memorial of its legibility. Clean the face gently with a soft brush and water, work carefully around the inscription, and never use anything abrasive that could disturb the painted or gilded detail.

York stone

York stone is a hard, durable sandstone long used for traditional memorials, kerb sets and ledger slabs. It is tougher than the softer sandstones and stands up well to time, but it is still a sandstone — porous, and capable of absorbing water, dirt and biological growth.

The rules are the familiar ones for natural stone: soft brushes, clean water, gentle biological treatment where needed, and absolutely no harsh chemicals or acids. Because York stone can hold moisture, allow it to dry naturally and avoid sealing in dirt or growth with aggressive products.

Portland stone

Portland stone is a fine, pale limestone, instantly recognisable on many of London’s older monuments and especially common on war memorials and Commonwealth war graves. Like all limestones it is soft, porous and reactive to acid, and it weathers by darkening with pollution crust and developing biological growth.

It deserves the same careful, reverent treatment as other limestones: no acid, no pressure washing, and only gentle, methodical cleaning. Given how many Portland stone memorials honour those who served, the bar for getting it right is rightly a high one — the goal is quiet restoration of dignity, never a harsh transformation.

Biological growth (lichen, algae, moss): what’s safe

London’s damp climate makes biological growth almost inevitable. Green algae, black spotting, lichens and moss take hold on every stone type, and on soft, porous stone they can root surprisingly deep.

The safe approach is patience, not force. A soft brush and clean water remove loose growth, and a properly diluted, stone-safe biocidal treatment can be applied to kill what remains, then allowed to weather away gradually over weeks. This is gentle on the stone and far more lasting than scrubbing. What is never safe is bleach, acid, wire brushes or pressure washing — these damage the surface, can leave the stone more vulnerable to regrowth, and on lichen-covered soft stone may pull away the very surface you are trying to save. Be especially cautious with lichen on limestone, sandstone and marble: hard scraping takes the stone with it. For a fuller list of products and methods to steer clear of, see what not to use on a headstone.

The golden rules for every stone

• Identify the stone first — the method depends entirely on what it is made of.
• Soft brushes only — never wire, metal or abrasive pads.
• Clean water as your default — add treatments only when needed, and only stone-safe ones.
• No bleach, no acid, no pressure washing — ever, but most critically on marble, limestone, sandstone and Portland stone.
• Work bottom-up, rinse top-down to avoid streaking and dirty runs.
• Be patient — gentle, repeated, slow cleaning beats one aggressive attempt every time.
• When the stone is soft, old, flaking or precious, get a professional opinion before you start.

Caring for the stone, and the memory

Cleaning a headstone is an act of love and remembrance, and the people who do it deserve to feel confident that they are helping rather than harming. The right method, matched to the right stone, restores a memorial gently and keeps it readable for the generations who follow.

If you are unsure what your memorial is made of, or you have a soft, historic or much-loved stone you would rather not risk, we are glad to help. Memorial Guardian London identifies the stone and cleans it with the correct, safe method, with our headstone cleaning service and full headstone cleaning available right across London’s 32 boroughs and out to the M25. Get in touch for a free, no-obligation quote — we will treat your memorial with the same care we would our own.