Cleaning blue stone: the right method for every stain
To clean blue stone, always start with clean water and a brush: outdoors, that is usually enough. Indoors, a neutral soap (Marseille soap or linseed oil soap) does the job. Avoid vinegar, lemon, limescale removers and any acidic product: blue stone is a limestone, and acid attacks it (source: the quarry company Pierre Bleue Belge).
Why is blue stone sensitive to acidic products?
Belgian blue stone, also known as Petit Granit, is a compact crinoidal limestone with a grey-blue colour, not a granite. The quarry company Pierre Bleue Belge (Les Carrières de la Pierre Bleue Belge, in Neufvilles) puts white vinegar, limescale removers, acids, lemon and acidic fruit at the top of the list of things to avoid. The manufacturer Lithofin says the same: blue stone is sensitive to acids, and a polished face can turn matt or show light marks (Lithofin, MN Protector sheet).
The trap is the “miracle” product. A cement remover for natural stone such as Lithofin MN Enlève Ciment dissolves cement film, efflorescence, limescale and rust… because it is highly acidic (pH below 1). Its own sheet states that it is not suitable for polished and honed limestone surfaces (Lithofin MN Enlève Ciment sheet). Before you buy, read the label: “acid”, “descaler”, “limescale remover” or “cement remover” mean “not on blue stone”.
A stone that has been marked cannot be put right by cleaning alone. For deep scratches, Pierre Bleue Belge refers you to a contractor or a marble craftsman who repolishes the stone, and adds that stonemasons can grind it to give it a new look. And if a piece is too damaged to save, a step or a threshold for example, Gripierres, the made-to-measure blue stone workshop linked to the quarry, can make the replacement piece to measure.
How do you look after blue stone day to day?
Outdoors, clean water, a hose and a brush are enough. No special product is needed: the stone dries quickly (Pierre Bleue Belge). Grease stains do not soak in deeply and often disappear with rain, sun and wind. If you would rather not wait: water with two spoonfuls of ammonia added, or a detergent for natural stone. Never in full sun (same source).
Indoors, after laying, let the floor dry for at least three months, using only a damp cloth and clean water. After that, wash it with warm soapy water made with a neutral linseed oil soap or Marseille soap, without rinsing. Soap darkens the stone and speeds up the patina: alternate with clean water (Pierre Bleue Belge).
How do you remove moss and green algae from a blue stone patio?
For the spring clean of a patio, Pierre Bleue Belge advises sprinkling washing powder over it: leave it to work, scrub, then rinse with clean water.
Against algae, the limestone care guide from the quarry company Polycor suggests diluted hydrogen peroxide (125 ml to 4 litres of water) or, if needed, vigorous scrubbing of the wet stone with a soft-bristled brush. The same guide rules out any bleach-based product: no bleach on blue stone (Polycor). Another option is a moss and algae remover free of chlorine and acid, such as Lithofin Algex (pH about 8), diluted 1:10, applied with a watering can, with no rain for at least 4 hours, between 10 and 25 °C and never on a hot surface. Its sheet classifies it as corrosive: wear gloves and safety glasses (Lithofin Algex).
In the joints, a joint scraper removes moss and green deposits (Kärcher). Whatever the method, try it first on an inconspicuous area.
How do you remove a rust stain from blue stone?
Rust can come from metal garden furniture, a metal fitting, an iron-rich fertiliser or a leaking heating system. Choose an acid-free rust remover that is compatible with limestone. Lithofin Rouille-Net, which is slightly alkaline (pH about 9), is suitable for limestone: apply a generous coat to a dry surface; it turns purple when the reaction starts; leave it for 10 minutes at most without letting it dry, then rinse. The surface must be at least 15 °C, and a test beforehand is still needed, as the sheet warns that leaving it on too long can dull polished limestone (Lithofin Rouille-Net sheet).
Where does white bloom come from, and how do you remove it?
Three very different cases:
- Efflorescence. Salts already present in the masonry, or carried in by moisture, can crystallise on the surface as the masonry dries out (Buildwise). Dry brushing is often enough for surface deposits (Futura). If the bloom comes back, it is the damp that needs treating, not the stone.
- Limescale from water around taps or in the shower. Wipe regularly; limescale that has already set comes off with steam, never with a descaler (Pierre Bleue Belge).
- Natural white marks. Fossils, white marks and veins, black spots: these are features of the stone’s appearance (technical sheet from Les Carrières de la Pierre Bleue Belge). They do not come off.
Can you pressure wash blue stone, and from what distance?
Yes, at reduced pressure, so as not to damage the joints (Pierre Bleue Belge). The producer gives no figures for pressure or distance, and we found no official value specific to blue stone.
The cautious guideline below therefore comes from a guide for limestone in general: a pressure of no more than 1,200 psi (about 83 bar), a 45° nozzle, held more than 12 inches (about 30 cm) from the stone, with no chemical product that could damage limestone (Polycor). Kärcher points out that too strong a jet can make slabs rough, and that a surface cleaner increases the distance between the nozzles and the stone for a gentler wash (Kärcher). Start well back, move closer gradually, and never aim straight at the joints.
Should you protect blue stone with a water repellent?
It is not essential. A clear water repellent can be applied after laying, on clean, dry stone, and is renewed about every four years (Pierre Bleue Belge). Lithofin notes that the dense structure of blue stone prevents the usual protection by impregnation, that the effect of its protector lasts from a few weeks to a few months, and that it can darken the colour: test it first (Lithofin). This product does not protect against acids.
New stone or weathered reclaimed stone: the same cleaning?
The principle does not change (water, a brush, no acid), but the aim does. New stone will change over time: it develops a patina depending on use, exposure, weather and care (technical sheet from Les Carrières de la Pierre Bleue Belge), and it lightens with the weather as the carbon on its surface is leached out, as the product sheet for our rustic blue stone steps explains. Black spots can appear and open up in the years after laying; outdoors, a hard winter can speed this up, after which things settle down (Pierre Bleue Belge).
Reclaimed stone, such as our reclaimed blue stone kerbs, already has that patina: it is often the reason people choose it. Acid or grinding removes it along with the dirt. Stick to water, a soft brush and low pressure. If, on the other hand, you want a regular surface that is easy to brush, sawn blue stone setts, with a sawn top face, are better suited.
Table: which method for which stain?
| Stain | Method | Avoid |
|---|---|---|
| Everyday dirt (outdoors) | Clean water, hose, brush | Unnecessary products, too much soap |
| Everyday dirt (indoors) | Neutral linseed oil soap or Marseille soap, warm water | Vinegar, limescale remover |
| Grease (outdoors) | Wait for rain and sun; otherwise water + 2 spoonfuls of ammonia | Treating it in full sun |
| Moss, green algae | Washing powder, scrubbing, rinsing; diluted hydrogen peroxide; or a moss remover free of chlorine and acid | Bleach, acids, a powerful jet on the joints |
| Rust | Acid-free rust remover compatible with limestone, 10 min max | Cement remover, acidic rust remover |
| Efflorescence | Dry brushing, then look for the source of damp | Acid, limescale remover |
| Limescale from water (bathroom) | Wipe, steam | Descaler |
| Natural white marks, black spots | Nothing: that is the stone | Sanding “just to see” |
FAQ
Can you clean blue stone with white vinegar?
No. Vinegar is an acid, and blue stone is a limestone: it dulls and marks the stone (Pierre Bleue Belge).
Can you use bleach on blue stone?
We do not recommend it. The limestone care guide from Polycor says never to use a bleach-based product (Polycor). Against moss and green algae, choose washing powder followed by rinsing, diluted hydrogen peroxide or a moss remover free of chlorine and acid, after a test on an inconspicuous area.
When can you wash a new blue stone floor?
Let it dry for at least three months, using only a damp cloth and clean water (Pierre Bleue Belge).
Can an acid mark on blue stone be put right?
Not by cleaning. Pierre Bleue Belge does not deal with this specific case; for deep scratches, it refers you to a marble craftsman who repolishes the stone, and says that stonemasons can grind it. If the piece has to be replaced, Gripierres can cut it to measure.
Your blue stone project
Are you planning a patio, a staircase or a kerb in blue stone, new or reclaimed? Ask for a quotation for the stone: we will tell you what we have in stock, and you can come and see the stone and collect it in Tavier.
Sources
- Pierre Bleue Belge (Les Carrières de la Pierre Bleue Belge) — “Caring for your blue stone: child’s play” (page in French)
- Les Carrières de la Pierre Bleue Belge — technical sheet 1.10, flamed finish (appearance, patina), in French
- Polycor — Limestone: care and cleaning guide (in French)
- Buildwise — Visual guide “Salt efflorescence and crypto-efflorescence” (in French)
- Kärcher Belgium — Cleaning patio slabs (in French)
- Lithofin — Rouille-Net technical sheet (in French)
- Lithofin — MN Enlève Ciment technical sheet (in French)
- Lithofin — Technical Information Algex
- Lithofin — MN Protector technical information for blue stone (in Dutch)
- Futura Sciences — Salt efflorescence (in French)
- Hazotte — product sheet: rustic blue stone steps
- Hazotte — product sheet: reclaimed blue stone kerbs
- Hazotte — product sheet: sawn blue stone setts