{"id":3493,"date":"2026-09-25T15:52:19","date_gmt":"2026-09-25T15:52:19","guid":{"rendered":"https:\/\/sagit-global.com\/?p=3493"},"modified":"2026-11-03T14:08:40","modified_gmt":"2026-11-03T13:08:40","slug":"tache-rouille-marbre","status":"publish","type":"post","link":"https:\/\/sagit-global.com\/en\/entretien-et-maintenance\/tache-rouille-marbre\/","title":{"rendered":"Rust stains on marble: deposited rust or native rust?"},"content":{"rendered":"

A rust stain on marble has two possible origins, and they are not treated in the same way: a deposit of iron from outside, or the oxidation of iron-bearing minerals contained in the stone itself. In both cases, the iron is extracted with a poultice, never with acid.<\/p>\n\n\n\n

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Key points<\/strong><\/p>\n

\u2013 Iron does not stay on the surface: it migrates into the pore network and precipitates inside<\/strong> the stone. Wiping does nothing.<\/p>\n

\u2013 A second origin, overlooked by French-language sources: the oxidation of iron sulphides, carbonates and silicates contained in the stone<\/strong>, visible 8 to 12 months after laying<\/strong>.<\/p>\n

\u2013 Reference protocol, U.S. General Services Administration: sodium citrate poultice<\/strong>, about 48 h<\/strong>. It can dull a polished surface.<\/p>\n

\u2013 A conservation laboratory has excluded oxalic acid<\/strong> on calcareous stone, pH too low. The solubility of calcium carbonate increases down to pH 5<\/strong>, and below that decomposition becomes irreversible.<\/p>\n

\u2013 Reversibility depends on the depth<\/strong>, not of time: diffusion measured to \u2248 1 mm<\/strong>, iron predominant to \u2248 800 \u00b5m<\/strong>.<\/p>\n<\/blockquote>\n\n\n\n

Where this rust comes from<\/h2>\n\n\n\n

Three practical origins, only two of which are documented by the sources used here. The diagnosis governs the rest: the same poultice succeeds in one case and has no effect in the other.<\/p>\n\n\n\n

A deposit of iron from outside<\/h3>\n\n\n\n

La imported rust<\/strong> is a deposit of iron from outside: a furniture leg, a tin can, iron-rich water. The stain is not deposited; it migrates<\/strong>. The iron goes into solution as soluble Fe\u00b2\u207a ions, water carries it into the pore network, then it oxidises to Fe\u00b3\u207a and precipitates as insoluble oxyhydroxides inside the stone. In marble, diffusion goes deeper than elsewhere and follows the grain boundaries (Reale et al., Molecules<\/em>, 2023).<\/p>\n\n\n\n

Iron-bearing minerals contained in the stone<\/h3>\n\n\n\n

La native rust<\/strong> corresponds to the oxidation of iron-bearing minerals contained in the stone itself: a stone laid new and sound can develop its own stains. The oxidation affects accessory minerals in the mass: iron sulphides (pyrite, marcasite)<\/strong>, iron carbonates (siderite)<\/strong>, ferromagnesian silicates (biotite, hornblende)<\/strong>, dispersed or concentrated along veins. Water carries this iron to the surface of the slab, where it precipitates as limonite.<\/p>\n\n\n\n

The decisive fact is chronological: these stains develop after laying, within 8 to 12 months<\/strong>. And the damage is not only cosmetic, since cracks appear, attributed to thevolume expansion<\/strong> of the oxidised and hydrated minerals (Bams and Dewaele, Materials Characterization<\/em>, 2007). Reale et al. (2023) confirm the mechanism on a different corpus, archaeological marbles against marbles stained in the laboratory.<\/p>\n\n\n\n

Two reading precautions. These statements come from the abstract<\/strong> of the 2007 paywalled article: nothing is extrapolated beyond it. And the criterion is mineralogical, not commercial<\/strong>. The sources reason in terms of pyrite or siderite, never in quarry names: no trade name can be presented as \u201cprone to pyrite\u201d.<\/p>\n\n\n\n

Iron rising from the substrate<\/h3>\n\n\n\n

A third possibility, to be treated as a diagnostic hypothesis<\/strong> : metal fixings, reinforcement, iron-bearing laying mortar. No institutional source used here documents it, but the symptom is clear: the stain reappears after each successful treatment. This is decided at the laying stage and by the thickness, well before maintenance: see our marble tiles<\/a> and our marble worktops<\/a>.<\/p>\n\n\n\n

The extraction poultice, step by step<\/h2>\n\n\n\n

Le poultice<\/strong> is a white absorbent filler, impregnated with a solution that makes the iron mobile, applied to the stain and then removed. The reference procedure is public: Poulticing Rust Stains from Limestone and Marble<\/em>, from the U.S. General Services Administration<\/strong>. It gives the formulations, the permitted fillers and the application times.<\/p>\n\n\n\n

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  1. Prepare a white, inert absorbent filler<\/strong> : modelling plaster, untreated white flour, kitchen paper, powdered chalk, talc, Sommi\u00e8res earth.<\/li>\n\n\n
  2. Mix the solution<\/strong> : 7 parts glycerine, 1 part sodium citrate, 6 parts warm water<\/strong>. The glycerine keeps the poultice moist while it is in place.<\/li>\n\n\n
  3. Apply thickly<\/strong>, then cover with a film to prevent drying.<\/li>\n\n\n
  4. Leave for about 48 h.<\/strong> The time depends on the product: 30 minutes per application<\/strong> for sodium hydrosulphite, 24 h maximum<\/strong> for a commercial rust remover.<\/li>\n\n\n
  5. Remove, rinse immediately<\/strong> with the sodium citrate solution, then dry.<\/li>\n\n\n
  6. Check the finish.<\/strong> The GSA warns that the treatment may dull polished surfaces<\/strong>, which can be corrected by repolishing. Its warning concerns sodium hydrosulphite and sodium hypochlorite, which are not acids: it is indeed a loss of polish, not acid etching. On a thin slab, the margin is small.<\/li>\n\n<\/ol>\n\n\n\n

    Three families of products, three chemical roles, and that is what explains the differences in application time.<\/p>\n\n\n\n

    Family<\/th>Chemical role<\/th>Practical consequence<\/th><\/tr><\/thead>
    Sodium citrate<\/strong><\/td>Complexing agent: sequesters iron in a soluble complex, without acidity<\/td>The safest route on a limestone, hence the long application<\/td><\/tr>
    Sodium hydrosulphite<\/strong><\/td>Reducing agent: brings insoluble iron back to a soluble form<\/td>Effective, but only 30 minutes per application<\/td><\/tr>
    Oxalic acid<\/strong><\/td>Acid: attacks the substrate at the same time as the stain<\/td>Reserved for light stains, and problematic on limestone<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    Warning: what not to apply<\/h2>\n\n\n\n
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    Read before buying a product<\/strong><\/p>\n

    A marble is made of calcium carbonate. Any sufficiently strong acid dissolves it: that isacid etching<\/strong><\/a>, with loss of polish, a whitish matt finish and micro-relief. No vinegar, no lemon, no descaler, no rust remover not approved for calcareous stone.<\/p>\n<\/blockquote>\n\n\n\n

    The best-established point concerns oxalic acid. A conservation laboratory withdrew it from its tests on calcareous stone<\/strong>, its pH being too low and damaging the stone surface (Cezar, 1998). The same article gives a quantified marker: the solubility of calcium carbonate increases down to pH 5<\/strong>, and below that threshold decomposition becomes irreversible.<\/p>\n\n\n\n

    This acid does more than dissolve. It reacts<\/strong> with calcite and precipitatescalcium oxalate<\/strong>, which is very poorly soluble and therefore impossible to rinse away. You swap a brown stain for an irreversible white haze, plus an etched surface.<\/p>\n\n\n\n

    Why are these products tolerated on granite or glazed ceramics? Because granite is made of silicates and contains no calcite accessible to a weak acid. But tolerance is not harmlessness<\/strong> : the Natural Stone Institute states that hydrofluoric acid, present in traces in many commercial rust removers, also attacks silicates, and that all stones including granite and quartzite are attacked if exposed to it. On a limestone the damage is immediate and visible; on a granite it is slower and less spectacular. Neither stone is a good candidate.<\/p>\n\n\n\n

    No source makes it possible to generalise about the composition of consumer rust removers sold in France. The only reliable information is the data sheet and safety data sheet<\/strong> of the specific product.<\/p>\n\n\n\n

    When the stain will not come out<\/h2>\n\n\n\n

    No institutional source consulted gives a threshold in days or weeks, and the GSA procedure states no case of irreversibility. The criterion is not a time; it is a depth<\/strong> and a mineral state<\/strong>.<\/p>\n\n\n\n

    The measurements by Reale et al. (2023) give two markers. On an artificially stained Carrara marble, iron diffusion reaches about 1 mm at most<\/strong>. On a historic sample, iron remains the most abundant element down to about 800 \u00b5m<\/strong>, and calcium only becomes predominant again at about 1.7 mm<\/strong>. The iron also evolves into more stable phases, which are harder to extract.<\/p>\n\n\n\n

    A poultice works on the first few tenths of a millimetre. Reversibility therefore depends on the depth reached and the stability of the mineral formed.<\/p>\n\n\n\n

    Can a rust stain be prevented?<\/h2>\n\n\n\n

    On imported rust, a water repellent limits liquid water<\/a> that carries the iron. The only experimental study available here concerns ornamental slates<\/strong> with sulphide inclusions, treated with silane-siloxanes: the products tested significantly increased the sulphides\u2019 resistance to oxidation<\/strong>, without significant colour change (Rivas et al., 2011). These are slates, neither a marble nor a limestone. The result is indicative; it proves nothing about a carbonate stone, and it speaks of increased resistance, never of elimination.<\/p>\n\n\n\n

    For native rust, a water-repellent cannot prevent the phenomenon<\/strong>. This is reasoning, not a quotation: the iron is already in the stone, and a surface treatment does not remove a pyrite inclusion.<\/p>\n\n\n\n

    The upstream lever is a standard, and nobody uses it: the mineralogical identification of a stone falls underEN 12407:2019, petrographic examination<\/strong>. That is where the question of accessory minerals arises, before purchase or before laying.<\/p>\n\n\n\n

    Frequently asked questions<\/h2>\n\n\n\n

    How do you remove ingrained rust stains?<\/h3>\n\n\n\n

    By extraction, not by attack: a white absorbent poultice loaded with a solution that makes the iron mobile, left in place, then rinsed off. About 48 hours with the GSA\u2019s mixture of glycerine, sodium citrate and warm water. Several short applications are better than one prolonged one.<\/p>\n\n\n\n

    How do you remove a stain ingrained in marble?<\/h3>\n\n\n\n

    The same logic applies to any stain that has migrated into the pore network<\/a> : you do not rub the surface, you draw the substance out. The product depends on the nature of the stain, and on a limestone it rules out acids. If the colouring comes from within the stone, no poultice will remove it.<\/p>\n\n\n\n

    Does white vinegar damage marble?<\/h3>\n\n\n\n

    Yes. Vinegar is an acid, and marble is calcium carbonate: the calcite dissolves and the surface dulls. Vinegar is even used as a reagent to identify a calcareous rock. On a rust stain, it extracts nothing and adds acid etching to the problem.<\/p>\n\n\n\n

    Which products should not be used on marble?<\/h3>\n\n\n\n

    Anything acidic: vinegar, lemon, descaler, rust stain removers not approved for calcareous stone. Oxalic acid deserves a special mention, a conservation laboratory having excluded it from its tests on limestone because its pH was too low (Cezar, 1998). Also avoid scouring powders, which are abrasive.<\/p>\n\n\n\n

    How do you clean marble the old-fashioned way?<\/h3>\n\n\n\n

    The recipes in circulation are almost all based on vinegar or lemon, and therefore on an acid: the wrong choice on a limestone. The only traditional method validated by an institutional procedure is the poultice, a white absorbent filler and a non-acidic solution left to act.<\/p>\n\n\n\n

    Key points<\/h2>\n\n\n\n

    Before choosing a product, establish the origin. Two questions are enough: did the stain follow contact with a metal object, or did it appear with no visible cause several months after laying? And does it come back in the same place after treatment? In the first case, the sodium citrate poultice is the safe route. In the second, the iron comes from the stone or from the substrate.<\/p>\n\n\n\n

    Sources<\/h2>\n\n\n\n