IRON STAINING IN MARBLE

by Geoff Quick, CSIRO Manufacturing & Infrastructure Technology, PO Box 56, Highett, Victoria 3190, Australia

Contents
Iron staining
Preventaion of iron staining
Recommendations for New Installations
Removal of iron stains

CSIRO has been asked to investigate several instances of "yellowing" and staining of marble. While several causes have been reported in the literature, iron staining is the most common. It is sometimes possible to treat such stains, but one has to proceed cautiously as some potential remedies will exacerbate the problem if the cause is not properly diagnosed.

Italian polished marble tiles (300x300mm) on bathroom floor showing distinct wellow iron staining. When installed the marble tiles were origninally whitish-grey in colour.

Marble, by its nature, is a porous medium, and the presence of staining is not uncommon. The translucency of some marbles can exacerbate the problem. The most common change of colour is to a yellowish brown. Although iron staining is the most common cause of yellowing in marble, the literature contains several other reasons this colour change may occur, the most common being:

• Improper maintenance - as a consequence of wear, polished surfaces may become rougher and attract dirt. If improper cleaners are used, the dirt begins to accumulate in the surface pores, discolouring the stone. In many cases, dirty mops are among the main contributing factors. The use of household bleach as a cleaning agent will also promote the oxidation of iron-bearing minerals. If a cleaner is acidic, etching will also occur. The process of buffing the stone with steel wool pads, while the floor contains moisture, may also cause the stone to yellow.
• Wax build up or coatings (waxes, acrylics, urethanes, etc.) - it is not uncommon for coatings to be applied in multiple coats. As coatings build up, dirt is easily embedded in the soft layer, thus discolouring the stone. Some products on the market are not compatible with marble surfaces, and may cause yellowing over time.
• The breakdown and release of natural gums and resins (terpenes) in damp underlying wooden subfloors can migrate upwards into the stone causing discolouration.
• In some two-part epoxy (screed additive) systems, the unreactive part, if the epoxy is not properly mixed, can migrate into the stone during moist conditions causing a yellow-brown discolouration.
• The migration of dyes from rubber mats or jute backing of carpets.
• A process for polishing marble known commonly as the 'crystallisation process' may produce, in some instances, yellow stains in marbles and limestones, especially if the stone contains moisture at application.
• Exposure to UV light or heat, can cause some stones, sealers and coatings to yellow over time.

Iron staining

Optical micrograph of small pyrite inclusions in polished thin section of marble Reflected light, width of field: 0.1mm.

Iron staining in marble can be caused from external and internal sources. Externally sourced iron may include the water supply, iron-bearing minerals (or abandoned nails or screws) in the setting bed, or adjacent fixtures.

Internally, iron can be sourced from naturally imbedded minerals. Many white marbles contain a random distribution of very small naturally occurring inclusions of iron-bearing minerals. It frequently occurs in the form of iron sulphide minerals, such as pyrite or marcasite. When exposed to oxygenated water or other oxidisers such as acids and household bleach, the sulphide minerals begin to oxidise to iron sulphate, then subsequently to iron hydroxide (rust). The release of sulphur as sulphate, in the presence of water, generally converts to sulphurous acid or to the more corrosive sulphuric acid. The solubility of iron markedly increases in localised acidic environments, despite the strong buffering action of the marble. The mobilised iron would then precipitate in the more alkaline regions of the marble, yellowing the stone. The rate at which the sulphide minerals oxidise can vary, depending on the amount of the mineral present, composition₁, surface area and morphology, and the environmental conditions the stone is subjected to (i.e. oxygen concentration, presence of autotrophic bacteria; moisture from daily cleaning, the setting bed, or simply by humid conditions). In some cases it may take years before any yellowing occurs. However, in water-saturated conditions the process would be greatly accelerated. It has been known that after a flood, yellowing can occur overnight.

Prevention of iron staining

The oxidation of iron sulphide minerals (and the resultant iron staining) is unlikely to occur if oxygenated water is excluded from the reaction. In other words, if the tiles (and grout joints) are sealed with a suitable deep penetrating sealer, staining may not occur. This assumption is based on the fact that all water available to the system is absorbed from the top surface, and grout joints, by way of cleaning procedures and water spills. Marble, however, is a porous medium, which is capable of absorbing water, not only from the proper surface but from other sources.

Recommendations for New Installations

In new bathrooms, stone should always be treated with a suitable deep penetrating (but 'breathable') sealer, which will assist in preventing the oxidation of the iron sulphide by eliminating moisture. The shower base and walls (including grout joints) should be made completely waterproof. There are many product formulations on the market that are suitable for various types of applications.

There is a great debate in the stone industry on whether all (top and bottom) surfaces should be pre-sealed before installation. In theory, this is the most appropriate method to undertake. However, at CSIRO we have found that in some situations there are bonding problems between some adhesives and the sealed tiles. It is thoroughly recommended that the adhesive bond between the sealed marble tile and the chosen adhesive be tested before adopting this technique.

Removal of iron stains

Once the staining has commenced it is very difficult to reverse, and replacement of the tiles may be necessary. One of the following stain removal techniques may be useful in the interim, but it is probable that the yellow staining may return in time. There may be some practical difficulties in using these techniques on walls. It is important that the affected tiles be completely dry before attempting any one of these procedures. If localised spot staining occurs, replacing an individual tile may be the simplest initial solution.

1. Prepare a solution of distilled water and the following chemicals: sodium hydrosulphite and sodium metabisulphite. These chemicals may also be available in specialist proprietary products. There are also other commercial rust removers on the market, but if one is chosen, make sure it contains sodium hydrosulphite and that it is only mildly acidic. Avoid ones that are excessively acidic because they will aggressively react with the marble. Whichever product is chosen, follow the mixing and application directions for the product, particularly looking for application instructions for marble. Mix a solution in water and apply using a soft brush or cloth to the affected tiles. Allow the solution to soak into the tiles, and keep wet for several hours. Do not allow the solution to dry. For spot stains on floors, the construction of wells surrounding the treated area may prove beneficial. After several hours, remove all the excess solution with a wet vacuum and rinse the floor thoroughly (several times) with water and a chelating agent, such as EDTA (ethylene diamine tetra-acetic acid) or equivalent. A chelating agent will keep the iron suspended in solution. Be prepared to rehone and repolish the marble, since these chemicals are often acidic and can cause etching.

2. If the above procedure fails, then prepare a poultice using diatomaceous earth together with the same chemicals listed above. Mix the poultice into a thick paste and apply to a small area. Cover the poultice with plastic and allow to sit, covered, for 24 hours. After 24 hours, remove the poultice paste and thoroughly rinse the area with clean water and a chelating agent. If the stain has been removed, the entire area can be treated.

3. Alternative remedies - mix a poultice using diatomaceous earth with one of the following: sodium citrate (with glycerine), or ammonium oxalate. If resigned to the fact that repolishing is required, then consider using dilute hydrofluoric acid with the poultice powder.

Observe all safety precautions in using any of these chemicals. It is recommended that the preparation and application of all these treatments should be done by professional specialists, who have well documented safe working procedures.

Be sure to test any of these procedures in a small area for suitability before beginning the whole project. Keep in mind that acid solutions will likely remove any polish (through etching), and whether any of these processes are successful or not, you will need to repolish. In case of incompatibility, do not mix treatments on the same tile.

If the stain remains after any of these treatments, then tile replacement is likely to be the only solution.

If stain removal is successful, the cleaned tiles should then be sealed with a suitable penetrating sealer specifically formulated for the stone type and application. However, ensure that the tiles are completely dry, clean and that any wax treatments or soap scum is completely removed before application. It may take 2-3 weeks before the interior of the tiles are sufficiently dry to receive the sealer, unless the marble is subjected to a suitable accelerated drying regime. The use of a moisture meter is recommended.

Geoff Quick is a geologist and microscopist at CSIRO Manufacturing & Infrastructure Technology, PO Box 56, Highett, Victoria 3190, Australia [Fax (03) 9252 6255; Tel (03) 9252 6139; Email Geoff.Quick@csiro.au]. He is a member of the Standards Australia committees on natural and reconstituted stone tiles, and masonry units and segmented pavers. He is also a member of the ASTM C-18 Committee on Dimension Stone, the Geological Society of Australia, the Australian Microbeam Analysis Society (AMAS), and the Australian Microscopy and Microanalysis Society (AMMS).

1. According to Midgley (Magazine of Concrete Research, pp. 75-78, Aug. 1958), there are reactive and unreactive forms of pyrite to oxidation. Pyrite is known to have a 'defect' structure and rarely possess the exact 1:2 ratio of iron to sulphur shown by the ideal formula, FeS₂. Midgley concluded that samples which have a greater sulphur deficiency tend to be the reactive forms.

Employment Opportunity