Staining of an Indoor Carrara Marble Floor at the Opera House Oslo Norway

Surface staining is common for several types of marble, including the Carrara marble. The indoor Carrara marble floor in the new Oslo Opera House suffered, shortly after laying, a yellow to brown discoloration. The stain occurs only on the surface of the marble tiles and cannot be seen on edges or fracture surfaces inside the marble. Chemical analyses of the stain shows mainly sulfur, potassium, sodium, and chlorine. These elements can be found in the organic parts of the marble, and are more easily leached compared to the dissolution of other minerals in the marble. The few pyrite grains observed are unaltered, and can by no means be related to the color changes. The main potassium and sodium source is the mortar over which the marble tiles were laid. During the drying process, the upward moisture has concentrated the dissolved S, Cl, and Br on the surface of the marble where a change in physical conditions caused oxidation of sulfide and precipitation of elementary sulfur, generating the yellow to brown discoloration. Our investigations show the surface stain to contain crystals of native sulfur (S) and aphthitalite (K,Na)3Na(SO4)2. The sulfur originates from the marble itself and possibly some from the mortar cement in dissolved state as sulfide ions. Elementary sulfur precipitated at low pressure, room temperature, and low relative humidity is unstable, and will oxidize to SO2 gas and disappear when the marble is dried. The aphthitalite crystallization is a reaction of the potassium, possibly also some sodium and sulfur, from the mortar and sulfur and sodium in the marble. Aphthitalite is stable at 1 atm pressure and partly dissolves in water. The surface stains can be removed by treatment with dilute H2O2, carefully avoiding the incipient disintegration of pyrite.