Why is My White Marble Turning Yellow?

The classic look of a Bianco Carrara or Calacatta Gold bath is popular, and the current availability of field (in a variety of finishes and textures like honed, polished, tumbled, hand antiqued, brushed, etc.), moldings and mosaics allow for many design options. Check out this stunning Calacatta Gold Bathroom on our Houzz page or another example below:

But why are you constantly cautioned with the warning to "Install With White Thinset Only"? 

White modified thinset is the standard recommendation for Bianco Carrara, Calacatta, Onyx, Thassos, most Limestone, and other light or translucent stones. It is also recommended for Bardiglio, Azul Macauba, and Azul Cielo. Using a grey-colored thinset can result in discoloration of the stone or the appearance of oily stains on the face of the stone. It would be prudent to recommend using white modified thinset for all natural stone installations, but that may not prevent yellowing from occurring completely. Yellowing may still happen in a shower area where mud work or wet bed has been done using Gray Portland cement. Why is white modified thinset insufficient in this application? The mud mix used in a shower pan is very dry, and not all cement particles are thoroughly incorporated. When the stone is exposed to water, moisture penetrates the stone and grout into the mud bed, activating the loose cement particles. The shower pan prevents further penetration, and the moisture slowly migrates to the drain. The resulting chemical reaction manifests as yellow staining within the stone. This moisture penetration will even occur on floors that have been thoroughly sealed. This oxidation process can only be minimized during initial installation. One option is to use white Portland cement with a low Akali content for the mud job; another would be to install a waterproof membrane, such as Laticrete 9235 or Hydroban, on top of the mud. Or a Kerdi shower system, which doesn't even use mud, is the easiest installation method. The tile would then be installed with white modified thinset directly on the membrane. This creates a barrier that eliminates water from the mud bed, thereby preventing chemical reactions and oxidation.

This yellow discoloration can be removed using a commercial poultice or Hydrogen Peroxide. Still, the stains will continue to appear as the process is repeated, so once the material is installed, there isn't a permanent solution. 

We sell Miracle Poultice products that can help if you contact our sales team at sales@architessa.com. The availability of so many wonderful marbles, granites, limestones, onyx, and others has greatly increased the use of natural stone in commercial and residential applications. Various post-installation issues usually do not reflect any defect in the stone but rather an incorrect installation or the use of a stone unsuitable for the specific application. Recommending white modified thinset as a setting, having printed cautions and instructions available for specific (such as shower pans), understanding the nature of various stones, and the specification of products specifically developed for stone care will result in successful jobs and satisfied clients.

In conclusion, ensure if you install white marble or light stone, use a white-colored thinset and Portland cement (or waterproof system). Do not use grey mud or thinset that can penetrate the light stone. Also, after installation, continue to seal your stone at least one time a year to prevent future staining. Note: for further technical information and specifications, please refer to the TILE COUNCIL OF AMERICA HANDBOOK and the DIMENSION STONE DESIGN MANUAL from the Marble Institute of America.