Like all tiles, natural stone tiles are typically grouted after installation. Because natural stone is porous, however, the surface of the stones needs to be sealed to help prevent it from absorbing some of the grout during the installation. If this step is overlooked, a filmy “haze” may be present on the stone after the grout dries. This is commonly referred to as grout haze, and while it can occur on any type of tile, it is the most difficult to remove on natural stone.
The most common way to get rid of grout haze is using an acid solution, but most natural stones are reactive to acids, which means using the acid solution to remove the grout haze may also remove some of the stone’s surface as well. Therefore the grout has to be dealt with using less invasive techniques.
Start by brushing any loose grout off of the surface of the stone with a dry cloth. Some grout haze is due to the way the grout dried on the stone’s surface; this haze may be able to be brushed away. If there is still haze remaining, you can usually remove it with a sugar solution.
Dissolve one cup of white sugar into one gallon of hot water. Soak some paper towels in the sugar solution and apply them to the stones affected by the haze. Let the sugar-soaked towels sit on the stone for about an hour, then clean the surface of the stones gently with clean water and a soft bristled brush. The grout haze should wash easily from the surface of the stone without harming the finish.
This technique works best if you can do it within 24 hours of noticing the haze, but can still be effective at removing haze for up to two weeks after the initial grouting job. So if you miss a step and forget to seal your stone before grouting, take care of any haze safely with a sugar solution.