Resined Stone and Bacterial Staining
By Frederick M. Hueston
Understanding Factory-Resined Stone
Most stone pros know that many slabs today are resined at the factory. The resin fills tiny pits, veins, and voids in the stone to strengthen it and improve appearance. It’s an accepted part of modern stone production, but it’s not without its problems. One issue that doesn’t get enough attention is bacterial staining.
How Bacterial Staining Develops
Here’s what happens. During the resining process, if any moisture, organic matter, or naturally occurring materials are present in the stone, they can become trapped beneath or within the resin. Over time, those trapped materials can feed bacterial growth, especially when combined with warmth and humidity. What starts as an invisible issue inside the stone can slowly turn into a dark or discolored stain that seems to appear out of nowhere.
The Role of Organic Material in Stone
Stone, by its nature, can contain organic materials. Some limestones and marbles formed from ancient marine life, plants, or other organic debris. Those remnants can react when sealed in by resin and later exposed to heat or moisture. Even granites, which are typically more stable, can sometimes hold trace organic minerals that react in a similar way.
Types of Bacteria That Affect Stone
Bacteria thrive on these trapped nutrients. There are two basic types of bacteria to consider:
- Aerobic bacteria, which require oxygen. These are usually found near the stone surface where air is present. They can grow around open pores, cracks, or areas where resin has aged or separated.
- Anaerobic bacteria, which don’t need oxygen. These can grow deep inside the stone or under the resin layer, especially where moisture collects. They’re often responsible for stains that seem to come from within the slab.
Addressing Bacterial Staining
Once bacterial staining appears, it can be difficult to remove. Normal stone cleaners won’t do much, and using harsh chemicals can make things worse by softening the resin or etching the surface. The best approach is to use a disinfecting cleaner that’s safe for natural stone, followed by a poultice to pull out any remaining stain. If the problem runs deep, re-honing and refinishing may be the only option.
Preventing Future Problems
Prevention starts long before installation. Factories should ensure slabs are fully dry and clean before resining, and installers should educate customers about proper care. Avoid trapping moisture under sealers or adhesives, especially on outdoor or wet-area installations. Keeping the stone clean and dry once in service will also reduce bacterial activity.
Final Thoughts
Resining makes stone stronger and prettier, but when organic material gets sealed inside, it can turn into a time bomb for future staining. Understanding how these issues start helps pros spot potential trouble early and explain to clients why that “perfect” slab might not stay that way forever.
