Preserving Indiana Limestone
By Fred Hueston
A Guide to Identifying and Treating Alkali Stains: Indiana limestone is prized for its soft, elegant color and time-tested durability. But if you’ve worked with it long enough, you’ve probably run into one of its more frustrating issues—those unsightly brown stains that seem to appear out of nowhere. These stains often leave homeowners scratching their heads and restoration professionals searching for solutions. The good news? These discolorations usually have a common cause—and once you understand what you’re dealing with, they’re often manageable.
What Causes the Brown Stains?
Those golden to dark brown blotches you see on Indiana limestone are typically the result of alkali staining. Here’s what’s going on behind the scenes: moisture from surrounding materials like concrete or mortar can carry with it soluble alkalis (think salts and minerals). When this moisture seeps into the limestone and eventually evaporates at the surface, it leaves behind those alkali deposits. Over time, those deposits oxidize or react with organic matter, giving the stone that stained appearance.
How to Keep Alkali Stains from Happening in the First Place
As always, prevention is your best friend. Here are a couple of proven ways to stop stains before they start:
- Damp-Proofing: One of the most effective ways to prevent alkali staining is to apply a damp-proof barrier to the backside and edges of the limestone. This is especially important when the stone will be installed near soil, concrete, or other moisture-prone areas. The Indiana Limestone Institute strongly recommends this step as a moisture block during installation.
- Smart Design and Detailing: Elevating stonework off the ground using design elements like raised curbs or separate sill courses can reduce direct exposure to moisture sources like landscaping or paving. A little thoughtful detailing during the planning phase can go a long way.
Dealing with Alkali Stains That Have Already Appeared
Once staining sets in, you’ll need to clean it off—but carefully. Indiana limestone is sensitive, so aggressive techniques can do more harm than good. Here are a few go-to methods that usually work well:
- Tri-Sodium Phosphate (TSP) Wash: This is a great first step for mild stains. Mix a TSP solution (follow product label instructions), apply it to the stained area, and give it a light scrub. Then rinse with medium-pressure water.
- Hydrogen Peroxide for Organic Stains: If the stains are caused by organics like algae or mildew, apply full-strength hydrogen peroxide to a pre-wetted surface. Let it sit a bit, scrub gently, and rinse thoroughly. This works well on biological discolorations.
- Controlled Water Pressure Cleaning: Sometimes a careful pressure wash is effective. Use water at no more than 1,200 psi and always with a 45-degree fan-tip nozzle. Keep the nozzle at least 6 inches from the surface to avoid damage. This is a good method when dealing with larger areas of staining.
By knowing how and why alkali stains develop—and how to prevent or remove them—you’ll be well equipped to restore Indiana limestone to its clean, natural appearance. Whether you’re maintaining a monument, building façade, or a custom stone installation, a little know-how goes a long way.
