How Does UV Exposure Affect Stone Surfaces?

Frederick M Hueston StoneForensics.com

Today, let’s dig into something you may encounter frequently in your work: how UV exposure impacts stone surfaces. This is more than just a cosmetic issue. UV light can play a role in the durability, color, and even the surface integrity of the stones we work with every day. Whether it’s an outdoor stone patio, pool coping, or a decorative indoor surface that gets a lot of sunlight, understanding how UV rays can affect stone is crucial.

1. The Fading Effect on Stone Colors

Let’s start with one of the most noticeable effects: fading. Stones come in such a wide variety of colors, from the subtle hues of limestone to the bold shades in marble and granite. But all of these colors can fade over time when exposed to sunlight, thanks to those persistent UV rays. Here’s how it happens:

• Absorption of UV Rays: Some minerals in stones react to UV light by breaking down over time, especially darker colors like blacks, reds, and blues. The result is a slow but steady fading. You might notice that the parts of a stone patio under shade retain their original color, while the exposed sections have dulled.
• Sensitivity of Certain Stones: Stones like marble and limestone, particularly when polished, can lose their rich colors relatively quickly in direct sunlight. Softer stones tend to be more porous, making them susceptible to UV-induced changes in color. Granite, although generally more resistant, can also fade—especially when it contains minerals sensitive to UV exposure.

2. Surface Integrity and Texture Changes

Beyond fading, UV exposure can change a stone’s texture. Over time, exposure to UV rays, along with weather conditions, can lead to a breakdown of certain minerals within the stone, resulting in surface damage. Here’s what to look out for:

• Crystallization Breakdown: Stones like marble contain calcium carbonate, which can be affected by UV exposure. This can lead to a change in the surface finish, especially when polished. Polished marble, for instance, might become rough or start to pit in areas with extensive sun exposure.
• Weathering and Erosion: When stones are exposed to UV light and moisture, they can experience accelerated weathering. Limestone and sandstone, being relatively softer stones, often exhibit this. Over time, the stone’s surface may begin to erode, and you might notice that it becomes more granular or roughened. This is particularly important to watch for on stone used in outdoor installations.

3. Discoloration Beyond Fading

While fading lightens the stone’s color, discoloration is another issue altogether. Some stones might even darken in places due to UV exposure.

• Organic Impurities: Stones containing organic materials—like those in some granites and sandstones—can darken over time in the sun. UV rays can oxidize these organic impurities, causing patches of discoloration.
• Resin and Filler Issues: Here’s another common culprit for darkening: some stones contain resins, particularly engineered stones or natural stones with surface resins applied to fill imperfections or enhance colors. Over time, UV exposure can cause these resins to darken, especially on stones with resins that aren’t UV-stable. Resin darkening can lead to uneven discoloration across the surface, affecting the stone’s overall look. It’s something to keep an eye out for on stones like travertine and even some granites where resin may be used as part of the manufacturing or repair process.

4. Microcracking and Microfracturing

In addition to discoloration, UV exposure can lead to microcracking, which can alter the structural integrity of the stone.

• Thermal Stress: UV exposure often goes hand-in-hand with heat. As stones heat up and cool down repeatedly, they expand and contract. This constant temperature fluctuation can cause tiny fractures within the stone. It’s not always immediately visible, but over time, these microcracks can lead to more significant structural issues.
• Increased Porosity: As microcracks develop, the stone becomes more porous. With greater porosity comes an increased susceptibility to staining and more frequent moisture absorption. This is especially concerning for outdoor stone installations in freeze-thaw climates, where water absorption followed by freezing can lead to more severe cracking and deterioration.

5. Loss of Protective Coatings

Many stones are treated with sealers or finishes to protect them. UV exposure can break down these protective layers.

• Sealer Degradation: UV rays can accelerate the breakdown of sealers and coatings, which are designed to protect the stone. When these protective layers degrade, the stone surface becomes more vulnerable to staining, water damage, and further UV effects. Resealing outdoor stone surfaces more frequently can help maintain protection, but it’s essential to use UV-resistant sealers wherever possible.
• Oxidation of Metal-Based Sealers: If any metal-based sealants or coatings have been used, UV exposure can lead to oxidation. The result? Unsightly staining and a surface that may be even more susceptible to future degradation.

What Can Be Done?

Fortunately, there are steps you can take to mitigate these effects for your clients. Here are some practical tips:

• Choose UV-Resistant Stones and Sealers: If you have input on stone selection, steer clients toward UV-resistant options for areas with heavy sun exposure. Granite and some engineered stones tend to hold up better over time. When it comes to sealers, always opt for those with UV-resistant properties.
• Regular Resealing: Keep in mind that even the best UV-resistant sealer will wear down with time. For outdoor surfaces, resealing every year or two is a good rule of thumb. This will help protect the stone’s surface, maintain its color, and reduce moisture absorption.
• Provide Shade Where Possible: Suggest using shading solutions, like awnings or strategically placed plants. This can help reduce the UV impact and prolong the life of the stone.
• Regular Maintenance and Inspections: By setting up a maintenance plan that includes regular inspections, you can catch issues like fading, microcracking, and coating degradation early on. This can save your clients money in the long run, as smaller repairs are generally easier and less costly than major restorations.

Understanding how UV rays impact different stones can help you take preventive action and give clients the best advice on stone care. In the world of stone restoration, knowledge is power, and the better you can protect these surfaces, the longer they’ll last. Keep those stones looking beautiful!