Selecting the Right Injection Grout for Historic Marble at Independence Hall
Independence Hall in Philadelphia
Philadelphia’s Independence Hall holds a storied place in American history, serving as the first capitol building of the United States and where the country’s founders signed the Declaration of Independence and the U.S. Constitution.
But after 270 years, some of the masonry, including the marble bases of multiple brick columns, showed heavy wear. Contractors encountered cracks ranging from one-sixteenth of an inch to one-eighth of an inch.
To fill gaps this size, many have used epoxy — which we would never recommend. In this instance, it was even more important to choose an injection grout, and one based on the historic nature of the marble. At the time of the building’s construction, it was common to use a softer marble than you may see in later buildings.
Unlike epoxy, M32 Injection Grout matches the softness of historic marble and is formulated to allow for moisture to escape.
The contractor avoided the common mistake of using an epoxy by selecting our M32 Injection Grout for its softer properties and extremely fine aggregate. Other contractors may have chosen our M31 Injection Grout, but that is most compatible with modern marble and harder stones.
I find that an architect’s or specifier’s first instinct may be to select an injection grout with high psi—thinking such a product would result in a longer-lasting repair. The problem is: that high psi material will damage the substrate.
As an epoxy hardens, it reaches its full PSI of 5,000 and sometimes over 10,000, putting too much pressure on the surrounding marble and acting as a wedge that moves against the crack. In contrast, our M32 Injection Grout moves with the stone. This is especially important in the Northeast, with its frequent freeze/thaw weather cycles. When trapped moisture freezes, it expands, putting pressure on the stone and substrate. The permeability of Jahn grouts allows moisture to be released, whereas an epoxy-based product traps moisture under the stone and grout, leading to repairs that don’t last and that actually damage the stone.
A Potassium Silicate Stain is applied using a small brush to replicate the veins in the marble.
The Final Touch
Once the injection grouting was complete, we then had to “hide” the injection grout holes. To do that we needed to replicate the look of the marble. Our injection grout is white and not color-matched before installation, making the repairs visible. The contractor brought us in to solve this extra challenge.
Our solution? Replicate the veins in the marble by using a small brush to apply a Potassium Silicate Stain. We custom blended a blue potassium silicate coating and a gray potassium silicate coating to match the color, and then we added a Liquid Silicate to create a translucent stain. We advised following the veining pattern of the marble with the stain for an authentic appearance.
Following the veining pattern of the marble with the stain for an authentic appearance.
Points to Remember
Whenever you’re restoring a historic property, especially one of such importance, consider the materials that would have been used during that time period to specify products accordingly—be sure to account for typical weather conditions, and don’t hesitate to call on an expert for consultation.
Whenever I participate in a historic project like this, I think ahead to the next generation. If I want my kids to be able to visit a building with the significance of Independence Hall, I advise that the products selected will lead to a repair that lasts as long as the stone. Contact me (tech@cathedralstone.com) if you want guidance on any aspects of your masonry restoration project.
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