Digital craftsmanship in stone design

The NMAAHC’s bench seats feature black granite in a polish finish.
Photos © DC Real Estate Photo

CNC technology was critical for creating the intricately crafted cubic wall of black granite encompassing the site’s perimeter. Most of the museum’s walls feature a highly polished finish, but a combination of polished coping and honed granite finish was selected for its north wall, which also serves as the main entrance. This wall spans approximately 103 m (340 ft) across the front of the museum, running parallel to Constitution Avenue. The less-polished finish was selected due to concerns over too much reflection off the wall at the highly traveled entrance.

Fabrication of the granite pieces for the north wall proved particularly challenging because of the wall’s slight radius and continuous bullnose. The fabricator had to produce each of these stones at precisely the same thickness to ensure an exact alignment at installation. Most of the wall’s cap pieces were milled by the fabricator’s five-axis CNC machine, then finished by hand to create the bullnose. With multiple workers applying a bullnose to hundreds of pieces, accomplishing the precision required for alignment required an exceptional level of coordination and craftsmanship.

Varying finishes were also used for additional granite elements at the site. The museum’s bench seats employ a polish finish, and the curbing features the same honed granite finish as the north wall. The paving—encompassing 2322 m2 (25,000 sf) of granite—and stair treads use a proprietary slip-resistant finish.

Modeling most of this project in 3D software allowed the fabricator to coordinate with the architect and make sure all parties were getting exactly what they wanted prior to fabrication. Using 3D drawing capabilities for a project like this provides virtually endless possibilities, and can help streamline the fabrication process.

Sandblasting achieves intricate imagery
When a project’s design calls for images to be etched into stone, high-tech sandblasting capabilities can bring the artist’s vision to life. Such was the case with the granite entrance to Boston Children’s Hospital, where an intricate dot-matrix stenciling and sandblast process transferred an artist’s images to polished gray-black stone. The architecture firm for this project was Mikyoung Kim Design.

CNC technology was critical for creating the cubic wall of black granite encompassing the National Museum of African American Culture and History’s (NMAAHC’s) perimeter in Washington, D.C.

Titled “Playful Nature in the City,” the 111-m2 (1200-sf) L-shaped wall greets visitors to the hospital with playful images of animals and foliage. The intricate ‘hide-and-seek’ design includes dragonflies, butterflies, and flowers, and provides a different view with each experience.

To achieve the complex etchings, the stone fabricator took the artist’s images in a vector format and then converted them to a mask, which was applied to the stone. Through the sandblasting process, various shades of gray were produced and recessed into the stone. White paint provided highlights and added depth.

More than 200 pieces of granite were created, with unique images on each piece. Each adjacent piece had to be carefully laid out and lined up to ensure the image flowed correctly across the entire wall.

Stone selection is a critical decision for a sandblast project. Typically, a dark-colored stone is used to create an effective and necessary contrast against the images inscribed into the stone. At Boston Children’s Hospital, black granite provides such a contrast, offering the ideal background to allow images of animals and foliage to shine through. Additionally, this granite’s grain structure and stark gray-black color appeal to many artists. The stone’s consistent appearance in many different finishes allows designers to achieve a contrast in color without the use of different stones. In addition to the feature entrance wall, the project includes more than 167 m2 (1800 sf) of surrounding curbing in the same stone.

As designers continue realizing the benefits of technology, the demand for digital craftsmanship in stone design is expected to rise. The abundance of equipment in the marketplace will likely lead to lower prices. As such, the design world will have more opportunities than ever to see fine, detailed stone work in more market segments and more applications. Ultimately, the industry will see more use of sustainable, natural stone materials in project design.

Dan Rea is senior vice president of sales and marketing for Coldspring. He is also a member of MIA+BSI: The Natural Stone Institute, and served as president of the Marble Institute of America (MIA) in 2015. He can be reached via e-mail at

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