I am starting a series looking at buildings in Seattle. I plan to focus initially on some lobbies that I have long liked. My first is the wonderful art deco Exchange Building. Ironically finished in 1929, the building, as the name suggests, was supposed to house the Northwest Commodities and Stock Exchange. Most of the elaborate motifs feature items that represented Washington state agriculture. It is a lovely building.
For many years, I have taken people to the building on my downtown building stone tour and focused on the exterior Morton Gneiss. If we went on a weekday, I would often go inside and show them the extravagant stone, although I had no clue where the stone originated.
Recently, I was showing Dave Tucker of Northwest Geology Field Trips around Seattle to help him put together a tour for a guidebook he is writing on regional geology. When we stopped at the Exchange Building, his excitement prompted me to try and find out a bit more about the stone. Here’s what I learned.
The stone is quarried in Italy near the town of La Spezia, about 14 miles west of the legendary marble quarries of Carrara. The best quarries are on the Tino islands. Initially quarried in the first century and used in religious buildings, the Romans also employed it to pave roads (now that would be wicked cool to see) and in the Luni amphitheater at La Spezia. Quarrying ramped up again after World War II but has slowed down of late. The ancient name was Portovenere and the modern name is Portoro (from Porta oro), with varieties labeled Portoro a macchia larga (large-veined), Portoro di Prima (the best variety), and Portoro a macchia fine (thin-veined).
Geologically, it is part of the vast sedimentary rocks of the Tuscan nappe. The Late Triassic Portoro Limestone is an aragonite mud deposited on a shallow carbonate platform. It is up to 260 feet and consists of black calcite with alternating layers of mixed dolomite and calcite. The complex folding and reworking of the micritic limestone during Miocene deformation, which also metamorphosed the Carrara marble, produced the distinctive and beautiful stylolitic veining that characterizes the stone. Four- to eight-inch-wide shear zones separate undeformed layers up to eight inches thick.
Sorry for the bad coloring. This stone is black. The purple is light reflected from Cherry wood paneling.
The black coloring primarily comes from small amounts of organics in anaerobic environments. Limonite and sulfides produce the yellow banding with dolomite mosaics and hematite forming more violet veins.
The building architect, John Graham Sr., did a splendid job of bookmatching the Portoro panels. He also incorporated Italian travertine and some sort of purple brecciated stone, which I know nothing about. Someday I hope to figure out that part of the story.
Curiously, the Portoro is a stone of warm places, such as northern Africa and Sicily. When weathered, it loses its brilliance and appears “irreversibly opaque, whitened and corroded,” according to one study of it. You can see this in Seattle at the entrance to Shucker’s Restaurant on 4th Avenue, just north of Seneca.
Next up will be the elegant lobby of the Smith Tower.
