Archive for the ‘Oakland stone’ Category

Oakland building stones: Granite variations

29 August 2016

granite-wilsonbldg

Oakland’s buildings embody history and geology in the different types of stone that compose their facades. So how about granite?

California began producing granite soon after statehood, in a stonecutting district near Sacramento where rail and barge transport was handy.

At the time Oakland’s third City Hall was being designed, around 1910, many large quarries in the Sierra were supplying the market, and the architects chose “Sierra White” stone from a quarry near Madera, probably the McGilvray Quarry. The same district still produces Sierra White granite for the great Coldspring empire.

granite-cityhall

This fine-grained granite is excellent for carving and for resisting the attacks of urban pollution. Its neutral color works well with the building’s superb ceramic tiles and marble. Many of San Francisco’s notable buildings (the St. Francis Hotel, Bank of California and Old Custom House among them) also use Sierra White.

Buildings of later vintage take advantage of stone from more distant sources. The Lake Merritt Tower building at 155 Grand Avenue, built in 1990, features a splendid granite with extremely coarse grains, not typical of California. The granite benches around the building use the same stone in a polished finish that displays it especially well.

granite-bench

The building itself employs the same granite with a flamed or thermal finish. It’s produced by running stone slabs through a set of gas-fired torches. The sudden thermal expansion shatters the minerals and, properly controlled, yields this rugged surface.

granite-lakemerritt-tower

The Caltrans building next door, formally named the Nicholas C. Petris State Building and finished in 1992, is faced with a pink granite from Texas in both polished and sandblasted finishes. Here’s a closeup.

granite-caltrans

What are we looking at here? Time for the geology part. First of all, to stone dealers “granite” means only one thing — a coarse-grained rock (1) hard enough to take a good polish and (2) not marble, serpentine or quartzite — but that thing is a very broad one. Stonecutters came centuries before geologists, and what they say goes.

Geologists don’t call every granite building granite. “Black granite,” for instance, is gabbro. Scrupulous geologists would call all the stones in this post “granitoid,” meaning one of a broad set of coarse-grained rocks made of feldspar and quartz in various proportions. Real petrological granite has a narrower set of proportions. None of these stones has the 20 percent quartz that would qualify it as true granite. I know you’re disappointed. But the feldspar makes a great building stone.

In the picture above, quartz is the clear stuff that looks gray. Feldspar comes in two main types. The white grains are plagioclase feldspar, and the pink ones are alkali feldspar. (A pink color is unusual; it ranges from white to buff to red.) The black grains are typically hornblende, biotite (black mica) or both.

The gorgeous Texas Pink granite of the Lake Merritt Plaza complex, at 20th and Harrison streets, shows these minerals more vividly.

granite-lakemerritt-plaza

I do think this is Oakland’s most beautiful building stone. Texas Pink is a brand (not the kind you burn on steers), also from the Coldspring empire. It’s quarried from Granite Mountain, about an hour out of Austin and near the amazing dome of pink granite preserved in Enchanted Rock State Natural Area.

Texas Pink granite was also used, a bit more adventurously, in the AC Transit headquarters building at 1600 Franklin Street.

granite-actransit

I don’t know in what year the building was constructed, but I have the impression that Oakland architects are finished with Texas Pink granite for the moment. Metal and glass is the default exterior of today’s large urban buildings. In their lavish lobbies and interiors, though, stone is still the right stuff.

Oakland building stones: Kaiser Center’s dolomite

18 July 2016

During yesterday’s OHA walk around some of Oakland’s building stones, I was pleased to bring out an underappreciated aspect of this great city landmark.

Kaiserctr

The Kaiser Center, like City Hall, is one of Oakland’s signature buildings. When construction finished in 1960 the curving, T-shaped office tower was so iconic that Ansel Adams is said to have photographed it, presumably giving it the same sheen of grandeur he gave Half Dome. (I couldn’t find that image, although Rondal Partridge also photographed it, which is just as good.)

The photo above shows the butt-end of the 7-acre Kaiser Center complex, at Webster and 21st streets. In the foreground is the parking structure/commercial space that holds up the wonderful rooftop garden.

The Kaiser Center is famous for its extravagant use of glass and metal, specifically aluminum, Kaiser Aluminum’s principal product and a design element dictated by Henry J. Kaiser himself. But I spoke to the group about the third element that embraces the metal and glass in a gracious unity — the white stone cladding.

As you walk around the Center, the cladding appears pleasantly rough yet perfectly homogeneous. It’s not a veneer of solid stone, nor is it some kind of textured concrete. Neither is it painted. It’s an extraordinary material. After 55 years of weather, it still looks white and crisp and fresh.

Nearly all of the cladding is out of reach. Some panels touch the ground along the curving roadway behind the main tower, but that’s too hazardous even for your typical roadside-loving geologist. However, at the left edge of the photo is a little section of wall where the architects felt obliged to extend the cladding to the sidewalk. That’s the only safe place to examine it.

Kaiserctrstone

The backing is concrete, but the aggregate that makes up the face is a pure white stone. Steel scratches it, therefore it is not quartzite, which was my first guess. It has the frosty luster of a carbonate, but it doesn’t fizz under dilute hydrochloric acid (I always carry some). Therefore it is not ordinary limestone or marble. It’s something much less common: coarse-grained dolomite, a stone with its own place in Kaiser’s history.

Whereas limestone and marble consist of calcite (CaCO3), dolomite rock or dolostone consists of the mineral dolomite (CaMg(CO3)2), in which magnesium takes the place of half of the calcium. It has many industrial uses, and the Kaiser conglomerate has mined it in several different quarries.

During World War II, magnesium metal was in great demand, especially for aircraft. The supply of magnesium ore, magnesite (MgCO3), was very tight. (Magnesite was mined at this time south of Livermore, in Del Puerto Canyon.) Kaiser sought a way to produce magnesium metal from dolomite using the Hansgirg process, but the process was plagued with difficulties. The magnesium program was known as one of Kaiser’s biggest headaches.

Henry Kaiser was closely involved in the design of the Center — indeed, it’s said he intended to live there. One of his conceits was that the materials, as much as possible, should be supplied by Kaiser companies. His architect, the Los Angeles modernist Welton Becket, was noted for his use of natural stone cladding.

Kaiserctrstoneclose

I imagine that Becket and Kaiser were pleased, each for his own reasons, to showcase the exceptionally pure dolomite from Permanente Metals’ large Natividad quarry, a few miles north of Salinas. I feel quite sure that’s what this is.

More reading:

“Looking Down on Creation” in The Monthly, Nov 2006

“Five Painterly Vistas from Kaiser Rooftop Garden” in The Urbanist, Mar 2011

The Natividad quarry (bottom of the page), at Quarries and Beyond

Working rocks

14 September 2015

Last week I made another visit to Knowland Park in preparation for my two upcoming Wild Oakland walks on September 20 (tickets still available) and October 4 (no tickets needed). I’m holding off on posting about the park’s geology until after these events, but it’s hard to wait.

Meanwhile, here’s something different. Oakland has plenty of excellent native rocks, but the days are long past when Oaklanders could shop for Oakland stone at an Oakland quarry. Today newer places must make do with more anonymous stones from distant sources. They do their jobs with stolid competence and the occasional dash of flair.

This roadside lot on Cameron Drive uses guard rocks to discourage parking and keep runaway vehicles from breaking the wall. They’re picturesque, but kind of brusque.

roadsidestones-cameron

Sobrante Park Elementary School has this splendid green boulder by its front entrance. It’s not serpentinite but, most likely, a beautifully chloritized basalt from the Franciscan Complex. The two neighboring boulders are sandstone. I think their job is keep runaway vehicles from the building, although neither Topanga Drive nor El Paseo Drive is a high-speed thoroughfare.

schoolstone

And down by Lake Merritt, the latest Measure DD improvement, the Sailboat House Shoreline Project, has made the shoreline more wildlife-friendly. The marsh vegetation will have to wait for the rainy season, but the infrastructure is in place.

lakesidestones

These boulders are undistinguished sandstone, but they’re laid with care. I predict that the gulls will be cracking mussels on them, if they aren’t already. The rocks will also allow people to step into the marsh without kicking up the reeds and mud. And they’ll keep runaway vehicles out of the lake. Hmm, there seems to be a common thread here.