The Idaho State Capitol: Geothermal Heat and Georgia Marble

The Idaho State Capitol sits on a gentle rise at the top of Capitol Boulevard, and it dominates the downtown skyline the way a capitol building should. What you might not notice from the street is just how unusual the building is, from the rock it's made of to the way it stays warm in winter.

Construction began in nineteen oh five, fifteen years after Idaho achieved statehood. The architects were John Tourtellotte and Charles Hummel, Boise men who looked east for inspiration, to the United States Capitol in Washington, to Saint Paul's Cathedral in London, and to Saint Peter's Basilica in Rome. The result is a Neoclassical building with a copper-clad dome, rising two hundred eight feet above the street.

The exterior sandstone came from a state-owned quarry right here in the Boise foothills, at Table Rock, a formation that had been supplying building stone since the eighteen sixties, when prison inmates first quarried it for the territorial penitentiary. The stone cuts easily and weathers to a warm buff color, giving the capitol a look distinct from the pale limestone and granite of most state capitols.

Inside, four kinds of marble line the halls. Red Georgia marble, grey Alaskan marble, green Vermont marble, and black Italian marble. A family of Italian artisans created the two hundred nineteen pillars, each one technically a scagliola, a blend of marble dust, gypsum, and glue, dyed to pass as solid stone. The technique is centuries old and almost indistinguishable from the real thing.

Then there's the heat. The Idaho State Capitol is the only state capitol in the country heated entirely by geothermal water, pumped from more than three thousand feet underground. The hot water circulates through the building's systems just as it has circulated through Boise's geology for millions of years. The building that looks most like tradition is, in this one crucial way, powered entirely by the earth beneath it.