Again, a fabulous view of Volterra, which is now nearer. The Saline di Volterra, is the intersection with SR68 - it's 10 km down and 10km back. Otherwise, have a look at Palazzo dell'Orologio and get back on the road. The road gets exciting again around Pomarance and there are many things to see: Parrocchiale di San Giovanni Battista, the home of Paolo Mascagni, the clock tower, Palazzo Pretorio, Palazzo de Larderel e Sillana fort. Try and visit this in September when there is the historical event, the palio.
The SS/SR439 prepares riders for a glimpse of the Valle dell'Inferno with its fumaroles and bubbling puddles in Larderello. All your questions about geo-thermics can be answered at the Geomuseo. Just ask.
Take a picture with the geo-thermic towers nearby on your way to Castelnuovo Val di Cecina. This town even smells like the medieval ages: three stone houses, small arches and stone roads. There is even "chiassino," the narrowest street in the world (at only 50cm wide). A mountain landscape now covers the area and allows the driver to concentrate and enjoy the road.
Last stop of the tour is Massa Marittima. Nestled on a high and isolated hill this town is enclosed and protected by the well-preserved city walls. Explore the central square and visit the Cathedral of San Cerbone which unusual position breaks traditional architectural perspectives to seem like an image straight out of a painting of Giorgio de Chirico.