Imagine diving into the placid surface of a painting by Vermeer, parsing apart Klimt's bejeweled surfaces, or untangling Jackson Pollock's knots of paint. Art historians, collectors, and restoration scholars have long sought to uncover the methods of great painters. Over the past decade, scientists have peered with light beneath the varnished surface of paintings to discover the chemistry of pigments, to identify the authors of unsigned works, or probe the crack depths from damage or age. Now, researchers at the University of Barcelona in Spain have used light at terahertz frequencies to uncover the hidden carbon signature of a painting previously thought to be unsigned. Though unsigned, the painting has been studied by art historians and confirmed to be painted by the Spanish artist Goya in 1771. Such secondary validation made the piece an apropos choice by the researchers, who published their findings May 14, 2013 on the arXiv . "Sacrifice to Vesta" at three