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A new preparation route to the intermetallic clathrate-I compound Na2Ba6Si46 is introduced, which allows one to make large amounts of product with standard laboratory equipment. The precursor Na2BaSi4 is oxidized with gaseous HCl at 673 K to Na2Ba6Si46, NaCl and BaCl2. Full-profile refinement of the crystal structure from the X-ray powder diffraction data revealed a composition close to Na2Ba6Si46 (Na1.94(1)Ba6.06(1)Si46, space group Pmbar 3n, a=10.281(1) Å). Differential scanning calorimetry showed an exothermic effect at 874 K, indicating that Na2Ba6Si46 is metastable. The product was additionally characterized by scanning electron microscopy. The electronic structure of Na2Ba6Si46 was investigated by a first-principles, all-electron full-potential method, predicting metallic conductivity. Na2Ba6Si46 obtained by oxidation with HCl shows Pauli paramagnetism; no bulk superconductivity was found down to 1.8 K in a magnetic field of 20 Oe.