Aluminosilicate containing 0.50 wt% of iron oxide as impurity yields metallic iron deposit by heating with sodium borate (3Na(2)O center dot B2O3) in H-2 at 950 degrees C, while it remains unchanged by heating without the borate under the same condition. The SEM and EDX measurements indicate that the metallic iron deposit is formed on the sodium borate glass layer. We first find that the reduction in minor constituents in oxides is enhanced by the addition of borate as a flux.