How are smooth, non-porous surfaces like steel swords or glass mirrors purified according to the text?

Methods of Purifying Visible and Invisible Impurities

• They must be soaked in vinegar for 3 hours.
• They must be washed and squeezed three times.
• By thoroughly wiping (Mas'h) them with a clean cloth or soil.
• They can never be purified once contaminated.