Oak contains higher levels of tannic acid than other types of wood, according to AntiqueRestorers.com. When this tannic acid comes into contact with water, specifically tap water which contains iron, a dark stain appears on the wood. These stains are often the result of drinking glasses that are set on top of unsealed oak tables or moisture around nail heads. Removing black stains from the oak wood can be accomplished using a bleaching solution that is specially formulated for use on wood.
Things You'll Need
Plastic drop cloth
Oxalic acid wood bleach
Put on gloves and safety glasses to protect yourself from acid splash. If possible, move the oak item to an outdoor space. If the oak item cannot be moved, lay a plastic drop cloth on the floor to protect it during the bleaching process.
Mix a solution of dry oxalic acid crystals with hot water according to the instructions on the oxalic acid package. Oxalic acid is the primary ingredient in wood bleaching solutions and is available at home improvement and hardware stores.
Dip a paintbrush into the acid solution and paint the entire surface of the wood with the acid. Make sure to cover every section of wood, not just the stained area.
Allow the acid solution to dry completely.
Examine the black stain on the oak wood to see if it is gone. If the stain is still present, apply a second coat of oxalic acid solution to the entire piece of oak wood, and allow it to dry. Continue applying coats of the acid solution until the black stain is gone.
Insert a sponge into a bucket filled with distilled water. Squeeze the sponge to remove excess water and wipe the entire surface of the oak wood to remove the acid residue. Rinse the sponge frequently in clean distilled water as you wipe.
Pour out the distilled water and add 1 qt. of fresh distilled water to the bucket. Add 2 tbsp. baking soda to the water and stir with a spoon to dissolve.
Insert a fresh sponge into the solution and squeeze out the excess water. Wipe the entire surface of the oak to neutralize any remaining acid residue and stop the bleaching process.
If the wood has been stained or sealed, apply a paint stripper to the surface and scrape it off using a putty knife before bleaching the wood.