So you’ve just had your oak floor fitted and you have sealed it with oil, and for some unknown reason there are now black marks on the boards. This occurs when the tannin in oak touches a wet or acidic environment and it causes a chemical reaction which leads to dark discolouration. The discolouration is caused by the following equation: Gallic acid + water + iron (III) iron (III) –gallate + acid.
Interestingly this same chemical reaction was used in the middle ages to create the Gal ink to write with. The iron (III)-gallate-complex has a dark colour and it is therefore responsible for the stains on the oak floor.
To prevent black or dark stains appearing on your oak floor prevent any metal or metal furniture from coming in contact with your oak flooring. Check all furniture for metal studs. Best would be to remove the studs and replace them with natural fiber pads. - See attached pictures.
Unfortunately the only way to remove the marks is to sand the floor back and re-finish the floor.
