LFPDC Publications :: Fact Sheets :: # 2

Iron Tannate Stain

The Issue

Recently, a number of people have complained of blackish or purplish stains which appear on their oak cabinets or furniture. At first glance, this looks like mildew. This problem can practically destroy the aesthetics of a new or remodeled set of red oak cabinets or furniture. If you encounter a stain similar to this, the first question to ask is "Was steel wool used to fine sand or polish the red oak?" Second, if steel wool was used, what type of stain (oil based or water-based) and finish was used. If either the stain or the finish was water-based, and steel wool was used, the problem is more than likely iron-tannate stain.

The Cause

Red oak has tannic acid in it. Iron tannate stain is a chemical stain which occurs as tannic acid oxidizes with the iron fragments from the steel wool. Steel wool fractures and leaves minute particles all over the wood. This stain typically occurs more frequently in the areas where the wood grain is coarser (more porous).

There are a few other ways to help decide if the stain is mildew or iron-tannate. The first of these is to wipe the surface with bleach, like Clorox or Purex. If the spots disappear, the problem is mildew. If not, it is probably iron-tannate stain. If you look at the doors of the cabinet, look to see if the color occurs on both the underside of the top rail and the top side of the bottom rail. If it does not occur on the underside of the top rail, it is iron-tannate stain.

When the steel wool is used to fine sand wood, small fibers of the steel wool will drop onto the floor, counter top, or lower door rails. If the top side of the bottom rail shows a much higher concentration of the stain, it is probably due to these fibers of the steel wool that have dropped and landed on the rail, and the prognosis is irontannate stain. Another visual method to determine if iron-tannate stain is the culprit, is to look closely at the spots. If the spots are odd-shaped (semi-circular or linear), then it is probably iron-tannate stain. Mildew generally only occurs as small splotches. The other way to accurately determine if mildew is present, is to look at a small section of the wood under a microscope. Mildew will have hyphae (root-like structures of fungi), which iron-tannate stain will not.

The Solution

The only way to remove iron-tannate stain from the red oak is to use oxalic acid. Oxalic acid is a strong bleaching agent, and will bleach the oxidization from the surface of the wood. It is recommended that the sealer (lacquer, varnish, polyurethane) be removed prior to using the oxalic acid for best results. The oxalic acid might or might not work through such a finish.

Author

Victor O. Harding
Assistant Specialist-Forest Products
Louisiana Cooperative Extension Service