Iron Stain on Wood 

Iron stain, an unsightly blue–black or gray

discoloration, can occur on nearly all 

woods. Oak, redwood, cypress, and cedar 

are particularly prone to iron stain because

these woods contain large amounts of 

tannin-like extractives. The discoloration 

is caused by a chemical reaction between 

extractives and iron in steel products, such 

as nails, screws, and other fasteners and 

appendages. Steel used in contact with 

wood must not corrode. This can be 

accomplished by using stainless steel or by

coating the steel. 

Coatings for fasteners, such as galvanizing 

(zinc) or ceramic coatings, give a wide 

range of performance. Shiny galvanized 

fasteners are electroplated with zinc. The 

zinc coating is very thin and fails quickly. 

Dull-gray galvanized fasteners are 

mechanically coated and can last longer 

than electroplated fasteners; however, 

because the zinc coating has significant 

amounts of iron, staining is likely. Hotdipped (double-dipped) galvanized 

fasteners, recognized by their “globby” 

appearance, give the longest protection to 

the steel. The globs of zinc on a hot-dipped 

galvanized screw can clog the head, 

making the screw difficult to use. 

Therefore, stainless steel is the best choice 

for fasteners, particularly screws. 

Problems have been associated with traces

of iron left on the wood from cutting or 

slicing; cleaning the surface with steel 

wool, wire brushes, or iron tools; using 

finishes stored in rusty containers; and 

using iron-containing or iron-contaminated 

finishes. Iron dust from metalworking and 

even plant fertilizers can be sources of 

iron. Urine on wood floors will hasten the 

reaction with iron in wood extractives, 

producing the discoloration. 

A simple test can be used to determine 

whether wood discoloration is caused by

iron. Apply a saturated solution of oxalic 

acid in water or a saturated solution of 

sodium hydrogen fluoride (sodium 

bifluoride) to the stained wood surface. 

If the solution removes the stain, then iron 

is present on the wood. If the solution does 

not remove the stain, try applying bleach 

to the stained area. If it removes the stain, 

the discoloration was probably caused by

mildew. 

If the iron stain is spotty, try to view the 

stained wood under a 10x magnifying 

glass. “Chunky” discoloration is usually a 

result of molten metal and looks like 

clinkers from a grinding operation. Stain 

that resembles slivers or flakes could be 

from steel wool. An even discoloration 

throughout the stain indicates that the iron 

was in solution when it contaminated the 

wood, probably in a contaminated finish. 

It is easy to contaminate wood. For 

example, a wood processor routinely

treated wood with a solution of oxalic acid 

to prevent iron staining, not realizing that 

the treatment tank itself contained iron, 

which contaminated the wood. Merely

striking wood with a hammer can cause 

iron stain on some wood. It is best to cover 

the head of the hammer when nailing 

redwood and western redcedar siding. 

Oxalic acid reacts with iron tannates to 

form a colorless complex. After treating 

wood with oxalic acid, thoroughly wash 

the surface with fresh, warm water to 

remove excess acid. If all sources of iron 

are not removed or protected from 

corrosion, staining will occur again. 

In other words, oxalic acid treatment is 

only a temporary solution if iron remains 

on the wood. In time, the oxalic acid/iron 

complex will break down, permitting the 

iron to react again with the extractives and 

form a dark-colored stain. 

Note: Oxalic acid is usually available 

from a pharmacy. Always apply a 

saturated solution; that is, if the 

chemical completely dissolves, add 

more in small amounts until the water 

will not dissolve the additional oxalic 

acid. For oak, a chemical reaction 

between oxalic acid and extractives

can leave a pink stain if the solution 

is left on the wood too long. 

Caution: Use extreme caution when 

using oxalic acid or sodium 

bifluoride. Irritation and burns of the 

skin, eyes, and mucous membranes 

can occur and ingestion of a few 

grams can be fatal. Sodium bifluoride 

(which will dissolve glass) is only

available to professionals in retail 

quantities from Aldrich Chemical 

(800–558–9160). 

R. Sam Williams and Mark Knaebe 

Forest Products Laboratory

USDA Forest Service 

One Gifford Pinchot Drive 

Madison WI 53705-2398 

For more information, consult our web-site 

(www.fpl.fs.fed.us).

May 2002