Oxalic Acid — Frequently Asked Questions

By chelating the iron rather than merely dissolving it. A simple acid attacks the surface of an iron oxide deposit but leaves iron in solution that can redeposit as the surface dries. Oxalic acid binds the iron into a stable soluble ferrioxalate complex that rinses away entirely, taking the stain with it rather than redistributing it.

They are real rather than nominal. Absorbed oxalate precipitates as calcium oxalate crystals in the kidneys and can cause renal damage, and it is hazardous by ingestion and by skin absorption as well as being corrosive. Gloves, eye protection and good ventilation are required, and it should not be used in enclosed spaces without airflow.

Because most dark staining in wood is iron reacting with natural tannins, particularly around fixings and where water has stood. Oxalic acid chelates that iron and lifts the stain without bleaching the wood's natural colour the way a peroxide or chlorine bleach would, which is why it is the traditional choice in furniture restoration and deck cleaning.

Usually as the dihydrate, in white crystals or powder. The anhydrous form exists but is less commonly traded. The hydration state should be confirmed for accurate dosing, since the dihydrate carries two water molecules per acid molecule and is correspondingly less concentrated by weight.