Removing Iron Oxide Stains from Rocks Safely with Oxalic Acid

01
of 06

The Rocks: Before Oxalic Acid Treatment

Gneiss but nasty
Click the photo for a 1200 x 1000 pixel version. Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

Many rocks are stained brown because of the presence of iron oxides. That stain can be gently removed without harming the rock using oxalic acid (C2H2O4). Oxalic acid is the preferred treatment for cleaning stains from quartz and pyrite crystals. It can be used (with care) on carbonate minerals, unlike the hydrochloric acid used in the acid test. Let me show you a typical treatment, starting with this assortment of gneiss specimens.

02
of 06

Put the Rocks in the Acid

Into the acid bath you go
Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

Oxalic acid is available at hardware stores, where it is sold as a bleach for untreated wood. It comes as a white powder that you stir into hot water. Read the instructions carefully and follow them: oxalic acid isn't especially strong, but it is poisonous and it damages metal. Using a plastic (not metal) container, cover the rocks with the solution, then put a lid on the container. The acid works slowly, so leave it overnight. The reaction produces some carbon monoxide, so ventilation should be good.

03
of 06

Oxalic Acid Treatment: Day 1

Signs of chemical activity
Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

After one day, the rocks looked the same, but the solution had begun to turn green. Oxalic acid takes the trivalent iron (Fe

3+

) in rusty stains, reduces it to divalent iron (Fe

2+

), and locks it up in an oxalate complex—an example of the chemical process called chelation.

04
of 06

Oxalic Acid Treatment: Day 3

Time to speed this up a bit
Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

After three days, the rocks were noticeably cleaner. The solution I used was on the weak side, and the rocks were heavily stained, so I stirred in more acid to double its strength. I also had been rotating the pail twice a day to mix the contents. The pros use a warm (not boiling!) acid bath of greater strength and an ultrasonic cleaner for faster results.

05
of 06

Oxalic Acid Treatment: Day 4

Clean enough for educational purposes
Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

After four days, I was ready to stop. I bottled the oxalic acid solution, which was not yet worn out, and rinsed the rocks in several changes of water over several hours. Gneiss is not a porous stone, so this treatment was sufficient. A porous rock, or fine mineral specimens, should be rinsed for at least as long as the acid treatment. If there is any doubt, use a baking soda solution to neutralize any remaining acid.

06
of 06

The Rocks: After Oxalic Acid Treatment

Rocks cleaned in oxalic acid
Click the photo for a 1200 x 1000 pixel version. Photo (c) 2012 Andrew Alden, licensed to About.com (fair use policy)

The result is a set of much cleaner specimens. Not every trace of iron oxides was removed, but that was OK for my purposes. The oxalic acid treatment can be repeated, after scrubbing off any loose rust. If the solution is dark, it may stain the specimen. In that case, neutralize the solution with baking soda before discarding it and make a fresh batch.

Oxalic acid can be rubbed onto carbonate minerals to clean rust stains, but exposures should be brief to avoid damaging the mineral.