Removing Iron from Clay

albercook · Post by **albercook** » Sun Feb 26, 2012 12:27 pm

My elementary school students found clay in the backyard of the school and want to make an art piece for the school from it. We slaked the clay and ran it through a 50 mesh screen. I have test fired it and it makes a very deep red that I have been assuming was all from iron. We want to us a little chemistry to make a second color for the artwork.

My understanding is that Iron (III) is water insoluble or weak acids. While Iron(II) is more soluble in water and acid. I added SuperIron out (Sodium Hydrosulfite and Sodium metabisulfite with citric acid) and heated the solution to attempt to convert the iron from iron(III) to iron(II). I boiled this and let the solids settle out. The solution did turn from slightly red to light tan. I decanted the solution and added more water. I did this many times. I also used a magnet to remove particulate iron. I have added Alum as a flocking agent.

The problem is that when I fire this treated clay the result is still almost the same color red. Any thoughts would be appreciated.

I am considering using EDTA as a chelating agent.

Thanks for any suggestions

expert · Post by **expert** » Sat Mar 03, 2012 10:24 am

Solubility of Fe(III) and Fe(II) compounds in the clay is essentially the same. For simplicity you can consider itâ€™s Fe(OH)₃ and Fe(OH)₂ after the reduction. I wouldnâ€™t try to reduce. Instead try EDTA and then water wash or citric acid. In the case of EDTA addition of acid will also help. You can use vinegar or citric acid. After the wash you should bring the clay to normal pH. Maybe wash with backing soda solution or something like that.

albercook · Post by **albercook** » Sat Mar 03, 2012 1:53 pm

Thanks for the reply. I will try that. Several questions.

1) So what is going on with SuperIron out if it is not reduction for increased solubility?

2) Any thoughts on the amount of EDTA vs. clay?

3) I should probably still use a strong magnet to remove particulate iron.

4) Do you think the EDTA will remove anything else from that clay that matters?

Thanks again for your reply.

expert · Post by **expert** » Sat Mar 03, 2012 4:17 pm

1) So what is going on with SuperIron out if it is not reduction for increased solubility?
Both EDTA and citric acid form soluble chelates with iron II and III. Moderate acidic pH required for both cases. No need to reduce Fe III to II

2) Any thoughts on the amount of EDTA vs. clay?
EDTA is very effective. Find experimentally. I would think 1% or less by weight.

3) I should probably still use a strong magnet to remove particulate iron.
I don't think you have a chance of metal iron in the clay. The magnet is useless.

4) Do you think the EDTA will remove anything else from that clay that matters?
All other metals that may give color or may not, such as Ni, Ca, Mg - only free ions.

albercook · Post by **albercook** » Sat Mar 03, 2012 5:01 pm

Thanks again. I will try citric acid and EDTA. I did actually find small iron particles in the clay. Why did you think that was unlikely?

expert · Post by **expert** » Sun Mar 04, 2012 10:33 am

Iron in metal form is not found in nature. If the clay was found on a site of former construction, then it's certainly possible. Still, it's most likely that the magnetic particles are not Fe but Fe₃O₄ that forms during welding. Also, there is natural Fe₃O₄ (magnetite)
http://en.wikipedia.org/wiki/Magnetite
Some soils like in Hawaii, for example, are very rich in Magnetite
http://www.ehow.com/info_7970486_ironrich-soils.html

This is a relevant paper if you want to learn deeper
http://www.clays.org/journal/archive/vo ... -1-133.pdf

I've made a search and found that reduction of iron helps removal, so you are right http://www.clays.org/journal/archive/vo ... -1-317.pdf
You may not have access to Na₂S₂O₄ but you may try to form it in situ by adding zinc powder to your bisulfite - citric acid mix. Any residual zinc will not affect the color. I see they also use oxalic acid, another chelating agent