Testing Iron purity in Red Iron Oxide. How to do Loss on ignition test.
I do quite a bit of glaze testing. When I'm developing a glaze with R.I.O (Red iron oxide) I often wonder to myself, “how pure is this iron?” I’m so picky bout sourcing my materials. Of course we all just buy whichever distributors iron we are most comfortable with or is the most convenient at the time … But not me. I pay close attention to my materials. If a glaze comes out differently within the same load, clay, and the materials are all the same except a shiny new bag of Red iron oxide, I know something is up; Something changed. Every other bag we clay people buy says “high purity” on it. Two questions; How can I trust the distributor? It is after all in their interest to say it's high purity and most of them will look at you sideways if you ask for a chemical analysis from the source. Secondly, what is the source or sellers definition of “high purity”? From my point of view 90% purity is ok but any less is bad. From the source / manufacturer’s point of view 90% might be considered high. Most people would consider 90% high on a test, right?
Not for me. I think about it like this; If I put 100 grams of Red iron oxide into a glaze batch and it’s only 95% pure, I might as well be putting 95 - ish grams in. Veteran potters will watch you fail at trying to make a good iron glaze and say “you know what the problem is, dontcha? You need a high purity iron.” When you ask, “ how do I test for purity in iron?” they look at you as if you just asked the meaning of life. Well i’m here to tell you testing for iron purity is very easy. A lot of my red glazes are very dependent on how pure my iron source is. It is very frustrating to make a glaze that partially depends on high purity iron, only to see a noticeable difference between the last bag of iron oxide to the new one. You can’t tell just by how red the hue of the iron is in the bag, you can’t trust the distributor, you need to test it yourself. Luckily for us R.I.O is close to 100% iron oxide.
Other materials like feldspars or spodumene aren’t very “pure” They have different elements they are comprised of, but not iron. Iron needs to just be iron, anything else within the material especially if its carbon based can, will, and should burn away. This is great because that means if we can find a way to burn off the things in the material that are not iron we would just be left with pure iron, then when we buy iron from the supplier we could test it. If only we had a way to heat the material up to that point………. Put that bish in the kiln. Here is what has been working for me and (if you don’t have third party testing or a fancy chemical analysis machine) is what you are going to do.
Get a gram scale (you should already have this because you are probably making glazes)
Get a small, already bisque fired petrie dish or bowl. It needs to hold about 100g of mineral
Put 100g of Red iron oxide in this buiqsue dish. It’s important you DO NOT TARE THE CONTAINER THE R.I.O is in. When you remeasure this on the gram scale you will keep the weight of the container. Otherwise you’d have to transfer the material and you might lose some iron in the process. The weight of the container should be included. If you have a container that weighs 300g and you add 100g of R.I.O you should be putting 400g total weight in the 022 fire.
Fire it at cone 022 (the lowest a kiln will probably fire). If you have a small tester kiln this is easy. It takes a day even with cool down.
When it comes out of the 022 fire weigh the container again. You should have less then the weight you put in. In this situation we have a 300g container, we put 100g of R.I.O in so we have 400g total weight in the kiln. When done firing we should have something like 390g by the end. Which means we lost 10 g from the firing. That 10 grams was taken from the 100f of r.i.o we added in.
Theoretically the firing process should burn off everything except the iron oxide and contiainer itself. You should be left with between 90 - 100g of iron. This is called loss on ignition or L.O.I
In our example we put in 400 g (300 g container + 100 g of iron oxide) You will notice you are getting 390g back (which means 10g burned off.) Or you will get 98 g back (2g burned off). This technically means even if you put 100g of iron in a glaze there is some percentage of organic matter that burned off; organic, non-pure, non-iron matter burned off, leaving you with just iron. The goal is to get a higher number back. If you go through this process and you get 399g back (meaning 1g of non iron material burned off) this means you have good, high quality 99%-ish iron oxide on your hands.
But if you go through this process and the scale reads 92g (you burned out 8g of non iron material) it’s trash; no matter what the bag or distributor says.
After making glazes that change with the difference between 2-5g or iron per every 100 g I started to notice my colors were off. I keep good track of my firing process and my glazes; those things are very consistent. I also started to notice different types of iron. Not only in color but in hue. One bag of R.I.O was a lighter red then the other I bought last year. The iron must come from different pits if not different distributors. After so many changes in my glazes and noticing this hue or color change I thought “there is absolutely no way these are all the same purity of iron”
Most of this blog is to teach you how to use LOI to test a material’s purity. But I do have to admit an unfounded conspiracy theory I have about RIO. We as ceramic artists are in a secondary market. By this I mean we are not the only ones that buy iron in this form. Not only that but we are not even the primary buyers; We are small frys in comparison to the make up, metal, and electronic industry. This means most of us get whatever we can from the ceramic shop without question. We technically do not NEED a better red glaze. Our want for high purity red iron oxide on clay is not fueling the worlds infrastructure, food, make up, or supplements.
This leads me to believe that we, ceramic artists, are being supplied with one of two things with respects to RIO.
There is such a high usage and turn over rate of Iron it makes it difficult to source a constant, standard, high purity source. So we often get random or random purity iron.
Or
2. We are getting the left overs of iron because we are not a primary industry. They probably source or buy it first and more frequently. Even if we were getting the same iron as bigger industries, most of the important ones such as food, medical, and construction probably test each batch for purity because systems depend on it . In comparison to us who are using L.O.I tests at best. I’ve only been doing this LOI test for 2 years and most potters I know do not do this.
This is the reason I use yellow iron oxide. I believe its’ turn over rate is much lower than that of Red iron oxide as it’s not used as much. On top of that I buy it from on supplier in large increments so I know it's consistent; Even if it’s not pure it’s from the same batch and pit.
Before I end this blog I want to make clear that I have no proof of the latter part of this blog. I will fully admit I have no proof of the ceramic art world being given low end RIO in place of the good stuff because industry buys it either more frequently or first. That’s why I call it a conspiracy theory. I have wishful correlations, but that alone is not proof. I felt it necessary to include this part because I want to be honest but also want to share the main reason I started to use and test different forms of iron. I believe it is partially what helping my iron saturated reds come out consistent.
Lastly I'd like to say I have been making glazes with calcined or fired through the loss on ignition iron and I see very little difference in color as long as the iron I'm working with is about 90% purity. Other iron I am working with have a slightly less red. When tested the lesser reds turn out to be amount 85% iron. Mixing your glaze Welland application has a much stronger effect than does the difference between 90% - 100% iron purity.
P.s The real weight of my test in the video was 432 not 431 (I rounded down by 1 g to make the map easier so realistically speaking The purity of my yellow arnoxide is most likely 89%)
