I Found Some Rocks

I was out mountain biking and came across a possible source of iron ore. I took small samples in my pocket.

At home, I carried out some tests on the pieces to determine what minerals I got. Mostly to refresh myself on what I learned in the Making of Money. Looking at these pictures now, I should have put something for scale, a ruler or a banana or something. The biggest pieces are roughly 50 mm / 2″.

By scratching the piece with a steel spike you get a rough measure of the hardness of the mineral on the Mohs scale. Ore minerals are quite soft and can be easily scratched. There are other red rocks that are not iron ore that are hard enough to resist scratching. For example, quartz can be red or can be stained red by iron oxide on the surface but is harder than steel and doesn’t contain iron. Of course, steel comes in many flavours as well and the spike that I used doesn’t feel very hard. I ground it from an old tool, maybe a screwdriver, that I found somewhere.

Scraping the specimen against unglazed porcelain – I used a fuse for this – leaves a streak of powdered mineral. The color of the streak is important. Limonite leaves a yellow, brown or reddish streak and hematite leaves a deeper red color. I think that most of my samples are limonite and one piece is hematite. What I believe to be hematite is the small flaky rock to the right of the fuse. The brightest red streak on the fuse is from that rock. It also partly has a metallic sheen, which is common with hematite.

I touched the rocks with a magnet but didn’t feel any attraction. Hematite and limonite are not magnetic but yet another iron ore, magnetite, perhaps not surprisingly, is. However, if you take a piece of hematite or limonite and heat it bright red, it becomes slightly magnetic. I think some of it converts into magnetite but I am not quite sure what happens. I tried to look it up, but couldn’t really find any sources now. No matter, I decided to try that anyway.

I set up a tiny firebrick “oven” and placed a sample in.

I heated the piece with a gas torch, turning it around with a steel pick to get an even heat througout.

Let it cool.

Now, can I grab it with a magnet?


Another sample seemed like it was inflating in the fire. The rocks were still moist since I had just brought them inside. I think the water in the cracks of the rock expanded and caused it to swell. Only a red shell was left intact with sandy texture inside.

Here is the same roasted piece on the right. On the left is the other half of the same rock that I didn’t roast. You can see the different composition on the inside on that one too. The roasted piece turned a much deeper red color, except for the inside which remained pretty much unchanged.

The roasted part is really crumbly. From these close up shots it looks to me like the insides are quartz sand weakly held together. Quartz is common in iron ores, but this seems to have a lot of it. It has a lower melting point than iron and becomes slag in the smelting process. That is normal, but too much is too much. I would try to remove most of it before the smelt. In this case, by picking up the good stuff with a magnet and throwing the quartz out or mixing it in with the clay to make the bloomery itself.

Here’s the flaky bit that I assumed to be hematite, which I broke to pieces after roasting to show the cross section. Seems nice, especially the top surface, but I still don’t have any kind of certainty of the identifications. I’ll go collect some more of this stuff and make a small bloomery again.

March 28, 2017 | Categories:  Miscellaneous | Keywords:  ,