A lot of what I’m using for this project doesn’t require much processing. Many of the rocks that I’ve collected can simply be ground up and used for colour (and I really enjoy that fact!). But, there are a couple elements that I’m curious to explore in a bit more depth to try and understand how some of the pigments I see in art stores come to exist. One such is the class of colours called Mars Pigments.
I’ve mentioned earlier that I was able to create a nice, pure Mars Black using the pyrite collected at Kettle Point. In theory, it should also be possible to create some other bright, strong colours, such as yellow, red and brown, using this same material.
Ultimately, I’m aiming to create iron salts which are the starting point for all Mars colours. The first step towards that is turning my pyrite (which is an iron sulfide) into magnetite (an iron oxide) and the best way I know to do this is by heating it. As you can see from the photo above there is no mystery to determining if the change is happening as the pigment turns a deep brown once it has changed.
One more note: This process really stinks! After cooking this small amount of pyrite, and driving off the sulphur, I have a whole new appreciation of hell being identified in Medieval literature as smelling like brimstone (which is an alternate name for sulphur). It is horrid smelling stuff. Next time I’ll be doing this cooking outside!
Before trying anything more grand, I thought it would be a good idea to test smelting a little bit of the calcified pyrite (now hopefully magnetite) to make sure that it would melt into good metal. But, melting iron takes a bit more effort than melting lead. Consider that lead has a melting point of 327 °C, whereas iron has a melting point of 1538 °C. So my first question was: Would I be able to smelt iron at all?
I measured out a bit of my ore and poured it into a cast iron pan, setting the pan onto a tree stump. For a heat source I figured I might have a chance if I used a MAPP gas canister on a plumber’s torch (which is advertised to produce a flame of 2210°C). So I hooked it up, fired it up, and got to work.
It took a long time. At first I had to be careful that the air flow from the flame on my torch didn’t blow the magnetite out of the pan. But, by carefully hovering the torch a little way above the pan I was able to create a hard surface of slightly melted iron; then it was time to turn up the heat and wait.
It was funny, but it seemed like I had to wait just a little longer than I thought it should take on any giving spot to see the ore melt. But, melt it did. The iron pan I was using also got hot. In fact, I now have a pan-sized circle on the tree stump I was using that is pure charcoal!
So I now know that the pyrite-turned-magnetite is good for smelting. But given that that little chunk of iron in the photo above took over an hour to make, I will be needing a better method if I’m going to use iron as a pigment source …
So, I can create iron using the pyrite I collected earlier! The question still before me is whether I can create enough to be useful as a pigment? In an attempt to do so, I am going to try and create my own bloomery …
What is a bloomery, you might ask? A bloomery is a kind of smelting furnace that was once widely used. In it’s most basic form it is basically a three foot long cylinder, filled with charcoal, and having air supplied near the bottom keeping everything really hot. Best of all, (for my purposes) a bloomery produces a bloom of iron, which is a porous mass of sponge iron. This is especially desirable in my future use, where a condensed mass of pig iron would be harder to work with.
Of course it is a lot of fun to build a bloomery; you get to have conversations with your neighbours such as: “What, that? Oh, that’s my blooming furnace!” (for some reason this is really funny to me …)
Given that this is my first attempt, I’m going to first try a simple, one-time use design that I hope will work. Using an old stove pipe on a nice dry bed of sand, I’ve filled up the pipe with charcoal. In olden times, air would have been supplied by bellows; I’m not afraid of a little hard work, but I am pretty glad that I have a small compressor to do that job for me!
With all that done, I’m ready to prepare my iron charges!
In retrospect, all I can say is that preparing the iron pyrite that I’m planning to use to make my iron ore was a lot of work.
First, I needed to crush up the pyrite nodules. For that I got to work using my cast-iron mortar and pestle. This isn’t really the work that a tool this size was created for, and while it did a great job, it was a process that took all afternoon using what is intended for use as field test equipment.
Once I had a batch properly powdered, I moved it onto the next step. The powdered pyrite had to be heated, which drove off the sulphur and changed my iron sulphide into iron oxide (from that the bloomery should be able to make iron ore). Even in the great outdoors this process stank! Regardless of taking every precaution, the odd whiff would seem to hit me out of nowhere and as of this writing I can still taste it in the back of my throat.
But now the dirty work is finished; let the fun begin!
On Saturday morning I fired up the bloomery.
The charcoal was a little troublesome to get started, but once it was going it got very hot in the bottom of the furnace. Gauging the firing consists of measuring how much charcoal is burning every ten minutes. Once I starting adding two pounds of charcoal every ten minutes I began to add charges of my prepared iron oxide.
Once this got going some beautiful (but unexplained) colours began to play within the flames. While this was beautiful to watch, I also caught the odd whiff of sulphur; this really worried me because it meant that the charges still had some of their sulphate nature left and I didn’t know if the iron ore would form if it wasn’t a pure oxide.
In summary, it was an afternoon of getting sooty, playing with fire and crossing my fingers.
After the charcoal was all burned and the fire had died down I began poking around the bloomery’s floor to see what the results were. At first I was encouraged when I hit something solid with my long steel pole. And, carefully, I drew out a large mass.
But, once this mass was removed I got the sense that something was wrong. The bloom didn’t have the sponge-consistency I expected and it seemed somehow less iron-like than I anticipated. Without thinking about it too much, I reached for my sledgehammer and gave the mass a whack: It broke into many small lumps …
On closer inspection, these lumps were made up of what looks to be pyrite sand. By carefully sifting through the pile I did find a few lumps of real iron and these are very useful for my attempt to make a Mars colour pigment. But, I’m afraid most of this mass isn’t going to be much use. My theory is that the pyrite I roasted wasn’t heated long enough to completely change it to magnetite so smelting wasn’t possible because only a crust of iron oxide had be created.
In the end my first attempt at firing a bloomery wasn’t much of a success, but I’ve learned a lot and I intend to try again with better results.
It has been raining quite a bit this year and this constant moisture has had an unexpected effect that I noticed today: The sand in the bottom of my bloomery is rusting! And, to my great surprise, when I put a magnet to the mass it sprang to meet it. So, it seems that the pyrite I put into the kiln did change. The heat from the furnace completed the change of the pyrite into magnetite. I tested this theory and indeed the sand in the bottom of the kiln jumped to the magnet when I offered it.
So, after collecting all the sand in a bucket, I have put a small dish of it outside. We’ll just have to wait and see what happens …
Over the past few weeks a slow but steady change has been happening to the bowl of bloomery sand that I’ve been weathering. As you can see from the photograph above, it has become a good raw sienna colour!
When it first began to rust (changing from an iron ore into an iron oxide) I noticed that the little blooms were a bright yellow and I had some hope that an even brighter colour might result. At this time all the dark iron sand from the Kettle Point bloomery seems to have changed and the results are a bit darker. Still, for an attempt that I had originally dubbed a failure, this is something that is really nice to have …