This was our most successful iron ore-to-wrought iron smelt to date, even though the weather was the worst we have ever had for doing a smelt.
We had a great crew.
Our biggest mistake was that we cured the outside of the furnace too much before burning out the furnace form. This caused the furnace to
crack vertically in a couple of places.
But, once we patched the cracks, we continued on with NO negative consequences.
Here is Mariah Clark's log of the event. Pictures still to come:
Thursday:
We made cobs of clay and peat moss, then let them cure under the roof of the shop overnight.
We roast ore in an open pan fire. The ore was only mildly magnetic either before or after firing. Here's hoping it is okay.
First, I want to note that I am pretty sure that the ore we use is Limonite, and I think it is certainly a bog ore.
It has occasional Geodes in it which supports my conclusion, in my mind, at least.
See this article, below. I think it pretty well describes the ore we use:
Limonite Iron Ore
(Posted on January 28, 2014 by ironore):
Limonite is another ore of iron, this time with a chemical formula of FeO(OH)·nH2O, although it can be rather variable in
compositions so could have other formulas, and is not considered a true mineral because of the variability of structure
and is a mixture of other minerals.
A lot of what is considered Limonite is actually Geothite, an iron oxide which is the most common component of iron rust.
It is named after the Greek word for meadow as it is sometimes found in bogs and marshes. As such is also otherwise known as bog ore.
The ore is quite dense and hardness is within the range of 4 to 5.5, so somewhat lower than Magnetite and the specific gravity
is between 2.9 and 4.3, which is slightly higher than average.
It ranges in colour from bright yellow to greyish brown but generally looks brownish on a white background.
The mineral is sometimes formed from the hydration of both Hematite and Magnetite; from the chemical weathering of other minerals
rich in iron or from the hydration of iron rich sulphide minerals.
It is always formed due to the alteration or solution of previously existing iron minerals and therefore,
it is often found in the run-off streams from iron ore mines. Limonite is very common worldwide with deposits notable in China, Italy, Spain and the US.
Limonite can be used in the production of iron and steel but this is much less common than for both Hematite and Magnetite.
It has been known to contain Nickel, however, and some producers have used the ore to make stainless steel.
It has been used extensively as a pigment in the past, forming yellow to brown hues and can still be used as a colouration
in paints of other dyes.
It has also been used for prospecting as it can sometimes signify the presence of gold ore.
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-We got started early with preparing the long-lead items. This gave us plenty of time for the cobs to thoroughly hydrate.
- We (Mariah) sorted the roasted ore into 2 piles:
-Highly magnetic (~10% of total?)
-Slightly magnetic
And we started the smelt with the highly magnetic stuff, thinking that we'd have the best chance of starting the bloom off well.
-Air blast:
We started off with about 4" WC and the bleed valve wide open.
The new thing was that we started slowly increasing the pressure as the charging rate slowed down a few hours into the smelt.
I think we ended up with full pressure from the blower (~6.7"WC or so) and the bleed valve fully closed.
Tapping Slag:
Not sure if this was different, but this time, we closed the tap arch after the first tapping and didn't open it again until
we were almost done adding ore.
Then we left it open until we pulled the bloom.
Things we did different (Wrong or Bad):
-Curing the outer skin of the furnace for too long a time:
This time we had a roaring fire going on the outside of the furnace for a relatively long time, maybe an hour and a half?
When I tapped the surface with a stick, the "skin" was not simply "firm" it was as hard as a rock. (BAD!!!)
By the time we finally got a good fire going inside (we had several cockpit errors here, I think), we had about 4 bottom to top cracks.
Pretty scary!
We should revisit the Ore to Axe DVD to see how long they cured the outside of the furnace before starting a fire inside.
I know think that we only need a slightly firmed up outer skin before starting the fire inside.
The problem is that, as the outer skin dries, it shrinks (no surprise). But, as it attempts to shrink, it can't because the wooden
furnace form is still in place.
Soooo----
In future, we need to get the form burned out ASAP before it causes the furnace to crack.
I am thinking that we only need 10 or 15 minutes with a decent external fire before lighting the innards.
After reading this, one might be tempted to use some "boy scout juice" to get that inside fire going, but we need to realize
that we don't want an immediate blaze in there; it needs to be a slow fire to avoid excessive thermal shock.