October 5, 6, and 7, 2017; Iron Ore Smelt at Tunnel Mill

Created: May 14, 2018
Last Revised: April 20 2020

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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.

Right below here, are Mariah Clark's notes, and down toward the end are Pete Stanaitis's post smelt summary

Here is Mariah Clark's log of the event. Pictures still to come:
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.

The weather is less than ideal. 60░ F and humid, intermittently heavier rain with near constant drizzle.
Setting up a big tarp to provide shelter. Also, building stack upon a big slab of local stone to get us off the wet earth. Then one layer of brick, clayed in with cobs, and packed with charcoal and old fines. Add the wood form which will be burned off after it is clayed in. Clayed with cobs, tied with jute twine at each layer.

Slag arch is approximately 10"x10", above the bricks, with two bricks un-mudded for slag.
40 3/8" tall stack form, tapered from 11" diameter base to 10" diameter at the top.
18-20 brick base.

1552 - Stack mudded. Splitting kindling to fire the stack. Raining in earnest. Thunder.

One 5-gallon bucket of charcoal = 4 lbs = one full charge
Starting with "the good ore," most magnetic, hoping to make a good solid to start for other ore to stick to and consolidate.
(After the ore roasting was complete, Mariah and crew separated the ore, eliminating everything that didn't look like ore, eg: rocks:
One group was the most magnetic.
The other group was "everything else but rocks".

Note: Blower Pressures are given in inches of H20 = WC.

1210 - Lit with newspaper, kindling, and blower. No diesel.
1228 - Topped with charcoal, blower on full but at 3.5 inH20
1240 - Third bucket charcoal
1245 - Approximately 4" drop
1315 - Burning charcoal at 1"/min
1324 - Valve on blower to 45░ --> = 4.1 WC
1335 - Burning approximately 4" in 3 minutes --> backing valve to ~ 67░
1346 - First 4 lb charge of ore, topped with charcoal
1353 - 2 lb ore, topped charcoal. Flames burst through cracks, quick patching done.
1357 - Burning slow, still topped from 4 minutes earlier, so opened valve to 45░
1401 - Almost a full measure of charcoal. 2 lb charge of ore.
1405 - 2 lb charcoal
1415 - 2 lb ore
Adding charcoal gradually rather than all at once, but still at a 1:1 ore:charcoal ratio, so whenever ore is noted as being added, so was an equal amount of charcoal.

1423 - 2 lb ore
1430 - 2 lb ore
1437 - 2 lb ore - half "good ore" and half regular. Suddenly burning faster.
1444 - 2 lb regular ore (from now on, only regular ore).
        - Moving from half charges to full charges of both ore and charcoal
1453 - 4 lb ore
1501 - 4 lb ore
1509 - 4 lb ore (Time per Dion)
1516 - 4 lb ore
1523 - 2 lb ore
1526 - 2 lb ore, valve to 60░ (3/4 full flow)
1534 - 4 lb ore
1536 - bleed at full flow, ~ 4.2 WC
1542 - 4 lb ore
1550 - 4 lb ore
        - 5.1 WC

** 50 lbs in **

1602 - 4 lb ore
1606 - First slag run - beautiful gooey flow, slag arch covered after.
        - 5.1 WC
1613 - 4 lb ore
1622 - 4 lb ore
1632 - 4 lb ore
        - 5.7 WC

1642 - 4 lb ore

** 70 lbs in **

1651 - 4 lb ore
1652 - 6.3 WC
1701 - 4 lb ore
1706 - Second slag run, now leaving slag arch uncovered.
1711 - 4 lb ore
1714 - 4 lb ore (Time per Pete)
1728 - 4 lb ore

** 90 lbs in **

1730 - 6.7 WC
1740 - 4 lb ore, then 8 lbs of charcoal

** 94 lbs total - FIN **

1808 - Last charcoal has been added, blower on, burning down. Plan is to work the bloom hot, immediately.

Total Bloom = 23 lb

23/94 = 24.4% yield

- Pete notes we could easily do with five batches of cobs, not six. (Ball clay and peat moss.) "Four and a half to five batches if you're not doing the hardening process."

- No ore was clinging to the walls of the furnace.

- Coarser fracture/crystal/appearance to the slag after cooled. Mildly- to non-magnetic slag. Good gooey flow, other years finer.

- Fairly porous bloom with inclusions - similar to previous years.
- Very even heat/color/movement while being consolidated by strikers. Very dense under hand hammers.
- Interestingly, the ore was only mildly magnetic at best. After roasting, it was still only mildly magnetic.
The most magnetic ore = 17 lbs. (There was also lots of good looking but non-magnetic ore left in the roasting pan.)
We roasted approximately 200 lbs total. It broke red/black, nicely crumbly:
        - 200 lb ore into roast
        - 150 lb out of roast
        - 94 lb into smelter, 56 lb leftover
        - 23 lb good iron out of smelter

- Started with the 17 lbs of the most magnetic ore - with the idea that starting with "the good stuff" might give us the best chance of a good glob forming and providing a base for the "less good"/less magnetic ore to glom onto after.

-The Smelter was built upon a 1 foot thick slab of limestone that was about 3 feet in diameter.

- Last charge (4 lb) or ore, followed by two buckets of charcoal (8 lbs total). Allowed to burn down some, then tore down after the last charge. Immediately removed and started working the bloom.

Post Smelt Summary by Pete Stanaitis

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)ĚnH2O, 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.

Things we did different (Good):
-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.
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.

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