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Saturday, October 29, 2011

Monday, October 24, 2011

Measuring the HEAT...

Iron Smelting Furnace Temperatures
Short Shaft over Slag Pit

October 9, 2011 / DARC Smelt Team, Neil Peterson recording

On our last smelt, we set up to record furnace temperatures over the duration of the experiment.

The furnace was our standard short shaft type, roughly 25 cm interior diameter, 70 cm total height.
Walls were clay and straw cobb, about 10 cm thickness.
Fuel was hardwood charcoal (mainly oak) graded to .5 through 2.5 cm diameters.
Air volume via the tuyere (set at 20 cm above base) was roughly 800 litres per minute.

Holes were drilled through the furnace walls at roughly every 10 cm, starting at 10 cm above the interior base.
Measurements were taken using an industrial quality digital pyrometer (Model HH12B from Omega with type K bare wire thermocouples).
The probes were inserted roughly 5 cm beyond the interior surface of the furnace wall.
Measurements were taken roughly every hour over the course of the smelt event.

Because the probes did not reach into the central core of the furnace, there is every possibility that the central furnace temperatures were even higher than what was recorded.
Our thermocouples failed (melted!) at roughly 1350 C. On several recordings, this temperature was reached.

Image : Neil takes readings, early in the smelt

Time Elapsedbasetuyereplus 10plus 20plus 30plus 40top

10 cm20 cm30 cm40 cm50 cm60 cm70 cm
13:501:503281051plus 1350119511891014660

11241265 *909700719

Note: It has been suggested by some theoretical researchers that temperatures above 1200 C are impossible to achieve inside a charcoal fired furnace...

Cross posted from Hammered Out Bits

Friday, October 21, 2011

Thanksgiving Smelt - fast overview

'Celtic Iron Age' slag pit furnace
October 9, 2011
DARC smelt team
(Darrell / Neil / Marcus)

Showing the initial layout of the 'pit'. A standard 20 L plastic pail was surrounded by dirt, then filled to top with cut willow branches (about 0.5 - 1 cm diameter). Use of concrete blocks would allow for easy excavation after the experiment.

Our standard short shaft furnace is constructed on top of the pit. Clay with straw cobb, 25 cm ID, 70 cm tall. Ceramic tube tuyere (2.5 cm ID), electric blower.

Total time : 5 3/4 hours
Total charcoal : 57.5 kg
Total ore : 48 kg

Slag block as excavated (furnace itself was removed in one piece and retained for further use) There was no actual bloom recovered!

A fragment of the slag block, showing how hot slag had dripped down between the sticks, solidified, the heat converting the wood to charcoal. This from the front side of the furnace, indicating lack of iron (pale green colour). Slag to the rear of the furnace was a black iron rich colour.

The purity of the ore was questionable.
There is a chance some iron may exist trapped inside the slag block. A check with a magnet at the usual location (under the tuyere) did not indicate any however.
It is possible that the existing iron rich slag might be recovered, then utilized in a second smelt attempt.

For now we want to retain the slag block itself as a sample.

The extracted slag block. In this shot the tuyere is located to the upper right, directly above the scale vertical line. The colour shift in the slag from the rear to the front of the furnace is easily seen. There is an extra bulge in the slag about at ground level (the clay furnace sat directly on the loose dirt here.

A full report is in the works!
(duplicate from Hammered Out Bits)

Saturday, October 1, 2011

Thanksgiving IRON SMELT at Wareham

One of the traditional iron smelt events at Wareham is over Thanksgiving weekend. This is 'Darrell's Smelt' (originally a sad replacement for Early Iron after that event was dropped). The DARC team normally takes part.

Because Thanksgiving is a family day for many, and because some people get involved with the archery stuff in KW also that weekend, Smelt Day is SUNDAY.

The tentative plan for the weekend will be this:

Saturday - Furnace Build and Open workshop (self directed)
Sunday - Smelt Day
Monday - Evaluate and clean up

Vandy and I will be prepared to welcome guests any time after 9 AM.
The primary working day is SUNDAY, for those intending on a one day trip.
As usual, this is a 'limited open' event - please drop me a post back if you are intending on coming up.

Going into 2012, I want to work towards a new furnace type. I have proposed to Goderich Celtic Festival that I undertake a smelting demo at their event next August. This would be a Celtic Iron Age, slag pit type furnace. The style applies to early Danish and Anglo Saxon as well.
The upper portion of the planned furnace is going to be much like our standard types (short shaft, clay cobb construction). I have a wide number of ore types on hand, and have not determined which I may use. (Likely one of the Virginia rock ores, as I have considerable of those materials.)

Any working advice from our friends in England and Denmark, who have worked with these type of furnaces, would be helpful!