Haffenreffer Museum of Anthropology - Brown University
April 20 - 23, 2011

Archaeology & Experiment - Iron Smelting
A hands on experience in Experimental Archaeology

Short Shaft Furnace using mixed ore types
Smelt Report

Reference images by Krysta Ryzewski (used with permission)
My images are outlined in green

In April in 2011 I was able to deliver a version of the 'Archaeology & Experiment' program - at Brown University. This was thanks largely to the interest of my friend, colleague and sometime mentor Kevin Smith of Brown's Haffenreffer Museum of Anthropology. The program was organized in association with Krysta Ryzewski, who was teaching a course on the Archeaology of Materials. A number of students from the Material Sciences program also took part. The smelting activities took place on the grounds of the Haffenreffer, in Bristol Rode Island.

The participants:

Note that this list was taken from Krysta's original student work schedule.
Some of the Friday build people turned out on Smelt Day (and worked then too).
Many of those who came on Smelt Day stayed for that whole sequence.

Individuals are identified by first name only as a security consideration. Where possible I have attempted to credit those people who contributed specific materials to this report.


Smelt Master : Darrell Markewitz
Instructors : Kevin - Krysta
FRIDAY - Build & Prep
Morning Shift: Seekay - 'Slash' - Kevin - Peter ( furnace building )
Afternoon Shift: Max - Nick - Morgan - Anya ( furnace building, ore crushing, drying )
SATURDAY - Smelt Day
Morning Shift: Ian - Alicia - Julieta - Matthew
Afternoon Shift: Zoe - Jeremy - Hiu

In keeping with the historic basis of the program, the furnace chosen was the basic (Norse) Short Shaft layout with which there is considerable experience at this point. Although the basic slag management system would be as a Slag Tapping furnace, the shaft would be elevated on a concrete block plinth packed with charcoal fines.
The air system would use the proven ceramic tube tuyere, inserted proud of the inner wall and set to 22 degrees down angle. Air would be supplied using the standard industrial electric blower, controlled via a sliding blast gate.
After some discussion, it was decided to use a combination of ore types. These were chosen both for expected effects, plus to allow the students the experience of working with different ore bodies. The majority of the ore would be a lower iron content rock ore from Virginia, enriched with a quantity of both DD 1 analog and high iron content haematite grit.
Provisions would be made for measuring air volume & pressure, plus internal temperatures of the furnace.

The students would be guided by an experienced Smelt Master. The intent from the very start was to merely demonstrate, then have the participants undertake as much of the actual building, preparation and operation of the furnace as would be possible.

Note that there are considerably more images on this report than is typical - this document records not just the smelt itself, but also the teaching process.

Layout of an 'Ideal' Furnace

Furnace construction details
Clay cobb on plinth

Furnace Internal Diameter
25 cm
Stack Height (above tuyere)
50 cm
Tuyere Size
standard 2.5 cm dia ceramic tube
Tuyere Angle
22 degrees down
Tuyere Penetration
about 4.5 cm
(at start)
Tuyere Height
16 cm
above base

A note on the ORES

Rockbridge Limonite - 25 kg
DD 2 Analog - 9.5 kg
Haematite Grit - 1.4 kg
The expectation was that the two higher iron content ore materials would create the actual bloom. The rock ore was expected to contribute less iron, but to provide the working slag system in the furnace. Extensive slag tapping was expected (ideal for teaching / experience purposes). Taken together, the yield expected was in the range of 2.5 - 3.5 + kg.

Actual Furnace Measurements
Drawing by Seekay Hui

The Construction Phase:
Friday - weather bright and sunny

Overview of the working area, at the rear of the education activities building at Haffenreffer. The sand base of these 'archaeology teaching areas' proved ideal for the workshop.

After equipment set up (from the opposite end). The wooden frame would prove our saviour - combined with tarp covers as the weather turned

Preparing the (well used!) sheet metal forms that would allow for the building of a 'standard' sized furnace.
Filling the concrete blocks that form the plinth with charcoal fines.
Another group chops hay into the required short pieces, using hand axes on wooden slabs.
Mixing powered clay with the chopped straw and sand to make the cobb for building the furnace. Everyone pitches in for this labour.
The metal forms are lined with heavy paper. This is to allow for easy removal of the forms once the furnace is built up. 'You have to pack it tight!' Using the handle end of a sledge to pack down clay blocks which are not pushed down into a solid mass. 'Best laid plans...' The paper had torn away from the inner form. It was decided to simply remove it (while it was still possible) and pack the growing walls by hand.
'How big?' Illustrating ancient measuring methods based on your own body - the 8 inch / 20 cm 'span'. 'Rubber Boot Archaeologist' - Kevin Smith, currator at the Haffenreffer gets 'covered in filth like the rest of us'... Cutting the tap arch and preparing to cut for and mount the tuyere. Explaining the mechanics at the bottom of the furnace.
Completed furnace, with arch and tuyere in place. Time for measurements. Students record the dimensions of their completed furnace. When in Doubt - PHOTOGRAPH!
Starting the drying fire. One student splits wood, another assumes the task of minding a slow gentle fire. Preparing Ore - Explaining the roasting process, and both what sizes are needed - and how best to do this. Pitching in for the ore breaking. A total of about 19 kg of rock ore needs to be crushed.
Ore and PIZZA! Absorbed students work while they snatch a bite. End of Day One. A metal disk is placed on top of the furnace to hold the heat in from the coals created from several hours of a small wood fire. This will continue the drying process over night. Snorri Assists - The widest travelling member of DARC keeps an eye on the drying fire.

Main Smelt Sequence :
Saturday - cold and rain

Setting up the air system. The T connection allows both viewing, and rodding clear, of the tuyere. Into the pre-heat. Detailing the action and sorting out responcibilities. A second group starts breaking and grading charcoal. About 6 x 20 lb bags were needed (total 45 kg after grading).
Filling the furnace with rough charcoal over top of the pre-heat wood fire. Initially there is incomplete combustion, which gives a 'greasy' smoke. Touching off the volatile gasses with a piece of burning paper. There is a plume of almost invisible flame off the top of the furnace now. Induction into the Mystery : Making Jiffy Pop has the intended effect of breaking the tension!
A fast peek down the tuyere. The sliding blast gate to control air flow (roughly calibrated) and the air pressure guage can be seen. Another student checks down the tuyere. Although *everyone* was required to wear safety glasses in the smelting area, some students are more nervous around the furnace than others. (Later) An 'Up the Kilt' shot - down the tuyere view port into the glowing heart of the furnace.
Using a standard measure of graded charcoal to keep the furnace full at all times. Time to burn that measure indicates relative temperatures inside. Taking internal temperatures. Thermocouples to a digital pyrometer were inserted through small holes drilled through the furnace wall. See the chart (by Ian Brownstien) Adding ore. Explaining how best to manage the individual additions as the heat in the furnace will change movement of fuel and ore over the course of the smelt.
Recording time of charcoal and ore additions. Time point between charcoal measurements is especially important.
Early in the smelt - opening up the bottom tap arch. The bottom of the developing slag bowl (bright yellow) is easily seen.
'Rogering' out a finger of solidified slag that is starting to block the tuyere. This is a normal requirement - but...
.. what?! While attempting to clear a blockage, the entire ceramic tuyere has been driven into the body of the furnace. Frantic work starts - first to attempt to hook out the tuyere with a narrow probe down the central hole. That didn't work! Now punching a hole into the side of the furnace to pull free the trapped tuyere and mount an alternative air blast method. If the blast is not restored in mere minutes, temperatures will crash and the smelt will be lost! After the excitement. Examining the withdrawn ceramic tube.
Slag can be seen clinging to the tube, with some errosion obvious on end that was originally inside the furnace (to the right here) End view - totally sealed with congealed slag! With a metal pipe tuyere now in place, there is still problems keeping the blast clear. The solution is to lower the slag bowl. The concrete blocks are pried open at the front, exposing the slag bowl.
By scooping away the supporting charcoal fines, hopefully the hot (and soft) slag bowl will sag downwards under its own weight. Poking a small hole in the bowl can also drain out some of the collected liquid slag. Care must be taken not to drain too much, or the developing bloom will be exposed to the air blast and cut away. "Looks clear now!" - Pushing the pile of fines back under the bowl to stop the draining of slag.
Note how the remainder of the team continues working.
"At least it's Happy Slag!" Examining the tapped slag for colour and consistency. A dark black, solid glass indicates iron rich slag, a sign the smelt is on track to producting a bloom. 'Still too much' Charcoal and ore addtions continue without interuption as one worker prepares to tap some slag off, and another keeps the tuyere clear. 'That doesn't sound right.' With experience, the sound of the air blast is as important an indicator of a problem as a visual check.
Kevin Smith attempting to punch a hole in the side of the bowl to drain slag. Note how he is not looking where he is trying to strike with the hammer.

'Maybe you had better let me in there' With fast action required to keep from drowing the tuyere, more experienced hands take over.

Place - THEN strike! A small hole starts to drain slag.
Generating a large pool of hot liquid slag below the bowl. Again pushing back fines and cold ash to seal the flow. In total there were at least three tapping sequences, twice attempting to 'sag' the bowl down. About done. At this point all the ore has been added, and burn down is started. Time to organize the extraction effort.

Extraction & Compaction Phase

The Extraction Team
The Hammer Team
'Poke it!' Probing the interior to determine the location and relative size of the bloom.
Scooping out the remaining burning charcoal to expose the top of the bloom and slag bowl. Using the 'Thumper' to both partially compress and also loosen the white hot bloom in place. With the bricks of the plinth open at the front, the thumping action has driven the slag bowl down through the open bottom of the furnace. As well a large triangular piece of the lower furnace wall has broken loose. (seen at lower left)
The hot materials are quickly shoveled away. The hot bloom mass is clear as the bright yellow material. Reaching in with the bloom tongs to grab the bloom mass.
Got it!
Moving quickly over to the nearby wooden stubb. Working the surface over quickly with the hand hammer to remove loos slag and 'mother'. The Hammer Team at work - compacting the hot bloom.
'What is that other thing?' Pulling clear the lower slag mass to double check Getting a better grip and rotating to clear the tongs. Light taps with the hand hammer revealling the nature of this material.
'Its only slag!' Note brittle pieces, fast cooling (colour shift).
Final bloom ! (with reference object)
Traditional finishing ritual for the working team.

Time Construction about 3 hours
Preheat about 1 hour
plus organization
  Main Smelt Sequence 4:45 hours
  Total Elapsed Time about 6 hours
Fuels Preheat wood about 2 milk crates full
  Graded Charcoal about 45 kg
Ore Total Added 29 kg
  High Fe ores 10.4 kg
Total Bloom Weight : Approximately 2.2 kg
Yield - Against Total : About 8 %
Against High Fe : 21 %

Smelt Data Sheet

As a Working Smelt
There were certainly a number of major problems. The Limonite ore certainly proved to be of low quality, at was seen to generate significant slag volumes. It is questionable how much iron (if any) this ore contributed to the bloom. The excess slag, coupled with the slow response time of first time workers, made effective slag control difficult. This was compounded with the 'disaster' - where the ceramic tuyere was pushed entirely inside the furnace itself. Correcting for this failure took frantic action, and it was actually remarkable that the smelt was 'saved' at that point. As predicted, the primary contribution to a bloom came from the higher iron content ores. At a yield of roughly 20 % against those ores, the results are quite in line with earlier smelts.

As a Teaching Experience
The students were able to undertake the majority of the working tasks of correctly building, preparing and firing the furnace. Most of those attending responded enthusiastically and with few reservations to the working efforts. Although as a production sequence, this smelt was less than ideal, as a teaching tool the sequence provided very good experience. The struggles to maintain a working slag level allowed for many opportunities for activity. Making sudden modifications to the tuyere system served to emphasize the 'experimental' nature of this work. Finally, the creation of a solid bloom, even if on the small size, gave a great sense of achievement to those involved.

All images below by Darrell Markewitz

Remains of Working Area

Note that due to the dynamics of sheduling, it was not possible for a student activity analysing the debris fields on the day following the smelt.
View of the furnace, showing the broken lower wall segment. If patched, the furnace could be fired again. Base of the furnace, tuyere side. The extensive gaps to the upper right were caused by poor packing of the original clay wall material. Close up of the tuyere space. The larger hole had been punched through to make use of the steel pipe when the ceramic tube was mis-placed. The steel pipe used as an emergency tuyere. The portion extending through the wall is clearly seen (black fire scale). The end has erroded to the normal 22 down angle.

The Smelting Area




The Compaction Area




Photographs by Krysta Ryzewski
Data Tables recorded by
Unless noted, other text and photography © 2011, Darrell Markewitz