Saturday, July 04, 2015

Furnace from BC/AD

I scammed this image from the 'Iron Smelters of the World' Facebook page
Originally contributed by
Timm Datengeiz 
via an image from Archäologisches Museum Hamburg

Measurements supplied by Michael Schmidt Nissen :
 It's 100 cm high, inner diameter at the height of the 4 tuyeres is 41 cm, inner diameter at top is 22,5 cm. Outer diameter at bottom rim is 50 cm.

At 100 cm - it might be almost high enough to allow for a passive air flow.
Beautiful furnace, from the rest of the archaeological block a slag pit type.

Friday, June 26, 2015

Icelandic 6 - excavated

Monday, with the remains of Saturday's smelt expected to be cool, I 'excavated' the remains.
Top view (tuyere to left)
3/4 view (tuyere to left)
Neil Peterson's correction from yesterday (see comments) - that the earth was seen to collapse significantly more as the furnace was cooling, had continued overnight. The earth surfaces would fall away at the lightest touch. This would distort the inner surface from the shaft size present during the smelt itself.
Some idea of how much material collapsed is seen in the 3/4 view above. At the rear of the shaft (opposite the tuyere) as much as 5 - 6 inches of material had collapsed in. The 'average' collapse was bout 3 inches. This material was baked visibly to a reddish brown colour (all organics missing).

Loose debris removed to expose slag mass (tuyere to left)
 Eventually I was able to scoop out all the loose debris (ash, charcoal, loose slag pieces, collapsed earth).  For the archaeologists, this was done by hand 'polishing' the bottom surface of the shaft. I did not touch the walls themselves other than to remove a couple of the top stones (to keep them from falling).
As expected from the burn pattern during the smelt, there were larger charcoal pieces remaining in the back 1/3 of the furnace. (Some of these were larger pieces from the first filling with ungraded fuel - and some still burning slightly.)

To free the slag mass, the front area above the tuyere was cut away. First the cap stone was lifted, then the sod pieces over the tuyere down to the tap arch top stone were cleared.
Then the sods / earth from the sides were cut back to form an open slot, down to the top lintel stone of the tap arch. At this point the slag mass was removed as one piece. 

Front view (towards the tuyere)

Rear view - from the back of the furnace

Side view - Right from the front of the furnace

Top View (tuyere seen to left)
Field drawing with measurements (imperial / not to scale)

What remains is a very large mass - the expected bloom still locked inside the slag.
• The bowl itself has formed a bit lower than expected. The 'soft base' of charcoal fines was set at 4 inches / 10 cm below the lower edge of the lintel stone, but you can see the slag burned down a further 5 cm. This puts the bottom of the slag bowl roughly 10 inches / 25 cm below the bottom edge of the tuyere.
• The usual hot spot in an oval pattern around the tuyere has resulted in a wall of slag, fusing earth behind it. It is a circular pattern, roughly 7 inches / 18 cm above and about 8 1/2 inches / 22 cm to each side of the tuyere. This wall varies in thickness, thinner at the top and side edges, to about 1 1/2 inches / 4 cm at its centre.
• The slag bowl penetrated to about 9 inches / 23 cm from the front of the furnace. This left a gap about  5 inches / 13 cm from  the back wall. Some fragments of partially sintered, but reduced, iron were gathered from this rear area. It does appear however that the burning pattern inside the shaft did in fact pull ore into the heat zone (no ore fragments were recovered).
• There is a central pillar of slag rising above the normal flat surface of the slag bowl. This pillar is about 4 inches / 10 cm wide at the base and about 6 inches / 15 cm high, coming to a rough point.
It is thought that this slag was dragged up off the liquid slag bowl during attempts to hook the edge of the bloom during the failed top extraction attempt.
 • The ceramic tuyere has a final length of 6 1/2 inches / 16.5 cm, from a starting length of 8 inches / 20 cm. This represents only about 3.5 cm of erosion, more typical has been 4 - 5 cm in past smelts.

Almost all of the slag has been recovered. There are surprisingly few 'gromps' (partially sintered and reduced iron encased in slag that did not fall to incorporate into the bloom).
It is likely that most of the slag present is the result of silica mixed with iron from the ore. In this case there was no clay involved, which usually does erode to add to the slag produced.  One remaining task is to weigh all the slag produced.


At this point it has been decided to ensure the size and weight of the slag mass is recorded. To determine the presence of the likely bloom, it will be necessary to break up the slag to free the metal.  (This most likely to be attempted over the weekend - not an easy process with the slag mass cold!)
The hope is that the comparison of ore against slag and bloom produced will be of some use in understanding the production cycle estimated at Hals.

Wednesday, June 24, 2015

Icelandic 6 - the Smelt

Here are some actual photographs from the smelt itself:

Down the interior of the shaft - with the metal form removed.

Furnace - front view. Stone framed tap arch below.

Working area - The build was framed 2 x 2 m by rail ties.

Start of main sequence. Ceramic tuyere installed with air supply. Tap arch blocked with sods.

Into main sequence, ore analog added.


End of smlet : View down the opened tap arch, showing the slag bowl in place.

Two views down the inside of the furnace at the end of the smelt:
Showing the variations in temperature.
Highlighting the hot spot of the bloom.
As was remarked in the comments on the last posting (by Neil Peterson) - As the charcoal level dropped during burn down, the earth walls were seen to collapse on to the top of the descending charcoal. The result was very heavy erosion of the earth walls.


One overall problem was a lack of full consideration of the diameter of the furnace - and what that implied in terms of both charcoal requirements and air volumes.
Our normal furnaces vary from about 25 to 30 cm internal diameter. This build started using a 33 cm diameter metal form (1). After the short (30 minute) pre-heat, the walls had shrunk back somewhat, resulting in an ID at the start of the smelt of  36 cm.

A) Effect on air delivery volumes:

ID = 30 cm : area at tuyere = 700 cm2
air required (at 1.2 to 1.5 l/m/cm2) = 840 to 1050 l/m
ID = 36 cm : area at tuyere =  1020 cm2
air required (at 1.2 to 1.5 l/m/cm2) = 1225 to 1530 l/m

With the start of the main sequence (charcoal addition)  the available electric blower was set wide open. (That volume is thought to be 1400 l/m, but this has not been tested 'in line'.)
Another variable is delivery pressure, which in this case could not be modified. (Pressure effects how far into the charcoal mass of the furnace the air will penetrate.) The concern was that even if sufficient air volume was available, the air might not penetrate right across the furnace diameter.

B) Charcoal requirements :
Although the height of various furnaces varies, internal volume is a cube function.
Our typical furnaces require 4 - 5 standard buckets (about 1.8 kg) of charcoal to fill.
For this build the volume of charcoal needed was almost double - a total of 10 standard buckets.
This would certainly effect the 'hang time' of any individual ore particle. (The time the material was inside the reduction chemistry of the furnace.)
The consumption of a standard bucket of charcoal was inside the normal range considered effective on past bloomery iron smelts. The furnace ran a bit hot at first, at a rate of 6 - 8 minutes per bucket. This shifted (as expected) with the addition of the first ore charges. The average was about 10 minutes per bucket, increasing slightly with the later additions of larger ore charges. Ore charges started at 1 kg per bucket, and increased smoothly to 2.5 kg per 1.8 kg charcoal at the final stages.

C) Tuyere :
One effect that was not anticipated was the shifting of the tuyere angle over the course of the smelt. At the start of the smelt, the tuyere was set to the standard 22 degree down angle. However as the smelt progressed, this angle flattened out considerably. By the 2 1/2 hour point into the main sequence, the angle was reduced to 17 degrees down. Measured at the end of the smelt, the tuyere angle was at only 12 degrees down.
This appears to have been caused by the sods collapsing as they dried / organic materials burned away. The tuyere had been placed so that the interior end was resting on the stone slab lintel of the tap arch, but the outside end was only supported by grass sods.
This shifting could have been prevented by placing a small stone support on the outside end of the ceramic tube used.

Next : 'Excavating the furnace & extracting the slag / bloom mass

(1) The reason for using the larger form was an initial intention to line the sod shaft with a thin clay layer. This was suggested by clay fragments found at Hals, ranging up to as much as 2 cm thick. With that clay liner, this furnace would have in fact had an ID closer to 30 cm. Time (and weather) limitations resulted in this clay liner not being added to this furnace.
A successful test of a thin clay liner in an earth cut cylinder was made with 'Icelandic 5', May 2012

Monday, June 22, 2015

Icelandic / Hals Iron Smelt - full build

Readers may have been following the recent progress of the bloomery iron smelt based on the work of Kevin Smith at Hals in Iceland. Hals was a long duration, 'industrial' level iron smelting site, dating from the Viking Age.

As described in recent posts, the experiment on June 20 was to build and operate a full version of the type of iron furnaces at Hals. These are basically a cone constructed of grass sods, the shaft dug into the centre of that cone. The cone would have been framed in a box made of timbers. The space above the cone would have been levelled off with earth, creating a flat working surface around the top of the furnace shaft.

In a departure from the usual reporting method, here are a pair of illustrations of the smelting furnace during the later part of the main sequence. Illustrations by Margaret Gissing:


• The interior diameter of the final build was in the range of 33 - 35 cm.
This represents a considerable (!) increase in volume over our standard layout (at closer to 25 cm).
• Stack height (at the start) was 50 cm.
• Tuyere was the standard ceramic tube (at 2 cm ID), set at 22 degrees down, about 6 cm proud.
• Air was supplied by the normal industrial electric blower, in this case wide open to maximum.

• A total of roughly 31 kg of slightly enriched DD1 ore analog was added.
• Total charcoal consumed was roughly 76 kg.
• Total elapsed time (from short pre-heat to extraction) was about 7 hours.
• Ore charging followed a fairly standard sequence, increasing smoothly from 1 kg up to 2.5 kg per charge.
• The consumption rate for a standard charcoal measure (graded maple at 1.82 kg each) was an average of 10 minutes.

• Initial extraction attempt was made via the top of the furnace. It was clear that there was a nugget of bloom present, but it did not prove possible to get a hook on to the bloom itself to pull it free of the slag bowl.
• At this point an attempt was made to extract via the tap arch. It became clear the slag bowl had solidly attached itself to the stones framing the tap arch.
• Tapping with a steel rod from the top suggests the bloom in place is a good size - perhaps as large as 15 x 20 cm. (Admittedly, this is a vague impression at best - and may represent wishful expectations!)
• A decision was made to leave the slag bowl with bloom in place. The overall intent of this experiment is to measure the functioning furnace so as to be able to gain insight how the excavated remains at Hals reflect the working furnaces there. This will be done best by more carefully examining / 'excavating' the complete furnace.

One thing that became obvious as the empty furnace cooled. The bare, dry earth walls started collapsing inwards as soon as the supporting charcoal burned away.

Expect a more detailed examination / photographs of both the smelt and the final furnace, shortly.

 

February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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