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Comparison of the Composition of Bloomery Iron
from the Viking Age and from the Modern Era

Hurstwic recently smelted iron using a Viking-age bloomery furnace. The smelt was successful, resulting in a 11+ kg (25lbs) bloom (right) containing what appears to be good quality iron.

A slide show of a more recent smelt is here which created much higher quality material than our first smelt. More information about Viking-age iron smelting is here.

I was curious to compare the composition of our iron bloom to that of a bloom created in the Viking age.

For comparison, I used measurements of bloomery iron excavated at a Viking-age iron-making complex at Skˇgar in Fnjˇskadalur in north Iceland, a short distance above the river shown in the photo. The site was excavated in 2011 and 2012 (Gu­mundur St. Sigur­arson and Gu­nř ZoŰga, 2013), and the analysis of the bloom and related funds was done shortly after (O. Nordland, 2015)

The bloom fragment found at Skˇgar is labeled S#101 and is an irregular shape about 10x8x2cm. Our sample, labeled H001, was cut from the bloom and is about 2x2x1cm.


S#101 (from O. Nordland, 2015)


S#101 (from O. Nordland, 2015)


H001

Both specimens are highly magnetic, and the sheen of metallic iron can be easily seen in both samples after cutting.

In both cases, a section was cut, mounted, and polished for analysis. The samples show iron with many inclusions of other material, typically slag. Our sample (right) was highly porous, with numerous voids.


H001 visible light with raking illumination


H001 SEM image

SEM-EDS was used to analyze the iron portions and the slag portions of each sample separately. The process illuminates a small spot of the sample with electrons and measures the spectrum of the X-rays emitted (left).

Characteristic energy levels corresponding to various elements can be identified in the spectra, identifying the elements present. The quantity of each energy level identifies the amount of the element present. From this information, the composition of the spot being illuminated can be estimated. The spectrum shown to the left is for the measurement of one of the slag instrusions in H001. The large peak at the left corresponds to oxygen. The height of the peak allows us to calculate the relative amount of oxygen compared to other elements.

One limitation of SEM-EDS is that it cannot detect elements lighter than boron. Additionally, light elements are less well estimated. A rule of thumb is that carbon needs to be present at levels above 1% to be reliably detected and measured.

The measured composition of the iron portion of the two blooms is shown in the table. For each element detected, the weight is shown (in percentage of total weight).

   

S#101
weight percent

 

H001
weight percent

         

Fe

 

100%

 

100%

         

The measured composition of the slag portion of the two blooms is shown in the table. For each element detected, the weight is shown (in percentage of total weight). An asterisk indicates the element was not detected.

   

S#101
weight percent

 

H001
weight percent

         

O

 

48.3%

 

42.2%

Na

 

0.1%

 

*

Mg

 

0.7%

 

3.4%

Al

 

0.7%

 

8.1%

Si

 

5.9%

 

31.6%

P

 

0.2%

 

*

S

 

0.3%

 

*

K

 

0.2%

 

2.5%

Ca

 

1.3%

 

6.5%

Ti

 

0.1%

 

0.5%

Mn

 

0.5%

 

1.5%

Fe

 

41.4%

 

3.8%

         

I was surprised by the purity of the iron in each case. I expected to see more impurities, at least at trace levels.

I was not surprised to see large differences in the composition of the slag intrusions. For one thing, we did not use natural ore for our smelt, but rather an ore analog that had fewer impurities than natural ore.

S#101 had a high level of iron in the slag intrusions, something noted by Nordland. H001 had surprisingly low levels of iron in the slag, generally thought to be indicative of poor quality iron in the bloom. Yet the iron in H001 seems to be of good quality. H001 is also notable for having higher than expectedly levels of silicon in the slag.

An examination of the surfaces of the two samples in visible light after etching with nital reveals the microstructure of the iron. The green bar indicates 1mm. The H001 sample shows many different structures, suggesting many different regimes of iron formation.

 


S#001 microstructure (from O. Nordland, 2015)


H001 microstructure

In no way is our comparison the final word. As our iron smelting project moves forward, we expect to create more blooms which we plan to analyze using these and other analytical tools. We are eager to fine-tune our analyses and compare them to this and to other Viking-age bloom samples.


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ę2018 William R. Short
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