For what it's worth, the spheres I've examined have been magnetic and contained dendritic magnetite, which made nice SEM pictures. Their compositions were slightly less iron-rich than the scale - more of a typical slag composition - whereas the scale was dominated by iron oxides. I couldn't say for sure though whether they were from bloom consolidation or later stages of smithing, although it was probably largely the former taking place. There were alot of magnetic clay particles included in with the magnetic fraction from the workshop floors as well as the slag / scale proper.
________________________________________
From: Arch-Metals Group [[log in to unmask]] On Behalf Of Peter Crew [[log in to unmask]]
Sent: 11 December 2013 17:02
To: [log in to unmask]
Subject: Re: Hammerscale in smelting sites
Dear Sebastien,
The spheres from (experimental) furnaces include a very wide range of material, from cast-iron droplets through to (more rarely) vesicular fuel ash slags. I would have to go back to lots of old record sheets for the proportion of magnetic/non-magnetic spheres but the latter are probably more common. The problem with data from archaeological sites is that many, if not the majority, of primary production sites would also have debris from bloom refining, so the two sets of spherical slags/spheroidal hammer scales could be intimately mixed and a very great deal of work would be needed to identify and separate the types.
Best wishes,
Peter
From: Arch-Metals Group [mailto:[log in to unmask]] On Behalf Of Sébastien Perret
Sent: 11 December 2013 15:48
To: [log in to unmask]
Subject: Re: Hammerscale in smelting sites
Dear Peter,
thanks for these clarifications. Concerning the formation of spherical slag droplets in the smelting furnace, I wonder to what extend they would react to a magnet... Of course, they may contain metallic iron, but being formed in a reducing atmosphere, shouldn't we expect magnetite to be rather rare in such slag drops, in opposition to flake hammerscale from smithies, for example?
Regards,
Sebastien
Le 11.12.2013 16:01, Peter Crew a écrit :
Dear Sébastien,
Some interesting points have arisen from this discussion of bloom smithing debris.
The 'smithing flats' to which Sarah refers are not in fact large pieces of 'flake scale' but are quite large pieces of slag, up to 10mm thick, with one surface flattish and the inner surface quite irregular. This type of debris forms at the earliest stage of the refining of a slaggy bloom. Typically the flattish surface can be cracked and distorted. We have made these experimentally and they were found on our excavations both at the prehistoric sites at Bryn y Castell and Crawcwellt, but not at our medieval site at Llwyn Du, despite the much larger quantity of material being refined at that site. I suspect that this is because the blooms at Llwyn Du were removed from the furnace whilst still hot and then refined immediately, so the slags being hammered off at the early stage are still quite soft and don't develop the flattish surfaces. Thus we regard this type of 'smithing flat' as diagnostic of smithing small blooms which were removed from the furnace when cold, still with a quantity of slag adhering to the bloom.
In fact, we do have some very thick pieces of flattened scale from Llwyn Du, typically 4 or 5mm thick, but we don't really know how they were formed. The implication is that perhaps the billet was re-heated for quite a long time for the oxidised zone to develop, but there doesn't seem to be a logical explanation for why that should have been done. Maybe a piece of billet was left in the hearth by accident, and refined later on.
There are several points relating to the survival of hammer scale on archaeological sites. It is very friable and can fragment to magnetic dust, thus hard to detect. From systematic sampling, sieving and floatation (for charcoal), at Bryn y Castell, we found that up to one third of the total slag weight can be in the form of very small debris (less than 5mm and mostly magnetic). So, if this material is ignored the hammer scale and the total slag weight/site production will be under-represented. The proportion of this very small debris would probably vary from site to site. See my 1988 paper on the quantification of the smithing debris from Bryn y Castell.
There is also the possibility that hammer scale, and other magnetic/iron-rich material, would have been collected and re-cycled. This is recorded historically but would be difficult to prove archaeologically of course. As an example, at Llwyn Du where a very large number of smelting/refining cycles were carried out, there are very few surviving smithing hearth bottoms (all iron-rich) and only small quantities of surviving hammer scale, implying that re-cycling was carried out on a systematic basis.
Finally, can I make a general plea that we should cease to use the nonesensical term "spheroidal hammer scale", which I was glad to see that you did not use. This term is incorrect in morphological terms (scale is flat, spheres are round) and it has a functional implication which cannot easily be sustained. Slag spheres are also a common product of the smelting process - droplets of slag liquating from the bloom and cooling in the furnace bed, with a wide range of morphology, sometimes in large quantity and of a similar size range to refining spheres - so spherical slags are not necessarily diagnostic of refining or welding. There is another complication that thin flakes of magnetic slag can also be found in smelting debris, from the broken shells of vesicular slag - when broken to small fragments this material can easily be confused with hammer scale. We call this material 'curly scale'! Again, see the 1988 paper for a brief description and discussion of these points.
Best wishes,
Peter
From: Arch-Metals Group [mailto:[log in to unmask]] On Behalf Of Sébastien Perret
Sent: 10 December 2013 09:39
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: Re: Hammerscale in smelting sites
Dear All,
Thank you for your very helpful responses. I had a look at the papers from Patrice and Peter Crew, and both seem to suggest that flake hammerscale is more likely to occur in the later stages of the bloomsmithing process, which indeed makes sense to me now. In our case, spheroidal hammerscale predominates in all samples rather clearly, but always occurs together with some flake hammerscale. Of course, I agree with Tim's word of caution, and we certainly can't pretend our hammerscale samples are representative of the whole site ; however, I guess this assemblage would fit the assumption that the blooms have been consolidated to more or less compact billets directly at the smelting site...
Kind regards,
Sébastien
Le 10.12.2013 08:29, Patrice Rijk a écrit :
Dear Sébastien,
Some 20 years ago I carried out a bloom smithing experiment in Lejre. During this experiment I collected all slag. The experiment showed that spheroidal hammerscale and larger slag pieces were mainly formed during the first bloom smithing stages whereas flakes formed when the iron was already (partly) consolidated. In general, all types of hammerscale can be found during all stages of bloom smithing. It is the ratio between the types of hammerscale that allows to differentiate between the different stages of bloom smithing, i.e. between working bloom and consolidated bloom. You can find the article (in German) on academia.
Kind regards,
Patrice de Rijk
Dr. P.T.A. (Patrice) de Rijk
ArcheoMedia BV
Essebaan 7
2908 LJ Capelle aan den IJssel
Netherlands
Tel.: +31 (0)10-2582360
Fax: +31 (0)10-2582325
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Département de Géosciences
Chemin du Musée 6
Université de Fribourg
CH-1700 Fribourg
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Département de Géosciences
Chemin du Musée 6
Université de Fribourg
CH-1700 Fribourg
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