Peter,

One. How does he know it was Roman?  Slag was often used as ship's ballast
Two. Are you sure it is slag?  Hard-pan, bog ore can look like slag, so 
can other rocks when weathered.


	The procedure for slag identification is first look at the morphology 
of a sensible sized sample of the collection (that is whole of a small 
collection, or a sample of a large deposit). With experience this will 
indicate if the material is the result of smithing or smelting, it might 
indicate where it is associated with copper or iron working - but not 
necessarily. At this stage you can all determine what is normal and 
abnormal in the collection. The abnormal may be important in tell what 
was going on so don't ignore it. Often for iron smelting and smithing 
sites this is sufficient to work out what was going on and a lot more if 
you are lucky. But if you only have piece identification of process is 
in many cases is impossible by morphology alone - there are exceptions 
but that is a complete lecture in its own right.
	Next, if there are any problems or you are unsure of your 
identification, it is necessary to cut and polish optical sections. This 
would immediately confirm whether it was slag or natural geology - or 
something else. Reflected light sections are usually sufficient and for 
the determination of whether the material is associated with copper or 
iron it is necessary in preference to optical thin sections. Optical 
thin sections can look great and for those with the training give a lot 
more information. Optical microscopy should be used in preference to any 
form of chemical analysis in the first instance, as it tells you whether 
copper is present, and whether it is in metallic, sulphide or oxide 
form. It also tells you the type of mineral phases that are likely to be 
present, (a thin section will give a better identification of the phases 
but this is not always necessary). The distribution and morphology of 
phases which tells you about the way in which the material formed.
	Finally, chemical analysis can be used to answer specific questions - 
the composition of metallic inclusions, the chemistry of phases, overall 
composition. This information may help sort out problems such as the 
sort of ore used, etc.

	The use of chemical composition alone is not a good idea as  copper 
smelting slags can have a very low copper content which can be missed. 
Iron slag can have copper present as a result of the original process or 
the result of post-depositional changes. Iron smithing slag 
s can be cross-contaminated as a result of the same hearth being used 
for non-ferrous metal-working.

	I sorry that this might sound a little negative as far as chemical 
analysis is concerned, but most bulk slags (of copper and iron) tend to 
have chemically similar compositions. This is due to the nature of the 
conditions in the furnaces involved and that there are relatively few 
places in the relevant multi-element oxide phase diagrams where slags 
can form free flowing (or relatively free flowing) liquids at the low 
temperatures available in the furnaces of the periods involved.

	I could go on at length as to how to get the most out of archaeological 
slag (and do occasionally), but to interpret slag you do need to 
consider the both archaeological context (local and period) and the 
metallurgical aspects of the physical chemistry of their formation, and 
with smelting slags a knowledge of their geological context is useful. 
That is if it is a slag.

Tylecote's 'A History of Metallurgy' only gives the composition of 
smelting slags. 'Slag' is produced during primary and secondary iron 
smithing, the composition of these slags and slag-like materials can 
vary very considerably depending what was being done, the composition of 
the fuel, and the hearth lining tuyere and how good the smith was. These 
slags can vary in morphology from material looking like tap slag through 
to a poorly consolidated iron-rich sandstone. Some of these would fall 
into the composition range given. Smithing hearth bottoms and 
hammer-scale are the most characteristic products of smithing, usually 
these have high iron contents than smelting slag. The lower density slag 
  forms associated with smithing and other high temperature 
pyrotechnical processes can have iron contents in the range you give.

Usually one has to handle the material, as density, surface texture, 
hardness, colour, reflectance, and even smell can be important in 
working out what you are looking at. In this case, given the 
composition, and the present information, this material could be 
anything from a wide range of materials - quite a lot of them not slags 
or clinkers.

	To limit possibilities I would need more information, an image might be 
of useful.
	

Peter Seinen wrote:
> Dear all,
> 
> Diving amateur archeologist found some Roman slag in the river Maas near 
> Cuijk. I was asked to find out what metal preparation process produced the 
> slag. I used XRF as well as XRD and found that all slag mainly consisted 
> of silica (~50wt%), alumina (~25wt%) ironoxide (~10wt%)and a few wt% of 
> magnesium, calcium and potassium and some minor substances. This result 
> almost led me to the conclusion that it is iron slag. The book of Tylecote 
> (page 56), however, showed that the ironoxide content of Roman period slag 
> is typically between 40 and 70 (I presume weight%), much higher than my 
> 10wt%. The types of element is, however, an exact match.
> So, I am not so sure anymore if I have got slag and if I have not, what do 
> I have ?
> Can anybody shed some light on this 'mystery' ?
> 
> Best regards,
> 
> Peter Seinen,
> Underwater archeology "Mergor in Mosam", Cuijk 
> 
> 

-- 
Chris Salter,
Oxford Materials Characterisation Service,
&
Electron Microscopy Research Support Group,
&
Material Science-based Archaeology Group,
Oxford University Begbroke Science Park,
Sandy Lane, Yarnton, Oxford, OX5 1PF
Tel 01865 283722, EPMA 283741, Mobile 07776031608