Thank  you . J F

I sent my answer to Dr Itanyi via email, but for anyone interested in the topic, this was my suggestion, I'm afraid it perhaps is not as helpful as he wanted. 

I'm sure you are familiar with the work of Peter R. Scmidt. His 1997 volume Iron Technology in East Africa: Symbolism, Science, and Archaeology, University of Indiana Press, Bloomington covers a variety of slags and discusses pit furnaces, but may not cover shaft furnaces. Schmidt's edited volume from 1996, The Culture & Technology of African Iron Production, University of Florida Press, Gainesville does have some information on slag variation, but again I believe most of the chapters address pit furnaces. Perhaps more recent work by Schmidt might touch on your question. Although he is not currently working on the ethnoarchaeology of iron production and has other recent interests, you may want to contact him directly at the University of Florida: [email protected].