Iron smelting in Africa from a drawing of 1831 and iron working in Madagascar in 1853

Note of the East African Steel Industry as described by Edrisi.
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Idris' statement: People of the Zabag (or Zanedj) come hither for iron, which they carry to the continent and islands of India where they sell it for good money, because it is an object of big trade and it has a huge market in India.
This statement finds some support in: Al-Kindi (850) A Treatise to some of his Brethren Concerning Swords: Swords forged from imported steel: Some swords were called non-native. They were forged from imported steel. Some Khurasani swords for example were forged from steel imported from Sarandib and this is the case in several other cities
Idrisi's description of East Africa as a collection of harbors fully engaged in the production of steel for export to India, stands against the traditional view of a coast that exports ivory and slaves. (Ibn Said mentions a century later also iron mines on the coast).

Although his statement can not be proven it is easy to prove that iron and steel were widely traded in the Indian Ocean. For a statement that East-African iron was traded in the Indian Ocean too little information is available right now. Archaeology shows iron production declining and then disappearing on the East African coast (except for Madagascar) in the middle ages.
In India it is known as UKKU, and the anglicized version of this is WOOTZ steel. In Medieval times it was known in Europe as Damascus steel. This because the crusaders knew it mostly from the weapons the Arabs made from it. The prophet Muhammad also had a sword made of this steel, most probably imported from India.

Iron Trade in the Indian Ocean

S.D. Goitein in his "Letters of Medieval Jewish Traders" has a letter written in Aden about the ship wreck in the entrance to the Red Sea (Bad al-Mandab) of a ship loaded with pepper and "soft iron". (12th century) The ship came from India. Many more letters give evidence in this book about iron products being transported over the Indian Ocean (mostly steel). And Idris himself mentions iron as an import into Arabia from India.

A letter on 11 sept 1149 from Aden to Mangalore by Madmud bin Hasan bin Bundar
...........for pepper....the price per bahar will be 30 dinars, and more, and as for refurbished iron, a bahar will be not less than 20 dinars, and that the raqs (shining glittering iron) which was in the city is completely exhausted. (Tell him also) to dispatch a ship from Manglore, if they can and to send in it any available pepper, iron, cubeb, and ginger..........  
Another letter: 1139 from Aden to Mangalore from another familie member.
I took notice my master (Abraham Yishu) of your announcement of the sending of refurbished iron in the boat of the nakhoba (shipowner) Ibn Abi 'I Kata'ib. the shipment has arrived and I received from him two bahars and one-third as you noted.  (Goitien)

Iron smelting in Madagascar

There is some archaeological evidence from Madagascar. (Cited in General History of Africa III p331-332). " Iron was exploited in the true sense of the word. (In south Madagascar). Here, the metal does not seem to have been worked on the spot, since the usual practice of re-use, attested to by ethnography, is not enough to explain the striking contrast between the abundance of traces of exploitation of the ore (ashes, coal, slag) and the virtual absence of iron objects....... No doubt the smelted products were largely exported through Talaky (harbor in extreme south of Madagascar) whose development, if not foundation thus appears to be linked to its role as an outlet to the sea of export  products from the interior, which, moreover, were apparently not limited to smelted products."

Taken from: Chantal Radimilahy; L'Ancienne Metallurgie du Fer a Madagascar.
Note: XV means XVth century
At Dembeni (Mayoyye-Comores) Xth are found leftovers of forging iron and bits of minerals.
North
Mahilaka: XII to XV the place is an immense workplace for iron
Kingany: Metallurgy from XIII to XIV
Antsoheribory: Metallurgy XIV
Bemanevika: Ironworking XV several cube meters of leftovers
Sandrakatsy:  Metallurgy XIII
Highlands
Ambohidahy: traces of Metallurgy in end XV
Andranonandriaana: Metallurgie XV
Ankatso: Metallurgie end XV
Fanongoavana: Metallurgy XV
Vohimasina: Metallurgy end XV
South
Andranosoa: Metallurgy XII to XVII
Lintamandanimerina: Metallurgy XV
Efangitse: Metallurgy end XV
Iabomanitra: Metallurgy XV-XVI
Maliovola: Metallurgy and forging XIV

Taken from: L'extraordinaire et le quotidien: variations anthropologiques: hommage au......by Pierre Verin, Claude Allibert, Narivelo Rajaonarimanana.

The coast of North-east Madagascar from 1400 to 1600.
There is not only evidence of cultivation and of maritime exploitation, but also of the keeping of large mammals, probably cattle. Traces of local iron working are widespread. There is evidence of widespread production and local transport of chlorite schist basins. We have found workshops near the Iazafo estuary.  

Taken from: Nyame Akuma (1995)
Sambava a small town on the north-east coast of Madagascar. A major iron smelting site (14 iron smelting locations) was found 6km from the town. Dated to AD 1300.

Iron Smelting in Tanzania-Kenya Coast

Taken from: Iron metallurgy along the Tanzanian coast by Bertram B. Mapunda in Southern Africa and the Swahili world.
As la Violette et al noted: The evidence for smelting declines in the later periods, until there is none at all in the sites that date to the second millennium AD.
Similar observations on the decline of iron smelting industry was observed along the southern shore of lake Tanganyika and in Bukoba.
Smidt attributes the decline to the overexploitation of resources.
Some archaeo-metallurgists conducted a microscopic analysis of metal objects from several sites on the Kenya coast. The results were that the specimens were of crucible steel, indicating they were from imported iron. Because this kind of steel production is unknown south of the Sahara.
The results of the research of Bertram B. Mapunda show that after the first millennium the coastal dwellers changed their production to micro-scale smelting with miniature bowl furnaces and minuscule quantities of ore and charcoal. This mostly because of resource (ore) depletion.  

Kilwa Chittick (1974)  There is evidence of iron smelting, mainly fragments of tuyères and slag and a few objects of iron  for the period between 800 and 1100 AD (1974:28) For the following period - until the late 12th century AD - only a complete tuyère could be recovered, which has been fired, but does not appear to have been used in a furnace The wide mouth was clearly used with bellows; it could equally have been employed for smelting or forging iron (ibid.:52) At Husuni Ndogo, Chittick uncovered a number of crucibles and fragments thereof, apparently unfiled, for the period from the late 13th century to c. 1400. Additional fragments of crucibles were recorded as having been found in the foundation trench. No furnace was, however, found (ibid :200ff.). The evidence leads Chittick to the conclusion that iron was smelted from the foundation of Kilwa onwards, but not on a substantial scale. In the period of the "Shirazi dynasty" - up until the late 13th - century the smelting of iron, as it was known in the earlier period, was still familiar to the inhabitants.

At Pate, Wilson and Lali Omar discovered two pieces of iron slag, the by-products of iron working from early levels at Pate, one from the late 8th/early 9th centuries, and one from the late 10th century. No comparable evidence for local iron smelting or iron working is found thereafter until deposits of the 14th and 15th centuries with nine specimens. Only few remnants of iron products were found. (1997:58). From this the authors conclude that the inhabitants of Pate practised iron working and probably smelting from the late 8th to the 9th century (ibid :64).

According to Horton, who carried out major excavations at the ruins of Shanga, this town was occupied between c. 850 and 1440 AD The earliest archaeological levels, dated to ca 850 till 920, contain evidence of  iron working.  Horton maintains that the early Swahili  were iron workers who moved to the Coast and began to trade their products with foreign merchants. Shanga was thus first settled by local iron workers, fishermen, and farmers (Horton 1984).

Haematite ore, used along with ilmenite sand to produce iron, has been recovered in Shanga at 10 -century levels Allen supposes that this haematite came along the trade routes from the interior, probably from the Mount Kenya region. The ore was traded for iron products made on the Coast (1993:25).

Mark Horton (Swahili Architecture, Space, Social Structure)

Writing about the archaeology of Changa: In the mid-thirteenth century trading patterns shift from the Gulf to southern Arabia. Chinese pottery also rapidly increases in popularity. At the same time, textile working declines (as measured by the overall number of spindle whorls) and ironworking disappears. This may represent a shift from a craft-based exchange system to a more fully mercantile economy.

Chittick found several lumps of iron slag at Manda, which points to the fact that smelting of the metal was done there. The particular form of some of the lumps (one datable to the last quarter of the first millennium AD, and one to roughly 1000 AD) suggests that these are residues from the bottom of furnaces with a cup-shaded base. The surveys did not yield any trace of such a furnace He concludes from the occurrence of these objects at a considerable distance from where the smelting was carried out, and in view of the very small scale of the excavations, that iron smelting may have been done on a substantial scale. (Chittick 1967:54).

The "History of Pate" mentions, in a chapter concerning a quarrel between Manda and Pate around 1340 AD, the existence of blacksmiths at Manda (Freeman-Grenville 1962:249).

Abdurahman Juma 2004 Unguja Ukuu on Zanzibar

Iron slag  kg    freq%  kg per m3

Period Ia 58    54.2   7    500-600AD

Period Ib 25    23.4   3    600-900AD

Period IIa 24   22.4   7   1100AD

Total      107            17

Iron Smelting moves out of the Swahili Town.

Taken from: The Archaeology of Tanzanian Coastal Landscapes in the 6th to 15th Centuries AD by Edward John David Pollard 2008.

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Physical examination and microscopic analysis of iron objects from the Kenya coast suggest specimens are of crucible steel, indicating that they were produced from imported iron, as no evidence exists for crucible steel production in sub-Saharan Africa (Kusimba et al, 1994: 64 in Mapunda, 2002: 80). However, attribute and microscopic analyses by Mapunda (2002: 80) shows iron smelting on the Tanzanian coast, at Kaole and Dakawa sites throughout the MIW (Middle Iron Working) period. There are similarities between the EIW and the MIW materials showing that similar resources were used.

In the 14th to 16th centuries, the volume of locally produced artefacts including iron slag and spindle whorls decreased indicating a decline in industry, which may have been replaced by cheaper imports from the interior or across the Indian Ocean (Kusimba, 1999: 37-8; Horton and Middleton, 2000: 82).

Although iron working was an important industry of the pre-colonial period, it was more conspicuous during the 6th to 10th century than later, possibly because of a technology change towards microproduction in the latter period.

Little evidence of iron production was found after the 13th century at both Kilwa and Kaole. Chami (2002c: 29) recorded slag on the southern part of Kaole Ruins. This probably dates to the 12th to 13th century when the area was used as a harbour. Iron smelting has been recorded west of Husuni Kubwa associated with celadon (ARDA, 1960: 5; Lindahl, 1992) suggesting iron production had moved away from the waterfront at Kilwa Kisiwani Port by the 14th century. However, the general pattern indicates a decline in iron production, which Kusimba (1999: 37-8) attributed to cheaper imports from the interior or across the Indian Ocean. It is, however, quite possible that archaeological excavations, which have tended to concentrate upon urban areas, have simply failed to locate production sites. Iron making has always been an activity located on the periphery of settlements, and particularly the waterfront in coastal sites, and may merely have been sited in this later period further from the settlement margins. Iron making at sites such as the mounds at Bwembweni would support such an interpretation.

Objects fashioned from the raw iron and copper were rare at Mso Bay in Kilwa and Maganbani near Kaole. However, both sites included a sub-conical copper bead very similar to one found by Horton (1996: 324-5, 334) at Shanga, dated to around AD800 to 1000, and the Early TIW sites of Misasa, Mpiji and Kiwangwa in Tanzania (Chami, 1994: 64-5), dated to 4th to 7th century AD (Chami, 1994: 91). At Mso Bay a copper bead and a copper bracelet were found amongst Early TIW. The source of the copper may have come from inland or from a southern Asian region (Horton, 1996: 358; Horton & Middleton, 2000: 99). Chittick (1974: 33) recorded copper artefacts (rod, wire and kohl stick) from deposits on Kilwa Kisiwani, which are probably contemporary with Mso Bay. Chittick (1974: 58) also recorded one example of copper production, a 13th century crucible with a cuprous deposit, together with small fragments of copper at Mbuyuni on Kilwa Kisiwani. Working of the raw iron or copper need not have taken place at the same site as its production. Horton (1996: 363) argued that the general absence of large numbers of tuyeres, heaps of slag and smelting furnaces at Shanga points to only the reworking of iron or smithing on-site, while the smelting process must have occurred off-site. However, Horton (ibid.) did record slag west of the Friday Mosque mostly from before the end of the 11th century, which was not located on the waterfront. Nevertheless, the area around the Great Mosque was considered a large open space where only a single house and a number of tombs were found (Horton, 1996: 38), and so would have provided a suitably safe area for the work. Chittick’s (1984: 42, 45, 210-1) records of 10th-century iron furnaces at Manda sited 5 to 20m from the modern HWM (High Water Mark) do, however, provide further support for waterfront production. As a result of later land reclamation at Manda, it is quite possible that these furnaces were even nearer the waterfront when in use. Kusimba and Killick (2003: 112) record no large-scale ironworking sites at Mtwapa, Ungwana, Mwana, Pate and Manda confirming the likelihood that iron-smelting industries were often located outside the settlement areas.

Radimilahy’s (1998: 69) work in Madagascar found these industries on small islands in the mangrove swamps.

Further supporting evidence derives from Harding’s (1960: 136) investigations at Kisiju, south of Dar es Salaam, which yielded cone shaped copper smelting crucibles buried in mangrove swamp, while others were found on a sandy surface fringing the swamp, an environment again not unlike Bwembweni Mounds. These locations show a dependence on open spaces perhaps outside the main settlement and close to a supply of firewood for charcoal that would be easily sourced by mangroves in sheltered estuaries.