Ironworking, Metallurgy and Material Science
What laboratory analysis of Yoruba iron, glass and dye actually shows, including the Igbo Olokun glass which turned out to be made rather than imported, and how the reducing chemistry of an indigo vat and a soap ash lye works.
Dis file dey report wetin laboratory instrument don measure. Na deliberate restriction, bikos claim about pre-colonial African technology dey always face exaggeration or total dismissal, and na di record from lab be di ground wey none of di two fit survive. Di archaeometallurgy and glass chemistry na unusually good evidence: dem involve counted samples, analytical techniques wey get name, and compositions wey dem don publish. Di strongest single result for di whole of Yoruba material science na one case where analysis turn around one century of confident scholarly assumption to support di tradition.
Di craft and social dimensions of dis trades, di blacksmith and Ògún, di guilds, di dye pits and di adìrẹ industry, na 07-arts/07-metal-pottery-calabash-leather and 07-arts/05-textiles. Dis file focus on di chemistry and analytical evidence.
Confidence level here dey high bikos di main claims rest on published instrumental analysis instead of mere report.
Iron: wetin di slag dey show
Di relevant evidence come from Modakẹ́kẹ́ for Ifẹ̀, wey Akin Ige and Thilo Rehren analyse and publish for 2003 . Dem use chemical and mineralogical techniques examine fourteen fayalitic bloomery slags, join with possible ore and furnace wall samples .
Di two findings wey matter.
Skilful operation. Di authors conclude say di slags come from bloomery smelting operation wey show high skill, wey extract di maximum amount of iron possible from di ore . Na technical judgement from slag chemistry: di composition dey show how much iron remain inside, and na very small remain.
Ore charge wey dem deliberately mix. Most of di slags show titanium oxide wey high well-well, reach around ten weight percent, wey dey show say dem use mixed ore wey get limonite iron stone and black sand wey rich in ilmenite together .
Di second finding dey more interesting and e make sense to explain am well. To mix two ore sources wey different from each other na fluxing decision. Bloomery furnace must produce slag wey dey fluid at di working temperature so dat e go separate from di metallic bloom and dem fit drain or tap am commot; if person get di chemistry wrong, di slag go too thick, e go trap iron, and di smelting go fail. To add beach sand wey rich in titanium and iron to limonite ore dey change di slag chemistry in a way wey smelter wey no understand wetin im dey do no go fit discover or maintain. Na empirical process control be dis, wey dem reach without any theory of oxides, and na one of di solid evidence for indigenous Yoruba metallurgical competence.
Wetin bloomery smelting be. Dem dey reduce iron ore with charcoal at temperatures wey dey below di melting point of iron, wey dey produce spongy solid bloom of metal mixed with slag instead of liquid cast iron. After dat, dem go heat and hammer di bloom many times to commot di slag and make di metal solid. E dey take plenty work, e dey produce wrought iron instead of cast iron, and na di general method everywhere before blast furnace come out.
Dating and scale, as dem explain carefully. Di Modakẹ́kẹ́ material establish competence, no be chronology; di publication no give solid date for di smelting operation. Estimates of di scale of Ifẹ̀'s iron industry for di historical period, like di figure say about 500 smelters, blacksmiths and other iron workers live within di radius of di city, come from written documents instead of archaeological sources, and make person treat am as rough estimate. Iron production for di region drop after di destruction of Old Ọ̀yọ́ around 1830, and e drop more as European iron start enter.
Wetin person no suppose transfer. Di very early West African iron dates wey dey circulate, around 2000 BCE for Lejja and 750 BCE for Opi, na from Igbo sites for Nsukka area, no be Yoruba sites, and scholars still dey argue about di earliest one inside African archaeometallurgy. Dem no be evidence about Yorubaland, and to apply dem there na common and serious mistake. 07-arts/07 make di same point.
Glass: di Igbo Olokun result
Na dis be di strongest single result for Yoruba material science, and e clear well-well bikos na case where wetin scholars believe before dey wrong and di scientific analysis correct am.
Di problem. People don recognise Igbo Olókun, wey dey northern periphery of Ilé-Ifẹ̀, as glass-working site for pass one century; Leo Frobenius describe glass-encrusted crucibles from im pit shafts there for 1913 . For most of di twentieth century, dem take di material as evidence of secondary processing: imported glass wey dem melt again locally come use make beads . Davison neutron activation and X-ray fluorescence analyses for 1972 find one main group wey get low soda and plenty potassium, wey suggest say na wood-ash flux dem use, and since wood-ash glass dominant for medieval glassmaking north of Alps, e conclude say di beads likely come from medieval Europe . For three decades after dat, dem assume say all Ifẹ̀ glass na import .
Di correction. Na only after dem don build comparative databases on glass composition, before di meaning of di very high lime and alumina levels (wey reach average of around 15 and 13 weight percent respectively) come clear as unique and defining signature . Lankton, Ige and Rehren propose for 2006 say dis high lime, high alumina composition, HLHA, represent glassmaking tradition wey dey unique to West Africa and possibly to southern Nigeria .
Di excavation and di analysis. Babalola do excavation for Igbo Olókun for 2011 and 2012, for four one-by-three metre unit inside or near di fenced reserve, and im recover almost 13,000 glass bead wey dem screen through 1.2 mm mesh, over 800 crucible fragment, almost three kilogram of glass waste and cullet, and about 14,000 potsherd . Fifty-two bead from di excavated assemblage na im dem analyse by laser ablation inductively coupled plasma mass spectrometry, LA-ICP-MS, and by scanning electron microscopy with energy-dispersive X-ray spectroscopy, SEM-EDS .
Di results. Di analysis dem confirm say HLHA glass plenty pass and e provide solid evidence of second compositional group wey get low lime and high alumina, LLHA. No imported soda-lime glass bead dey at all among di sample dem wey dem analyse . Crucible evidence show say dem work LLHA glass together with HLHA glass for di site and e fit be say na local people make am as part of di same technological tradition .
Di conclusion, as di authors take talk am. Dem state say dem dey confident now beyond reasonable doubt say both di HLHA and LLHA group represent glass wey dem produce for early Ilé-Ifẹ̀ using local recipe, raw material and technology, and say dis glass dem dey different from di soda-lime glass of di Islamic world, from di wood-ash or potash glass of medieval Europe and from Indian high-alumina glass .
Di raw materials. Wetin likely pass na say granitic sand, weda dem add calcium carbonate or not, na im dem use produce di two type, with colorant wey rich in MnO, Fe2O3, CuO and CoO wey dem intentionally add . Freestone bin suggest say di high alumina fit come from feldspar and di lime from limestone or shell . Work wey Ogundiran and Ige do for Ọ̀ṣogbo, wey be seventeenth to eighteenth century site about 40 km north of Ilé-Ifẹ̀, recover HLHA bead and production waste and e argue say di manufacturing tradition continue dey use snail shell and local sand wey come from pegmatite .
Di dating. Radiocarbon determination from di excavation include 840 ± 30 BP (1058-1264 cal AD) for 0.60 m depth, 570 ± 30 BP (1304-1423 cal AD) on basal deposit for unit IO-C, and 610 ± 30 BP (1295-1404 cal AD) from di lowest level of unit OO-A . All of dem combine to support chronology of eleventh to fifteenth century for di glass bead and di workshop, wey match di eleventh to fifteenth century date from Garlake excavation dem for Woye Asiri and Obalara's Land and with thirteenth to fifteenth century thermoluminescence date on top di fired clay core of cast brass sculpture from Ita Yemoo . Most scholar identify di eleventh to fifteenth century as di period wey Ilé-Ifẹ̀'s flourish pass . Di authors state say compositional and archaeological data wey dey ground strongly show say dem bin dey make and use HLHA glass bead for Ilé-Ifẹ̀ at least by di eleventh century AD .
Di distribution. Dem don identify HLHA bead for Igbo-Ukwu for ninth to twelfth century context, Essouk for twelfth to fourteenth century context, Gao Ancien for tenth to twelfth century context, and for Kissi, Bura, Diouboye and Kumbi Saleh . For every case dem appear beside bead from at least one oda production area . Di authors describe di scale of production for Igbo Olókun as sometin wey unique for West Africa .
Wetin make dis matter and wetin e no show. E establish primary glass production, meaning glass wey dem make from raw material instead of imported glass wey dem melt again, for West Africa from at least di eleventh century, using recipe wey no dey anywhere else for di Old World, on a scale wey supply distribution network wey reach Middle Niger. Dat na major finding about indigenous technology and na only careful analysis e take establish am.
E no show say Ifẹ̀ invent glassmaking, say Ifẹ̀ glass old pass glass for oda place, or say di technology dey independent of any outside influence, and di paper no claim any of dis. Di soda-lime bead wey dem find for Orun Oba Ado and Ita Yemoo show say Ifẹ̀ bin dey connected to long-distance network wey dey carry glass from Middle East, Egypt and Syria-Palestine . Local tradition with unique recipe wey develop for place wey dey connected na wetin di evidence show, and e dey more interesting pass isolation.
Brass
Brass casting for Ifẹ̀ na di Yoruba technology wey people know pass and di sculpture belong to 07-arts/02-ife-sculpture. Three technical point belong here.
Di method na lost-wax casting, cire perdue, wey dem dey cover wax model with clay, melt di wax comot and pour metal inside di empty space, wey go scatter di mould wen dem dey comot am. Di thermoluminescence date on top di fired clay core of di Ita Yemoo casting dem fall into thirteenth to fifteenth century .
Di alloy dem dey vary and di terminology no tight. Copper wey dem mix with zinc dey give brass; with tin e dey give bronze; West African casting dey use varying proportion of copper, zinc, tin and lead, so "bronze" as dem commonly take call di Ifẹ̀ and Benin material many times no dey accurate.
Di copper no be local. Significant copper source no dey for Yorubaland and di metal reach dia through trade, wey dem usually attribute to trans-Saharan route. Di technical achievement therefore dey inside di casting instead of di metal production, wey be di opposite of iron case and e good make person keep am straight.
Di ẹdan Ògbóni, brass wey dem cast over iron core, dem treat am for 07-arts/07; di material argument wey e show, brass wey no dey rust joined to iron wey dey cut, na case where di metallurgy na di meaning itself.
Indigo: di chemistry of reduction vat
Yoruba indigo dyeing na redox process and di practitioners control am without any theory of oxidation. Di craft, di adìrẹ industry and how e take collapse dey for 07-arts/05-textiles; di chemistry dey here.
Di plant. Yoruba indigo na èlú, Philenoptera cyanescens, wey dem bin dey call Lonchocarpus cyanescens bifor, na one West African vine from Fabaceae family, and no be Indigofera tinctoria, di Asian indigo wey dey world trade . Plenti common book and article dey mix dem up, but e dey wrong.
Di problem wey di process dey solve. Indigotin, wey be di blue pigment, no dey melt inside water. Any pigment wey no fit melt no fit enter inside fabric fibre, so e no fit dye anything as e dey so. Di dyer must convert am to soluble form first, make e enter di fibre, before e go change am back.
How di traditional process dey do am. Dem dey mash di leaf and young branch to pulp, mould dem into balls wey reach about ten to twelve centimetres, dry dem, den sell dem for market . Dem dey soak di balls for water with alkali, and dat alkali na wood-ash lye, wey dem get by leaching water pass burnt hardwood ash so dat e go give potassium carbonate solution . Dem dey leave di vat make e ferment, from wetin record show, na from about six to eight days reach three weeks .
Di chemistry. Di fermentation wey dey happen for di vat na anaerobic bacterial action, and na reducing environment. Under alkaline reducing condition, insoluble indigotin dey change to leuco-indigo, wey be di soluble reduced form wey get yellow-green colour. Di fibre dey absorb dis leuco form from di solution. Wen dem bring di cloth comot into air, oxygen from air dey reoxidise di leuco-indigo back to insoluble indigotin, wey don trap inside di fibre now. Dis na why cloth dey comot from Yoruba dye pot with yellow-green colour and turn blue wen dem hang am, and na why di colour dey deep wen dem dip and oxidise am plenti times instead of just dipping am once for long time.
Di control problem. Di alkalinity and di reduction must balance well, even though di dyer no get any instrument. Wetin di records show na dat dyer wey get experience dey check whether e don ready through di colour of di foam wey dey top of di vat . Dis na trained observation proxy for redox state, and e show real analytical skill.
Wetin make di industry die. Caustic soda replace ash lye because e dey work within hours instead of di days or weeks wey fermentation vat dey take, and e cheap pass; e still be much harsher alkali wey dey damage cotton fibre. Wen dem mix am with synthetic indigo, e produce cloth wey fine for market stall but e no dey last wen pesin wear am, and di fall in quality standard destroy di Abẹ́òkúta industry between 1937 and 1939. Dem treat dis matter fully inside 07-arts/05-textiles and e base on Byfield's The Bluest Hands.
Chemical characterisation. Scientific analysis on traditional Yoruba indigo-dyed material show indigotin join with 2-hydroxynaphthoquinone and mineral ions like Al³⁺, S²⁻, Na⁺ and Fe³⁺ .
Soap: saponification with potash lye
Ọṣẹ dúdú, black soap, from ọṣẹ, soap, and dúdú, black, start from Yoruba communities for southwestern Nigeria . How dem dey make am na controlled chemical synthesis and e make sense to explain am clearly, because na di clearest example for Yoruba practice wey show two-stage process with intermediate step wey di makers no fit see with eye.
Stage one, to make di alkali. Plant material, especially plantain peel, palm bunch, cocoa pod or hardwood, dem go dry am den burn am to ash. Dem go leach di ash with water and filter di solution. Wetin dis dey produce na lye wey rich in potassium carbonate and related alkaline salts . Di kind material wey dem use dey matter for how strong di lye go be: plantain peel ash dey produce lye wey get stronger alkali pass cocoa pod ash .
Stage two, saponification. Dem go mix di lye with plant oil, especially palm kernel oil, palm oil, shea butter or coconut oil, den heat am. Saponification na di alkaline hydrolysis of di triglycerides inside di oil, wey dey split dem into glycerol and potassium salts of fatty acids, wey be soap . Potassium-based lye dey give soft soap, and na why traditional black soap dey usually soft or be like paste instead of to hard like modern bar soap.
Wetin make am special. Di person wey dey make am no fit see di alkali, no fit measure di concentration, and di reaction fit spoil easily: if di alkali too small, unsaponified oil go remain and di soap no go work; if e too much, free alkali go remain and di soap go dey burn skin. Di traditional process dey control dis by using fixed ratio of specific raw materials wey dem prepare in a specific way, wey be exactly how chemistry work before instruments dey. Di medicated soap ọṣẹ wey dem dey use for Yoruba traditional medicine, wey dem mention for file 01, na dis same process dem use, but dem add medicinal plant material join.
Pottery and other materials
Pottery na work wey women dey do, dem dey mould am without wheel, and dem discuss am for 07-arts/07. Di technical point wey concern dis matter na say di fired clay crucible wey dem use for glass industry for Igbo Olókun na pottery product, wey link di two industry together directly, and dem recover over 800 crucible pieces for di 2011-2012 excavation alone . Glass industry need refractory ceramics industry wey fit make container wey go survive continuous heating with hot molten glass inside dem, so di crucible evidence show competence for ceramic matter and glass matter join.
Wetin dem fit claim
If we talk am according to wetin scientific instruments support: Yoruba people smelt iron with high competence, and dem control slag chemistry by mixing ore deliberately . Dem make glass from raw materials, no be just to rework glass wey dem import, and dem use recipe wey special for di Old World, from at least di eleventh century, on a scale wey no get equal for West Africa, den trade di product across western Sudan . Dem run alkaline reduction vat and saponification reaction through trained observation without instruments . Dem cast brass through lost-wax method to standard wey nobody fit doubt, using imported metal.
None of dis need di word "advanced" or any comparison with Europe. Na just description of wetin di scientific analysis find.