Microstructures and compositions of ironmaking slag found at 42 sites where known technique was practiced show how the different processes for making wrought iron can be identified from the wastes they produced. Fayalite, wustite (or magnetite), and glass are common to all the slags. Puddling slags contain characteristic bands of iron sulfides. Minerals present only in bloomery slag identify 41 of 57 bloomery and finery slags examined. In the absence of diagnostic minerals, the higher content of wustite in finery slag usually distinguishes it from bloomery slag. Unbroken pieces of bloomery and finery slag indicate hearth size and shape. Diagnostic characteristics of puddling and bloomery slags are also found in inclusions present in wrought-iron artefacts. Comparisons with mechanical tests show that inferences about the strength properties of wrought iron can be drawn from the microstructure. Slag formation rates and metal loss in bloom smelting and fining are calculated from analyses of ores and slags collected at forge sites. Comparison of process details for bloom smelting and fining at New England forges shows that ironmasters chose the indirect process to control phosphorus in their product rather than to reduce fuel or ore consumption.
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