Fayalitic Slag

Material · committed · confidence 0.72

Generated from the Hyphae knowledge graph. Drafted by claude-sonnet-4-6 · Reviewed by claude-opus-4-7

The primary slag produced during bloomery iron smelting; composed predominantly of fayalite (Fe₂SiO₄, iron(II) silicate) with variable amounts of wüstite (FeO), glass, and minor gangue-derived oxides. Forms as iron oxides react with silica (from ore gangue and furnace wall materials) at smelting temperatures. Dense, glassy to granular, dark grey-green to black in color. High FeO content in slag indicates poor iron recovery. Archaeologically, slag composition is a primary diagnostic for bloomery vs. blast furnace operation.

Common sources

• Byproduct of bloomery iron smelting
• Also produced in Catalan forge and early finery processes

Composition

Primary phase: fayalite (Fe₂SiO₄). FeO content in slag typically 40–70 wt% (expressed as FeO); SiO₂ typically 15–30 wt%; minor Al₂O₃, CaO, MnO from gangue. FeO content inversely related to iron recovery efficiency. Composition ranges are site-specific and vary with ore source and furnace practice; these are indicative ranges only.

Hazards

• Liquid slag at approximately 1100–1200 °C causes severe thermal burns on contact
• Slag spatter during bloom extraction and shingling — hammer blows on partially solidified slag eject droplets

Properties

• color: Dark grey-green to black; glassy luster when fractured rapidly
• specific_gravity: Approximately 3.5–4.0 g/cm³ for bulk archaeological bloomery slag (denser than water; pools below bloom in furnace); pure fayalite mineral density is ~4.39 g/cm³ — bulk slag is lower due to porosity and minor low-density phases
• diagnostic_feature: Fayalite mineralogy and high FeO confirm bloomery origin; blast furnace slags are typically calcareous (CaO-rich) and lower in FeO
• liquidus_temperature: Approximately 1100–1200 °C for typical fayalitic slag compositions — deliberately below process temperature to maintain fluidity during smelting

Claims

• Bloomery slag mineralogy (fayalite, high FeO) is archaeometrically distinct from blast furnace slag (calcareous, lower FeO); this diagnostic feature is used to identify smelting technology in archaeological assemblages. (confidence 0.9)
  • cites bachmann-1982-the-identification-of-slags-from-archaeo
• High FeO content in slag is inversely related to iron recovery efficiency — it indicates a larger proportion of iron was lost to slag rather than reduced to metal. (confidence 0.9)
  • cites tylecote-1992-a-history-of-metallurgy
  • cites bachmann-1982-the-identification-of-slags-from-archaeo
• Fayalitic slag has a liquidus temperature of approximately 1100–1200 °C, deliberately below process temperature to maintain fluidity during smelting. (draft) (confidence 0.78)
  • cites verein-deutscher-eis-1995-slag-atlas
• Fayalitic slag is composed primarily of fayalite (Fe₂SiO₄) with FeO typically 40–70 wt% and SiO₂ typically 15–30 wt% in bloomery slags; these are indicative site-variable ranges. (draft) (confidence 0.78)
  • cites bachmann-1982-the-identification-of-slags-from-archaeo
  • cites tylecote-1992-a-history-of-metallurgy
• cm³. (draft) (confidence 0.72)
  • cites bachmann-1982-the-identification-of-slags-from-archaeo
• cm³ for archaeological bloomery slag (draft) (confidence 0.5) — ⚠ non-blocking verification: Pure fayalite density ~4.39 g/cm³ is a standard mineralogy value. The lower bulk range is plausible (porosity reduces bulk density) but needs explicit citation from Bachmann (1982) or equivalent. CIT-16 is assigned as the source; confirm the specific pages cover density.
• FeO and SiO₂ wt% composition ranges (40–70 wt% FeO, 15–30 wt% SiO₂) as indicative of bloomery slags generally (draft) (confidence 0.5) — ⚠ non-blocking verification: These ranges are site-specific and vary with ore source. Bachmann (1982) provides a useful survey but the ranges may not be representative of all traditions. Acknowledged in the composition field as indicative only.

Connections

Outgoing

• Has hazard → Molten Slag Splatter Burns — Fayalitic slag is liquid at ~1100-1200 C and is the direct agent in slag splatter burns. The hazard inheres in the material itself when in its molten state.
• Manufactured by → Bloomery Iron Smelting

Incoming

• Produces ← Bloomery Iron Smelting — Byproduct; primarily fayalite (Fe2SiO4). High FeO content indicates poor iron recovery. 60-80% of ore mass becomes slag. Source: Tylecote (1992), p. 30.

Sources

• bachmann-1982-the-identification-of-slags-from-archaeo (draft) · Bachmann, H-G. (1982) The Identification of Slags from Archaeological Sites. 0905853105.
• tylecote-1992-a-history-of-metallurgy (draft) · Tylecote, R.F. (1992) A History of Metallurgy. ISBN:978-0-901462-88-6. https://openlibrary.org/books/OL9376811M/History_of_Metallurgy
• verein-deutscher-eis-1995-slag-atlas (draft) · Verein Deutscher Eisenhüttenleute (VDEh) (1995) Slag Atlas.