Material · committed · confidence 0.72
Generated from the Hyphae knowledge graph.
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
- 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. (confidence 0.78; sources: CIT-16, CIT-01)
- Bachmann (1982) is the primary archaeometric reference for bloomery slag composition. Ranges are indicative — ore source and furnace practice vary widely.
- Fayalitic slag has a liquidus temperature of approximately 1100–1200 °C, deliberately below process temperature to maintain fluidity during smelting. (confidence 0.78; sources: CIT-17)
- The Slag Atlas (VDEh, 1995) is the standard reference for the FeO–SiO₂ phase diagram. The pure fayalite (Fe₂SiO₄) melting point is approximately 1205 °C; typical bloomery slags are off-stoichiometry so their liquidus can be lower. Confidence is moderate pending page-locator verification.
- 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; sources: CIT-01, CIT-16)
- Standard metallurgical principle; high confidence.
- 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; sources: CIT-16)
- Bachmann (1982) specifically addresses slag identification; standard archaeometallurgical criterion.
- Bulk archaeological bloomery slag specific gravity is approximately 3.5–4.0 g/cm³; pure fayalite mineral density is approximately 4.39 g/cm³. (confidence 0.72; sources: CIT-16)
- The pure fayalite mineral density of ~4.39 g/cm³ is a standard mineralogy value. The bulk slag range of 3.5–4.0 g/cm³ is plausible given porosity and minor lower-density phases, but the specific bulk range needs verification in Bachmann (1982) or a similar reference.
Needs verification
Slag Atlas (VDEh, 1995) page/section for FeO–SiO₂ phase diagram supporting the 1100–1200 °C liquidus range (non-blocking)
The Slag Atlas is the appropriate reference but no page locator has been confirmed. The 2nd edition (1995) should contain the binary FeO–SiO₂ diagram. Verify and add locator before promoting.
Bulk slag specific gravity 3.5–4.0 g/cm³ for archaeological bloomery slag (non-blocking)
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 (non-blocking)
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.
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
- CIT-01 · Tylecote, R.F. (1992) A History of Metallurgy. 2nd ed., Institute of Materials, London, p. 30. — Cited for slag formation chemistry, FeO diagnostic, and slag occurrence in bloomery process.
- CIT-16 · Bachmann, H-G. (1982) The Identification of Slags from Archaeological Sites. IAMS Occasional Paper No. 6, Institute of Archaeology, London. — Primary reference for slag composition ranges (FeO, SiO₂ wt%) and archaeometric diagnostic criteria for bloomery vs. blast furnace slag.
- CIT-17 · Verein Deutscher Eisenhüttenleute (VDEh) (1995) Slag Atlas. 2nd ed., Verlag Stahleisen, Düsseldorf. — Contains FeO–SiO₂ phase diagram data used for liquidus temperature range (approximately 1100–1200 °C for fayalitic compositions). Specific page/section for the FeO-SiO₂ binary not verified — see needs_verification.