Hyphae

Iron Bloom

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Material · committed · confidence 0.68

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The primary solid product of bloomery iron smelting: a spongy, porous mass of metallic wrought iron with approximately 30–50% occluded slag (fayalitic iron silicate) by mass. The bloom forms in the lower portion of the bloomery shaft at temperatures of approximately 1100–1300 °C where iron oxides are reduced to solid metallic iron but never melt. The bloom requires immediate hot hammer-working (‘shingling’) after extraction to expel slag and consolidate the iron fibers into workable wrought iron.

Common sources

  • Product exclusively of bloomery iron smelting; not produced by blast furnace route (which yields liquid cast iron instead)

Composition

Metallic iron (low carbon — wrought iron portions are typically at the hypoeutectoid end of the iron-carbon diagram, but distribution is heterogeneous; zones adjacent to charcoal may be carburized to higher carbon levels) with inclusions of iron silicate slag (fayalite, Fe₂SiO₄) and wüstite (FeO) in incompletely reduced zones. Carbon distribution within a bloom varies by furnace design and operator practice. Specific wt% ranges are not recorded in this node pending a citable source — see needs_verification.

Hazards

  • Extreme heat hazard — bloom at approximately 800–1100 °C on extraction; direct contact causes deep thermal burns
  • Molten slag ejection during shingling — liquid slag droplets expelled by hammer blows at smelting temperature

Properties

  • density: Below that of wrought iron (~7.87 g/cm³) due to porosity and slag inclusions; no well-characterized general bulk density figure exists for raw bloom across furnace types
  • workability: Plastic and weldable when hot (above ~900 °C); brittle if cooled before consolidation due to slag embrittlement
  • slag_fraction: Approximately 30–50% by mass in raw bloom — indicative range, highly variable across furnace designs and traditions
  • distinguishing_feature: Fibrous slag inclusions visible in polished cross-section after consolidation — diagnostic of bloomery (direct reduction) origin vs. cast iron or blast-furnace iron
  • temperature_at_extraction: Approximately 800–1100 °C; must be extracted while still hot — occluded slag re-solidifies as temperature drops and bonds more tightly with the iron matrix

Claims

  • Iron bloom forms at approximately 1100–1300 °C in the bloomery shaft — below iron’s melting point (1538 °C) — accumulating as a solid spongy mass rather than a liquid. (confidence 0.9; sources: CIT-01, CIT-03)
    • Well-attested across both primary references; thermodynamically consistent with solid-state iron accumulation below the iron melting point.
  • Raw bloom contains approximately 30–50% occluded slag by mass; this is an indicative range, highly variable with furnace type and tradition. (confidence 0.68; sources: CIT-01, CIT-03, CIT-06)
    • Aligned with Bloomery Iron Smelting node CLM-08 (confidence 0.68). Crew (1991) and others document high cross-tradition variability; no single generalizable precise figure. Retained as non-blocking caveat because the claim is explicitly qualified as an indicative range.
  • Bloom must be extracted and shingled immediately while hot (approximately 800–1100 °C); as the bloom cools, occluded slag re-solidifies and bonds more tightly with the iron matrix, increasing consolidation effort. (confidence 0.88; sources: CIT-01, CIT-03)
    • Consistent across both primary references; well-established practical metallurgical constraint with a clear physical mechanism.
  • Bloomery iron (wrought iron from shingled bloom) is distinguished from blast-furnace iron by fibrous slag inclusions visible in polished cross-section. (confidence 0.9; sources: CIT-01)
    • Standard archaeometallurgical diagnostic criterion; high confidence. The fibrous inclusion morphology is specific to the solid-state direct-reduction origin.
  • Carbon content distribution within a raw bloom is heterogeneous: zones adjacent to charcoal may be carburized, while the bulk of the wrought iron portions is at the low-carbon (hypoeutectoid) end of the iron-carbon diagram. (confidence 0.72; sources: CIT-01)
    • The heterogeneity of carbon distribution is well-acknowledged in the literature but specific wt% ranges vary by furnace type and operator practice. Claim is intentionally qualitative here; specific numbers have not been sourced to a citable reference and are tracked in needs_verification.
  • Wrought iron has a density of approximately 7.87 g/cm³; raw bloom density is lower due to porosity and slag inclusions, but no well-characterized general figure exists. (confidence 0.8; sources: CIT-15)
    • The 7.87 g/cm³ figure is a standard ASM handbook value for wrought iron. The inference that raw bloom density is lower follows from the presence of porosity and low-density slag (fayalite ~4.39 g/cm³ pure mineral). No source located that provides a general bulk bloom density range. The claim as stated is verifiable and honest about its limits.

Needs verification

Carbon content in bloom: specific wt% ranges (e.g., <0.1 wt% C in wrought iron portions; 0.2–0.8 wt% C in carburized zones) (non-blocking)

Specific numerical ranges were present in an earlier draft but lacked per-number citations. Tylecote (1992) pp. 27–32 or Pleiner (2000) may provide these ranges; the carburization wt% range in particular needs a traceable source. The claim has been softened to qualitative heterogeneity only in CLM-IB-05. This entry tracks the gap for a future cycle that adds a citable wt% source.

Raw bloom slag fraction (30–50% by mass) as a generalizable figure across furnace types (non-blocking)

Crew (1991) and others document high variability; no single published measurement spans all furnace traditions. Aligned with Bloomery Iron Smelting node CLM-08 (non-blocking). The claim is already qualified as an indicative range.

Raw bloom bulk density — no specific well-characterized figure located in general terms (non-blocking)

Bloom density depends heavily on slag fraction and degree of reduction. No source located that provides a general bulk density range. CIT-15 only covers wrought iron density as a reference baseline. CLM-IB-06 is already honest about this limitation.

Connections

Outgoing

  • Has hazard → Molten Slag Splatter Burns — The iron bloom at extraction temperature (800-1100 C) contains occluded liquid slag; hammer blows during shingling expel this slag as droplets, causing splatter burns.
  • Has hazard → Radiant Heat Burns from Furnace Operations — Iron bloom at 800-1100 C radiates intensely; operators shingling the bloom are exposed to sustained radiant heat flux at close range.
  • Manufactured by → Bloomery Iron Smelting — Iron bloom is the exclusive product of bloomery direct reduction smelting; it cannot be produced by blast furnace (which yields liquid cast iron) or any other currently existing process node.

Incoming

  • Extracted from ← Wrought Iron — Wrought iron is produced by hot-working (shingling) an iron bloom. The shingling process expels occluded slag and consolidates the spongy bloom mass into dense bar iron. Wrought iron is the direct solid-state transformation of bloom iron; it is not smelted separately. The EXTRACTED_FROM edge captures that wrought iron is derived from iron bloom, while MANUFACTURED_BY (from Wrought Iron to Bloomery Iron Smelting) captures the full production chain.
  • Produces ← Bloomery Iron Smelting — Primary desired product. Metallic iron yield 20-40% of ore mass charged (confidence moderate; source: Tylecote 1992). Bloom contains 30-50% occluded slag and requires immediate shingling to consolidate into wrought iron.

Sources

  • CIT-01 · Tylecote, R.F. (1992) A History of Metallurgy. 2nd ed., Institute of Materials, London, pp. 27–32. — Primary reference for bloom formation, extraction temperature, shingling description, and slag inclusion diagnostic.
  • CIT-03 · Sauder, L.; Williams, S. (2002) A Practical Treatise on the Smelting and Smithing of Bloomery Iron. Historical Metallurgy 36(2), pp. 122–131. — Experimental reconstruction providing empirical data on bloom yield, slag fraction, hot-working window, and shingling practice.
  • CIT-06 · Crew, P. (1991) The Experimental Production of Prehistoric Bar Iron. Historical Metallurgy 25(1), pp. 21–36. — Archaeometric data on slag fraction variability in bloom across experimental reconstructions; supports the indicative-only status of the 30–50% range.
  • CIT-15 · ASM International (1990) ASM Metals Handbook, Vol. 1: Properties and Selection: Irons, Steels, and High-Performance Alloys. ASM International, Materials Park, Ohio. — Standard materials-science reference for wrought iron density (~7.87 g/cm³); provides the baseline for the density comparison claim. Does not give raw bloom bulk density.

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