Iron Bloom

Material · committed · confidence 0.68

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

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

• 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)
  • cites tylecote-1992-a-history-of-metallurgy
• 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)
  • cites tylecote-1992-a-history-of-metallurgy
  • cites sauder-2002-a-practical-treatise-on-the-smelting-and
• 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)
  • cites tylecote-1992-a-history-of-metallurgy
  • cites sauder-2002-a-practical-treatise-on-the-smelting-and
• cm³; raw bloom density is lower due to porosity and slag inclusions, but no well-characterized general figure exists. (confidence 0.8)
  • cites asm-international-1990-asm-metals-handbook-vol-1-properties-and
• 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. (draft) (confidence 0.72)
  • cites tylecote-1992-a-history-of-metallurgy
• Raw bloom contains approximately 30–50% occluded slag by mass; this is an indicative range, highly variable with furnace type and tradition. (draft) (confidence 0.68)
  • cites tylecote-1992-a-history-of-metallurgy
  • cites sauder-2002-a-practical-treatise-on-the-smelting-and
  • cites crew-1991-the-experimental-production-of-prehistor
• 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) (draft) (confidence 0.5) — ⚠ non-blocking verification: 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 (draft) (confidence 0.5) — ⚠ non-blocking verification: 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.

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

• asm-international-1990-asm-metals-handbook-vol-1-properties-and (draft) · ASM International (1990) ASM Metals Handbook, Vol. 1: Properties and Selection: Irons, Steels, and High-Performance Alloys.
• crew-1991-the-experimental-production-of-prehistor · Crew, P. (1991) The Experimental Production of Prehistoric Bar Iron. Historical Metallurgy 25(1), pp. 21–36.
• sauder-2002-a-practical-treatise-on-the-smelting-and · Sauder, L.; Williams, S. (2002) A Practical Treatise on the Smelting and Smithing of Bloomery Iron. Historical Metallurgy 36(2), pp. 122–131.
• 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