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Make a Clay Crucible

Tech Level 2

Last edited · 92a9d6f · tewelde

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Summary

A crucible is a small cup built to do what ordinary pottery cannot: sit inside a charcoal bed at furnace heat, hold molten metal, and survive being lifted and poured. The differences from a pinch pot are a heavily tempered clay body, thick walls, a rounded bottom, and a deliberate pour lip.

This article covers making and proving the vessel. Melting and casting copper with it is its own later process — but the crucible must exist first, and it also serves immediately as a tougher container for ore tests.

Terms used in this article

  • Crucible — a heat-proof cup for melting things in.
  • Pour lip (spout) — a small pinched channel in the rim that lets liquid pour in a controlled stream instead of dribbling.
  • Thermal shock — cracking caused by fast temperature change; coarse temper and thick rounded walls are the defenses.
  • Temper / bone dry / open firing / spall — see Find and Prepare Clay and Fire a Clay Pinch Pot (First Pottery).

Prerequisites

Materials

  • Clay body tempered far coarser than for pots: about 1 part coarse sand and crushed fired clay (grog) to 2 parts clay. Crushed shards of a broken fired pot are ideal grog.
  • Enough for a fist-sized lump per crucible — make several; crucibles are consumables and some will crack.
  • Tongs sized to grip it: Make Green-Wood Tongs

Steps

1) Form a thick small cup

  1. Pinch a cup with an inside about the size of an egg to a small fist — small batches only; large melts are a later technology.
  2. Make the walls and bottom two fingers thick, far thicker than feels right for a pot.
  3. Shape a rounded bottom, no sharp inside corners — corners concentrate cracks.

2) Add the pour lip

  1. While the rim is soft, pinch one spot of the rim outward between two fingers into a small V channel.
  2. Smooth it well; a ragged lip dribbles.

3) Dry and fire hard

  1. Dry to bone dry as for a pinch pot, then give it a full extra day — thick walls hide moisture.
  2. Open-fire it long and hot: warm at the edge, then keep it in the hottest part of a strong fire for a good hour, glowing red, and cool it slowly in the ashes.

4) Prove it before trusting it

  1. Inspect: no cracks, rings (dully — thick walls ring low) when tapped.
  2. Heat the empty crucible to glowing in a charcoal bed, grip it with tongs, lift it out, set it on a stone, and let it cool on its own.
  3. Repeat the heat-and-lift cycle once more. A crucible that survives two empty runs without new cracks is proven.

Verification

  • Survives two empty glow-heat cycles with no new cracks (hairline surface lines that do not grow are acceptable).
  • Walls are two fingers thick everywhere; the tongs can grip and tilt it positively.
  • Holds water overnight without falling apart (seepage is fine; crumbling is not).

Safety

  • Everything about hot pottery applies, doubled: a thick damp wall spalls violently — never rush the drying.
  • Practice every lift and pour motion cold, then empty-hot, before there is ever metal in it.
  • A cracked crucible is firewood: never use a crucible with a through-crack for molten material; failure pours fire at your feet.

Troubleshooting

  • Cracks during firing: not fully dry, or temper too fine — add more coarse sand/grog and dry longer.
  • Cracks on the proving lift: thermal shock from cooling too fast, or thin spots — check wall evenness, cool in ashes not open air.
  • Crumbly after firing: too much temper or underfired; reduce temper slightly and fire hotter, longer.
  • Tongs cannot hold it: form a slight waist or rim overhang next time so the tongs have a shoulder to grip.

Variants