Alloying Bronze (Cu + Sn) — The Bronze Age Begins

Why Bronze is Superior

Bronze is a solid solution alloy — tin atoms dissolve into the copper crystal lattice, distorting it and blocking dislocation movement. Result:

Property	Copper	Bronze (10% Sn)	Improvement
Vickers Hardness	40-50	70-150	2-3× harder
Tensile Strength	210 MPa	300-500 MPa	2× stronger
Melting Point	1085°C	~950°C	Easier to melt!
Castability	Poor (gassy)	Excellent (fluid)	Complex shapes
Corrosion	Forms green patina	Highly resistant	Lasts millennia

The Ideal Ratio

Ancient smiths converged on ~10% tin, 90% copper through trial and error:

• <8% tin: Too soft, barely better than copper
• 10-12% tin: Optimal hardness, good castability, golden colour
• 15-20% tin: Very hard but brittle — good for bells and mirrors, bad for tools
• >20% tin: Extremely brittle, shatters on impact

Preparation

1. Weigh your metals: 900g copper + 100g tin for classic 10% bronze.
2. Cut copper into small pieces (1-2cm) for faster melting.
3. Tin can be in any form — it melts so fast it dissolves almost instantly.

Melting Sequence (CRITICAL)

1. Melt the copper first. Load copper into pre-heated crucible in the kiln. Bring to full liquid (1085°C+).
2. Add tin LAST. When copper is fully molten, add tin to the surface. Tin melts instantly (232°C) and dissolves into the copper.
3. Stir with a pre-heated dry stick or ceramic rod. Ensure uniform mixing — 10 seconds of stirring is enough.
4. NEVER add copper to molten tin — the temperature differential causes violent boiling and spatter.

Signs of Good Bronze

• Surface should be bright and mirror-like when fully liquid
• Colour: golden-yellow (not coppery red = too little tin, not silvery = too much tin)
• Flows smoothly when poured — bronze is more fluid than pure copper

Pour

1. Skim slag from surface.
2. Pour into pre-heated mold in one continuous stream.
3. Bronze has excellent castability — it fills fine details that pure copper cannot.

Post-Casting

1. Allow to cool naturally in the mold.
2. Remove from mold, break off sprues.
3. Cold-work the edges: bronze work-hardens even more effectively than copper.
4. Grind and polish the working edge.

Testing Your Bronze

• Ring test: Strike with a stick — good bronze produces a clear, bell-like ring. Dull thud = bad alloy or porosity.
• Edge test: A bronze axe keeps its edge 3-5× longer than copper.
• Colour: Golden-yellow when polished. Develops green patina over time (same as the Statue of Liberty).

The Civilization Impact

Bronze changed everything:

• Agriculture: Bronze ploughshares broke harder soil → more food → larger populations
• Warfare: Bronze swords and armour dominated for 2,000 years
• Art: Bronze casting enabled the first complex sculptures (lost-wax casting)
• Trade: Tin scarcity created the first long-distance trade networks

But bronze has a fatal flaw: tin scarcity. When the Bronze Age trade networks collapsed (~1200 BCE), civilizations that couldn't get tin were forced to master a harder, more abundant metal — iron. See Blueprint 08: Building a Bloomery & Smelting Iron.