Why Melting Control Decides Casting Quality
Melting is not merely a matter of turning solid copper alloy into liquid metal. The way a crucible is preheated, the order in which charge is loaded, the moment deoxidation is carried out and the time the bath spends above its liquidus all leave permanent marks on the finished casting. Gas porosity, oxide inclusions, zinc loss and hard spots begin in the furnace, not in the mould. This guide sets out practical melting practice for three families that behave very differently: pure copper, brass and cast bronze.
Melting Pure Copper
Preheat the crucible until it glows a dark red, then charge a dry charcoal cover 30-50 mm thick at the bottom. Charge scrap, blocks and bars in sequence, finishing with electrolytic copper on top so the highest-purity material enters the bath last. Preheating additions may be placed on the furnace mouth, but cold material must never be pushed into liquid metal. Keep the charge moving during melting to prevent bridging and local overheating.
Bring the bath to full melt at 1200-1220 °C and deoxidise with phosphorus copper at 0.3-0.4% of the bath weight. The deoxidation reaction is:
5Cu2O + 2P → P2O5 + 10Cu
The P2O5 leaves as gas, while residual phosphorus forms a phosphate slag that floats and is skimmed off before pouring. The frequently copied side reaction Cu2O + P2O5 = 2CuPO3 is chemically incorrect and should not be reproduced in training material. Skim the slag and pour at 1100-1200 °C.
Melting Brass
Brass is copper alloyed principally with zinc. Ordinary binary Cu-Zn alloys are used for wrought processing, while special brasses contain silicon, aluminium, manganese, lead, iron or nickel; most cast brasses are special brasses.
Charge make-up: aim for at least 30% new material and no more than 70% returns by weight. Well-sorted returns can reach 90% or more with excellent results when spectrographic verification confirms composition, but a high return content demands close attention to impurity accumulation.
Cleanliness: every piece of charge must be dry and clean; sandblast dirt and rust off before charging. Metal pieces should not exceed one third of the crucible diameter or four fifths of its depth.
Tools: crucibles must be crack-free, new crucibles heated slowly, used crucibles cleaned of slag. When changing alloy families, wash the crucible with a melt of the same series first. Stirring rods are graphite or refractory coated, never plain iron, which contaminates the melt. Ingot moulds are cleaned, coated and preheated to 100-150 °C.
Preheat the crucible to dark red, lay 20-40 mm of charcoal, add electrolytic copper and melt fast. Add master alloys with the highest melting point first, then returns, keeping the bath covered with charcoal. Deoxidise with phosphorus copper at 0.04-0.06% P of the copper weight at 1150-1200 °C; deoxidised castings show measurably better surface quality. Add alloying elements by grade: aluminium-copper master alloy at 1100-1120 °C; zinc and aluminium in batches at 1100-1150 °C with the power off while stirring. For silicon brass add silicon before zinc; for lead brass add zinc before lead. Never add zinc above 1200 °C, where volatilisation loss becomes severe.
| Cast brass grade | Typical pouring temperature |
|---|---|
| ZCuZn38 | 1100-1130 °C |
| ZCuZn40Pb2 | 1080-1100 °C |
| ZCuZn31Al2 | 1120-1140 °C |
| ZCuZn16Si4 | 1100-1140 °C |
Cross-contamination deserves particular care. After melting aluminium-bearing alloys, crucibles and tools must not be used for tin bronzes without a wash melt. Aluminium is harmless, even desirable, in aluminium bronze, but it is the most damaging impurity in tin bronze.
Melting Cast Bronze
Cast bronzes divide into tin bronzes, where tin is the main alloying element and provides good wear and corrosion resistance plus useful strength and plasticity, and tin-free bronzes such as the aluminium, lead and silicon types.
For tin bronzes such as ZCuSn10Pb1 and ZCuPb10Sn10, preheat the crucible, add 20-40 mm of charcoal, melt electrolytic copper fast, add returns under cover, then add phosphorus copper at around 0.5% of the charge for phosphor-tin bronze. Add zinc, tin and lead in sequence with continuous stirring, adjust to 1100-1150 °C, skim, add about 0.1% phosphorus copper for final deoxidation, sprinkle straw ash on the surface and pour at 1130-1180 °C.
For aluminium bronzes such as ZCuAl10Fe3 and ZCuAl10Fe3Mn2, never use a crucible that has melted other alloys. Preheat, add flux, then preheated low-carbon steel sheet at about 200 °C together with returns. Stir and raise the bath to 1150-1180 °C, deoxidise with 0.3% phosphorus copper and add flux. Add preheated aluminium and manganese in batches, pressing each batch under the surface with the stirrer, adjust to 1120-1220 °C, cover with straw ash and pour at 1160-1200 °C.
Aluminium bronze and silicon bronze tend to form high-melting-point Al2O3 and SiO2 films that cause slag inclusions, so keep the bath covered and avoid violent stirring close to the surface.
Cross-Cutting Rules: Time, Stirring and Cover
Shorten melting time: a longer melt means more element burnout and more gas pickup. Preheat the charge and keep furnace operations compact.
Stir with graphite rods: iron rods dissolve into the bath and release oxide inclusions, while graphite stirrers keep composition uniform without contamination.
Covering agents: glass or borax at 0.8-1.2% of charge with a 10-15 mm cover, or charcoal at 0.5-0.7% with a 25-35 mm cover. Remove covers only just before pouring; charcoal may be left in place to act as a pouring dam.
FAQ
Q: Why is a charcoal cover used at all?
The cover excludes air from the bath, limiting oxidation of copper and the loss of volatile elements such as zinc, and it also helps hold heat in the crucible.
Q: How much phosphorus copper should be used for pure copper?
Add 0.3-0.4% of the bath weight at 1200-1220 °C, then skim the phosphate slag before pouring at 1100-1200 °C.
Q: Why must zinc never be added above 1200 °C?
Zinc volatilises rapidly at that temperature, so a large part of the addition is lost as fume, the grade drifts off specification and the working environment deteriorates.
Q: Can brass be melted from 100% returns?
Well-sorted returns above 90% can work, but only when spectrographic verification confirms the composition and impurity levels are watched closely between heats.
Q: Why does aluminium bronze need its own crucible?
Aluminium left in a crucible is beneficial in aluminium bronze but is the most damaging impurity in tin bronze, where it degrades ductility and promotes inclusions.
Q: Why avoid iron stirring rods?
Iron dissolves into the copper bath and releases oxide inclusions; graphite or refractory-coated rods keep the melt clean and the composition uniform.
