Silver melting in a workshop
Use of borax and management of oxidation during silver melting.
At a glanceMelting silver in a jewelry workshop poses distinct metallurgical challenges. The primary obstacle with liquid silver is its extraordinary affinity for atmospheric oxygen. This causes the well-known phenomenon of spitting (*spratzen* in German, *rochage* in French), where the solidified ingot ejects gas blisters, forms craters, and shatters during rolling. This guide outlines how to melt and cast pore-free silver ingots.
The Spitting Phenomenon: Why Molten Silver Erupts
Liquid silver (above 962 °C) dissolves up to 22 times its own volume in gaseous oxygen from surrounding air. As the metal solidifies in the mold, oxygen solubility drops by over 95%. Trapped oxygen violently escapes through the freezing surface skin:
- Glowing droplets spray across the bench (serious safety hazard).
- The ingot interior turns into a sponge-like honeycomb of pores that crack during rolling.
Four Strategies to Prevent Oxygen Absorption
1. Strictly Reducing Torch Flame
Adjust your torch to a soft, slightly gas-rich (reducing) flame. The flame envelope must completely blanket the crucible opening to block ambient air.
2. Charcoal Powder or Graphite Cover
Sprinkle a layer of crushed hardwood charcoal or graphite powder over the melting silver. Burning carbon reacts with residual oxygen to form carbon monoxide ($CO$), maintaining a protective inert gas shield.
3. Balanced Flux (Borax + Boric Acid 50/50)
Borax alone is insufficient for melting silver scrap. The ideal workshop blend consists of 50% anhydrous borax and 50% boric acid powder:
- Borax dissolves copper oxides formed in sterling silver.
- Boric acid lowers slag viscosity at moderate temperatures, sealing the pool.
4. Deoxidizing Master Alloys
Modern jewelry casting houses add 0.2% to 0.5% of a master alloy containing silicon, phosphorus, or germanium to tie up dissolved oxygen into stable, non-gaseous slag oxides.
Silver Ingot Defect Troubleshooting Guide
| Ingot Surface Defect | Root Metallurgical Cause | Workshop Remedy |
|---|---|---|
| Cratered surface, blister eruptions (Spitting) | Dissolved oxygen expelled on freeze | Add charcoal cover, melt with reducing flame |
| Edge cracks during first rolling pass | Intergranular copper oxide ($Cu_2O$) | Re-melt with 1% copper-phosphorus master alloy |
| Heavy black crust, hard to pickle | Overheating or insufficient flux | Pickle in warm 60 °C sodium bisulfate bath |
| Smooth subsurface pinholes | Cold mold or excess oil in cavity | Preheat mold to 200 °C, lubricate lightly with beeswax |
Pickling and Annealing Protocol
Quench the solidified ingot in water and pickle in a warm sodium bisulfate solution. Sterling silver work-hardens rapidly and should be annealed at 650 °C (dull red) followed by water quenching.
Frequently Asked Questions (FAQ): Silver Melting
Can hydrochloric acid be used to pickle silver after melting?
No! Hydrochloric acid ($HCl$) forms an insoluble, adhering white coating of silver chloride ($AgCl$) on the metal surface. Always use bisulfate-based pickling salts.
Why does silver take longer to melt than gold despite a lower melting point?
Silver has the highest thermal conductivity of all known metals (429 W/m·K vs. 318 W/m·K for gold), rapidly dissipating torch heat into the crucible.
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Melt loss calculator
Melt Loss (Merma)
Typical losses: Gold 0.5–1.5% · Silver 1–3% · Platinum 0.8–2%