Precious Metals Sample Preparation

Introduction

Precious metals including gold, silver, and platinum are important materials in jewelry, electronics, catalysts, and medical applications. These metals are characterized by their softness, high ductility, and excellent corrosion resistance. Proper preparation is essential to reveal the true microstructure without introducing artifacts such as smearing, contamination, or edge rounding. Precious metals are particularly challenging due to their extreme softness and tendency to work-harden. This guide will walk you through the complete preparation process.

Common precious metals include pure gold (24K), gold alloys (14K, 18K), pure silver, sterling silver (92.5% Ag), and platinum alloys. These materials are very soft (typically 20-100 HV) and highly ductile, making them prone to smearing during preparation. The key to successful preparation is using appropriate abrasives, maintaining very light pressure throughout all steps, and avoiding contamination throughout the process. Special attention must be paid to polishing techniques, as these materials are among the softest commonly prepared for metallographic analysis.

Sectioning

When sectioning precious metals, use a slow cutting speed to minimize heat generation and deformation. These materials are very soft and ductile, requiring careful handling to prevent deformation and contamination. Precious metals are particularly sensitive to heat and mechanical stress, so precision wafering saws are often preferred over abrasive cutting for delicate samples.

MAX-D series abrasive cut-off blades suitable for precious metals. For very delicate samples, precision wafering saws with diamond blades may be preferred. Thin blades (0.3-0.5 mm) minimize heat generation and deformation.

• Use MAX-D series blades for general precious metal alloys, or precision wafering saws for delicate samples
• Use a thin abrasive cut-off wheel (0.3-0.5 mm thickness) to minimize heat and deformation
• Apply very light, steady pressure - precious metals are extremely soft
• Use adequate coolant to prevent overheating and maintain low temperatures
• Allow the wheel to do the cutting - avoid forcing or excessive pressure
• Clean sample immediately after sectioning to remove cutting fluid and prevent contamination
• For very thin or delicate samples, consider precision wafering with diamond blades

For more information on sectioning blades, visit our Abrasive Blades collection.

Mounting

Mounting provides edge retention and easier handling. For precious metals, castable mounting with epoxy resins is strongly preferred to avoid heat-related issues that could affect the microstructure. Compression mounting can also be used with very low-temperature settings, but castable mounting is generally safer for these soft, heat-sensitive materials. Ensure the sample is thoroughly cleaned before mounting to prevent contamination.

Castable Mounting (also known as Cold Mounting) — Recommended

1. Clean and dry the sample thoroughly to remove all cutting fluid and debris
2. Place sample in mounting cup with epoxy resin (transparent epoxy allows visual inspection)
3. Allow to cure at room temperature (typically 4-8 hours, depending on resin)

Castable mounting avoids heat that could affect precious metal microstructure or cause deformation. For very soft metals, consider using a softer mounting resin to minimize edge rounding.

Compression Mounting (Alternative)

1. Clean the sample thoroughly to remove cutting fluid and debris
2. Place sample in mounting press with low-temperature epoxy resin
3. Apply low pressure: 1000-2000 psi (much lower than typical mounting pressures)
4. Heat to 100-120°C (lower than standard mounting temperatures) and hold for 3-5 minutes
5. Cool under pressure to room temperature

Important: Precious metals are extremely soft and heat-sensitive. Castable mounting is strongly recommended to preserve the true microstructure. If compression mounting is necessary, use the lowest possible temperature and pressure settings.

For more information on mounting equipment, visit our Compression Mounting Equipment page.

Grinding

Grinding removes sectioning damage and prepares the surface for polishing. For precious metals, start with finer grits (240 or 320) rather than coarse grits to avoid excessive deformation. Precious metals are very soft (typically 20-100 HV) and can work-harden, so maintain very light pressure and avoid excessive grinding time. The goal is to remove damage without introducing new artifacts.

Silicon carbide (SiC) grinding papers in various grit sizes for progressive grinding. For precious metals, start with 240 or 320 grit to minimize deformation. Rotate sample 90° between each grit to ensure complete scratch removal.

Grinding Sequence

1. 240 grit: Remove sectioning damage (30-45 seconds per step) - start here for most precious metals
2. 400 grit: Remove previous scratches (30-45 seconds)
3. 600 grit: Final grinding step (30-45 seconds)

Important: Rotate the sample 90° between each grit to ensure complete removal of previous scratches. Use water as a lubricant and maintain very light, consistent pressure. Precious metals are extremely soft, so excessive pressure can cause smearing or deformation. Clean the sample thoroughly between steps to prevent contamination. Avoid excessive grinding time as precious metals can work-harden.

Note: For very soft precious metals (pure gold, pure silver), you may skip the 240 grit step and start directly with 400 grit if sectioning damage is minimal.

For more information on grinding supplies, visit our Silicon Carbide Grinding Papers collection.

Polishing

Polishing removes grinding scratches and prepares a mirror-like surface. For precious metals, diamond polishing followed by oxide polishing typically yields excellent results. Use soft pads and maintain very light pressure to avoid smearing, edge rounding, and relief. Precious metals are extremely soft and prone to deformation, so careful control of pressure and time is critical.

Polycrystalline diamond compound provides consistent cutting action for precious metals. Use with soft pads and very light pressure to prevent smearing.

Soft polishing pads are essential for precious metals. Harder pads can cause smearing and edge rounding due to the extreme softness of these materials.

Diamond Polishing

1. 9 μm diamond: 2-3 minutes on a soft pad (e.g., Micropad, Atlantis)
2. 3 μm diamond: 2-3 minutes on a soft pad
3. 1 μm diamond: 1-2 minutes on a soft pad

Final Polishing

1. 0.05 μm colloidal silica: 30-60 seconds on a soft pad
2. Rinse thoroughly with water and dry with compressed air

Critical Considerations: Use very light pressure throughout all polishing steps - precious metals are extremely soft and prone to smearing. Over-polishing can cause edge rounding, especially at sample edges. Use soft pads exclusively; harder pads will cause deformation. Clean the sample thoroughly between steps to prevent contamination. Precious metals can work-harden, so avoid excessive polishing time.

Tip: For very soft precious metals (pure gold, pure silver), you may reduce polishing times by 30-50% to minimize the risk of smearing and edge rounding.

For more information on polishing supplies, visit our Diamond Abrasives and Polishing Pads collections.

Etching

Etching reveals the microstructure by selectively attacking grain boundaries and phases. Precious metals typically require mild etchants due to their softness and reactivity. The choice of etchant depends on the specific precious metal and what features you want to reveal. Common etchants include aqua regia (for gold), potassium cyanide solutions (for silver), and various dilute acid solutions.

Common Etchants for Precious Metals

• Aqua Regia: For gold and gold alloys. Mix 3 parts concentrated HCl with 1 part concentrated HNO₃. Etching time: 5-15 seconds. Warning: Extremely corrosive and produces toxic fumes. Use in fume hood with proper PPE.
• Potassium Cyanide Solution: For silver and silver alloys. 10% KCN in water. Etching time: 10-30 seconds. Warning: Highly toxic - use with extreme caution and proper safety equipment.
• Dilute Nitric Acid: For platinum and platinum alloys. 10-20% HNO₃ in water. Etching time: 5-20 seconds. Less aggressive than aqua regia.
• Iodine-Potassium Iodide: For gold and gold alloys. Milder alternative to aqua regia. Mix 1g I₂, 2g KI, 100ml H₂O. Etching time: 10-30 seconds.
• Ammonium Persulfate: For silver. 10% (NH₄)₂S₂O₈ in water. Etching time: 10-30 seconds. Safer alternative to cyanide solutions.

Etching solutions and reagents for precious metals. Common etchants include aqua regia (gold), potassium cyanide (silver), and dilute nitric acid (platinum). Etching time typically ranges from 5-30 seconds depending on the etchant and metal. Warning: Many precious metal etchants are extremely hazardous and require proper safety equipment and fume hoods.

Etching Procedure

1. Ensure sample is clean and dry
2. Apply etchant with cotton swab or immerse sample (depending on etchant)
3. Etch for 5-30 seconds (time varies by etchant and metal - start with shorter times)
4. Immediately rinse with water, then alcohol
5. Dry with compressed air

Tip: Start with shorter etching times (5-10 seconds) and increase if needed. Precious metals are soft and can be over-etched easily, which can obscure fine details and create pitting. For gold alloys, aqua regia is the most commonly used etchant but requires careful handling. For silver, consider safer alternatives like ammonium persulfate before using cyanide-based solutions.

For more information on etchants, visit our Etchants collection.

Troubleshooting

Common Issues and Solutions

• Smearing: Too much pressure or wrong pad. Use softer pads (e.g., Micropad, Atlantis) and reduce pressure significantly. Precious metals are extremely soft and require very light pressure throughout preparation.
• Edge rounding: Excessive polishing time or too soft a pad. Reduce polishing time by 30-50% and ensure you're using appropriate soft pads, not overly soft ones that cause rounding.
• Contamination: Clean between steps thoroughly. Use fresh abrasives and separate polishing stations if possible. Precious metals can easily pick up contamination from other materials.
• Work-hardening: Excessive grinding or polishing can work-harden precious metals. Use lighter pressure and reduce preparation time. If work-hardening occurs, the material may require annealing before re-preparation.
• Over-etching: Reduce etching time or dilute etchant. Start with shorter times (5-10 seconds) and increase gradually. Over-etching can obscure fine details and create pitting.
• Poor edge retention: Consider using castable mounting with epoxy resin or softer mounting materials. Compression mounting at low temperatures may also help preserve edges.
• Scratches remaining: Insufficient grinding/polishing time or skipped grits. Ensure complete scratch removal at each step. For very soft metals, you may need to extend polishing times slightly, but with very light pressure.
• Deformation during sectioning: Use very slow cutting speeds and very light pressure. Consider precision wafering for delicate samples.
• Inconsistent etching: Ensure sample is clean and dry before etching. Surface contamination can cause uneven etching. Agitate the etchant gently during application for more uniform results.