Pure Gold

Pure gold is an extremely soft (25 HV) precious metal with FCC crystal structure and a melting point of 1064 degrees C. It is one of the most challenging metals for metallographic preparation due to extreme softness and ductility. Severe smearing, scratching, and abrasive embedding dominate all preparation stages. The material does not work harden appreciably, so deformation layers propagate deeply. Very light pressures and napless polishing cloths are essential throughout. Vibratory polishing is strongly recommended for final polish to remove subsurface deformation without introducing new damage.

Use a low-speed precision diamond saw with continuous coolant at 100-200 RPM. Apply minimal feed pressure; the extreme softness means the blade will progress readily. Avoid standard abrasive cut-off wheels, which embed abrasive particles into the gold surface and cause excessive deformation. For thin specimens (wire, foil, sheet), precision wafering with a thin diamond blade (0.3-0.5 mm) is preferred. For bulk specimens, a standard diamond saw is acceptable. Leave 2-3 mm allowance for grinding away cutting damage. The material will deform plastically rather than fracture, so clamping must be firm but padded to avoid indentation marks.

Cold mounting with castable epoxy is preferred for best results. Use a low-shrinkage epoxy resin with mineral filler (glass beads or alumina) to create a harder mount that provides mechanical support during grinding. The high melting point (1064 degrees C) means hot compression mounting is technically feasible, but the extreme softness makes cold mounting preferable to avoid pressure-induced deformation. For wire bond cross-sections or thin gold layers on substrates, vacuum impregnation is recommended to fill gaps. Edge retention is critical when examining gold coatings or thin gold features.

The extreme softness (25 HV) demands very careful grinding with fresh, sharp abrasives at each step. SiC papers embed readily into the soft surface, so diamond grinding discs are strongly preferred. Use very light pressure (10-15 N per 30 mm sample). Disc speed: 150-250 RPM. Complementary rotation to reduce deformation. Thorough ultrasonic cleaning between each step is essential to remove any embedded abrasive particles.

Grinding sequence:

• 320 grit: Remove sectioning damage (15-30 seconds). Very light pressure. Fresh paper only. Monitor constantly for smearing.
• 400 grit: Remove previous scratches (15-30 seconds). Rotate specimen 90 degrees.
• 600 grit: Further refinement (15-30 seconds). Continue very light pressure.
• 800 grit: Prepare for polishing (15-30 seconds). Check under microscope for embedded particles.
• 1200 grit: Final grinding step (15-30 seconds). Ensure all previous scratches are removed before proceeding to polishing.

Rotate specimen 90 degrees between each step. Use abundant water lubrication. Grind only long enough to remove the previous scratch pattern; over-grinding embeds abrasive deeper into the soft matrix.

Napless or very low-nap cloths are mandatory throughout all polishing steps. Napped cloths cause severe orange peel, relief, and rounding on gold surfaces.

Diamond polishing sequence:

• 6 micrometer diamond: 2-3 minutes on a napless hard synthetic pad with very light pressure (10-15 N per 30 mm sample). Use oil-based diamond extender. Monitor constantly for smearing and orange peel.
• 3 micrometer diamond: 2-3 minutes on a napless synthetic pad with very light pressure (10-12 N). Continue monitoring for deformation artifacts.
• 1 micrometer diamond: 1-2 minutes on a napless pad with minimal pressure (8-12 N). Surface should appear nearly scratch-free under the microscope.

Final polishing:

• 0.05 micrometer colloidal silica: 1-2 minutes on a soft final polishing pad with very light pressure. Vibratory polishing with colloidal silica for 4-8 hours is strongly recommended for best results, as it removes residual subsurface deformation without introducing new damage. This is the most effective method for achieving a true, artifact-free surface on pure gold.

If orange peel develops, reduce pressure immediately or switch to vibratory polishing. Check surface frequently under the microscope.

Pure gold is chemically inert to most single acids. Aqua regia is the traditional and most effective etchant. The material etches relatively slowly compared to base metals; times may need to be extended.

Aqua Regia (Chemical Etching) - Primary choice:

• Composition: 3 parts HCl (concentrated) + 1 part HNO3 (concentrated). Mix fresh immediately before use; solution loses effectiveness within hours.
• Application: Immerse for 5-30 seconds. Swab etching is also effective for controlled etching.
• Reveals: Grain boundaries, annealing twins in recrystallized material, subgrain structure in deformed gold.
• Rinse: Water, then ethanol. Dry with warm air.

Potassium Cyanide Solution (Chemical Etching) - Alternative for selective etching:

• Composition: 10 g KCN, 100 ml water
• Application: Immerse for 10-60 seconds. Slow, controlled etching. Produces very clean grain boundary delineation.
• Reveals: Grain boundaries with excellent clarity. Preferred for grain size measurements.
• CAUTION: Potassium cyanide is extremely toxic and potentially lethal. Must be used in a fume hood with full PPE. Never mix with acids, which generates lethal HCN gas. Waste must be treated with bleach before disposal.

Safety: Aqua regia produces toxic fumes (NOCl, Cl2); work in a fume hood. KCN is lethal; strict safety protocols required.