Pure Platinum

Pure platinum is a soft (40 HV) precious metal with FCC crystal structure and a very high melting point of 1768 degrees C. While harder than gold or silver, it is still soft enough to require careful preparation techniques. Smearing is less severe than with gold but remains the primary concern. The extremely high density (21.45 g/cm3) means specimens are deceptively heavy. Platinum is chemically very inert, requiring hot aqua regia or HCl-based etchants.

Use a low-speed precision diamond saw with continuous coolant at 100-200 RPM. Platinum is somewhat harder than gold and silver (40 HV), so it tolerates slightly higher feed rates, but caution is still warranted. Standard abrasive cut-off wheels can be used but diamond saws produce less deformation. The high density means specimens exert more force on the blade than expected from their size. Leave 2-3 mm allowance for grinding. Clamping must account for the high density to prevent slippage.

Cold mounting with castable epoxy is preferred for optimal results, though the high melting point (1768 degrees C) makes hot compression mounting fully acceptable. If using hot mounting, phenolic or epoxy resins at 150-180 degrees C and 2000-3000 psi are appropriate; the material will not be affected. Use mineral-filled epoxy for cold mounts to provide adequate support during grinding. For platinum catalyst specimens (e.g., gauze, mesh, or supported catalysts), vacuum impregnation is essential to fill the porous structure.

Platinum at 40 HV is moderately easier to grind than gold or silver but still requires care to avoid smearing. SiC papers are acceptable, though diamond grinding discs reduce embedding risk. Use light pressure (15-20 N per 30 mm sample). Disc speed: 150-300 RPM. Complementary rotation. Clean ultrasonically between each step.

Grinding sequence:

• 320 grit: Remove sectioning damage (20-40 seconds). Light pressure.
• 400 grit: Remove previous scratches (20-30 seconds). Rotate specimen 90 degrees.
• 600 grit: Further refinement (20-30 seconds).
• 800 grit: Prepare for polishing (15-30 seconds).
• 1200 grit: Final grinding step (15-30 seconds).

Rotate specimen 90 degrees between each step. Use abundant water lubrication.

Napless or low-nap cloths recommended to minimize relief and smearing.

Diamond polishing sequence:

• 6 micrometer diamond: 2-4 minutes on a napless synthetic pad with light pressure (15-20 N per 30 mm sample).
• 3 micrometer diamond: 2-3 minutes on a napless synthetic pad with light pressure (12-18 N).
• 1 micrometer diamond: 1-2 minutes on a napless pad with light pressure (10-15 N).

Final polishing:

• 0.05 micrometer colloidal silica: 1-2 minutes on a soft final polishing pad. Vibratory polishing with colloidal silica for 3-6 hours is recommended for producing the best possible surface finish. Platinum polishes to a brilliant mirror finish when properly prepared.

Platinum is extremely chemically inert, requiring aggressive etchants and often elevated temperatures.

Hot Aqua Regia (Chemical Etching) - Primary choice:

• Composition: 3 parts HCl (concentrated) + 1 part HNO3 (concentrated). Mix fresh immediately before use.
• Application: Warm to 50-70 degrees C and immerse for 15-60 seconds. Cold aqua regia etches very slowly; warming is usually necessary. Swab etching with warmed solution also effective.
• Reveals: Grain boundaries and annealing twins. Subgrain structure in deformed material.
• Rinse: Water, then ethanol. Dry with warm air.

Hydrochloric Acid Electrolytic Etch (Electrolytic Etching) - Alternative for more controlled results:

• Composition: 10% HCl in water (electrolyte)
• Application: 3-6 V DC, 5-30 seconds. Platinum anode, stainless steel cathode.
• Reveals: Grain boundaries with excellent clarity. Preferred when precise etching control is needed.

Safety: Hot aqua regia produces toxic fumes (NOCl, Cl2). Must be used in a well-ventilated fume hood. Standard PPE required.