Pure Silver

Pure silver is an extremely soft (25 HV) precious metal with FCC crystal structure and a melting point of 962 degrees C. Preparation challenges are nearly identical to pure gold: severe smearing, abrasive embedding, and deep deformation layers. Silver tarnishes quickly in air, so prepared specimens must be examined promptly. The material does not work harden appreciably, making deformation management the primary concern throughout preparation.

Use a low-speed precision diamond saw with continuous coolant at 100-200 RPM. Apply minimal feed pressure. The extreme softness means standard abrasive cut-off wheels will embed particles into the surface; diamond saws are preferred. For thin silver specimens (sheet, foil, plating), precision wafering is recommended. For bulk specimens, a standard low-speed diamond saw is acceptable. Leave 2-3 mm allowance for grinding away sectioning damage. Clamp firmly but with padding to avoid indentation.

Cold mounting with castable epoxy is preferred. Use a low-shrinkage epoxy with mineral filler for support during grinding. The high melting point (962 degrees C) allows hot compression mounting, but cold mounting is recommended to avoid pressure deformation of this very soft material. For silver brazing alloy interfaces or silver plating cross-sections, edge-retaining compounds and vacuum impregnation are recommended to preserve interface integrity.

The extreme softness (25 HV) requires the same careful approach as pure gold. Diamond grinding discs are strongly preferred over SiC papers to minimize abrasive embedding. Use very light pressure (10-15 N per 30 mm sample). Disc speed: 150-250 RPM. Complementary rotation. Thorough ultrasonic cleaning between each step is essential.

Grinding sequence:

• 320 grit: Remove sectioning damage (15-30 seconds). Very light pressure. Fresh paper only.
• 400 grit: Remove previous scratches (15-30 seconds). Rotate specimen 90 degrees.
• 600 grit: Further refinement (15-30 seconds).
• 800 grit: Prepare for polishing (15-30 seconds).
• 1200 grit: Final grinding step (15-30 seconds). Ensure all previous scratches are removed.

Rotate specimen 90 degrees between each step. Use abundant water lubrication. Do not over-grind.

Napless or very low-nap cloths are mandatory throughout. Silver is as susceptible to orange peel and smearing as gold.

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). Oil-based diamond extender.
• 3 micrometer diamond: 2-3 minutes on a napless synthetic pad with very light pressure (10-12 N).
• 1 micrometer diamond: 1-2 minutes on a napless pad with minimal pressure (8-12 N).

Final polishing:

• 0.05 micrometer colloidal silica: 1-2 minutes on a soft final polishing pad. Vibratory polishing with colloidal silica for 4-8 hours gives the best results for removing residual deformation. Examine promptly after final polishing, as tarnish begins forming within minutes in ambient air.

If orange peel develops, reduce pressure or switch to vibratory polishing.

Silver responds to several chemical etchants. The surface tarnishes rapidly after etching, so examine immediately.

Potassium Dichromate Solution (Chemical Etching) - Primary choice:

• Composition: 2 g K2Cr2O7, 8 ml H2SO4 (concentrated), 4 drops HCl (concentrated), 100 ml water
• Application: Immerse for 5-30 seconds. Swab etching provides more controlled results.
• Reveals: Grain boundaries and annealing twins with excellent contrast. The standard etchant for silver and silver alloys.
• Rinse: Water, then ethanol. Dry with warm air. Examine immediately.

Ferric Chloride Solution (Chemical Etching) - Alternative for rapid etching:

• Composition: 5 g FeCl3, 100 ml water
• Application: Immerse for 3-15 seconds. Etches more aggressively than dichromate solution.
• Reveals: Grain boundaries, subgrain structure, and deformation bands.
• Rinse: Water, then ethanol. Dry immediately.

Safety: K2Cr2O7 is a known carcinogen; handle with care and dispose of properly. Work in a fume hood. Standard PPE required for all etchants.