Pure Tin

Pure tin is an extremely soft (5 HB, ~5 HV) elemental metal with a very low melting point (232°C). Similar prep challenges to pure lead: severe smearing, abrasive embedding, and sensitivity to heat. Cold mounting is required. The material is non-toxic but the very low melting point demands careful temperature control throughout preparation.

Extremely soft material. Use a low-speed diamond saw or precision wafering saw with continuous coolant at 100-200 RPM. Minimize feed rate. The very low melting point (232°C) means heat generation must be carefully controlled. Standard abrasive cut-off wheels can be used with very low feed rates and generous coolant. For thin tin plate or tin coatings, precision wafering is preferred. Leave 1-2 mm allowance for grinding.

Cold mounting with castable epoxy is required. The very low melting point (232°C) makes compression mounting temperatures (150-180°C) risky, as the sample could soften or deform under pressure near its melting point. Use a low-shrinkage epoxy resin. For tin coatings or plating cross-sections, use edge-retaining mounting compounds and consider vacuum impregnation to fill any gaps at the coating interface.

The extreme softness (5 HB) requires very careful grinding. Start at 320 or 400 grit SiC. Use very light pressure (10-15 N per 30 mm sample). Disc speed: 150-250 RPM. Progress through 600, 800, 1200 grit. Fresh papers essential at each step. Thorough ultrasonic cleaning between steps to remove embedded SiC particles. Diamond grinding discs preferred over SiC paper to reduce embedding.

Grinding sequence:

• 320 grit: Remove sectioning damage (15-30 seconds). Very light pressure.
• 400 grit: Remove previous scratches (15-30 seconds).
• 600 grit: Further refinement (15-30 seconds).
• 800 grit: Prepare for polishing (15-30 seconds).
• 1200 grit: Final grinding step (15-30 seconds).

Rotate specimen 90° between steps. Abundant water lubrication is critical.

The extreme softness requires very careful polishing. Use napless or low-nap cloths throughout.

Diamond polishing sequence:

• 6μm diamond: 2-3 minutes on a napless synthetic pad with very light pressure (10-15 N).
• 3μm diamond: 2-3 minutes on a napless pad with light pressure (10-12 N).
• 1μm diamond: 1-2 minutes on a napless pad (8-12 N).

Final polishing:

• 0.05μm colloidal silica: 1-2 minutes on a soft final polishing pad. Vibratory polishing (2-4 hours) with colloidal silica gives the best results for removing residual deformation.

Prone to orange peel if over-polished. Check surface frequently.

Pure tin responds to several etchants. The material etches quickly; start with short times.

2% Nital (Chemical Etching) - Primary choice:

• Composition: 2 ml HNO₃, 98 ml ethanol
• Application: Immerse for 5-10 seconds.
• Reveals: Grain boundaries in beta-tin matrix.
• Rinse: Ethanol, then dry with warm air.

5% HCl in Ethanol (Chemical Etching) - Alternative:

• Composition: 5 ml HCl, 95 ml ethanol
• Application: Immerse for 5-15 seconds. Good grain boundary contrast.
• Reveals: Grain boundaries and substructure clearly.

Safety: Use fume hood for all etching. Standard PPE required.