Pure Lead

Pure lead is an extremely soft (5 HB, ~4 HV) elemental metal with a low melting point (327°C) and high density (11.34 g/cm³). It is one of the softest metals encountered in metallographic preparation. The extreme softness causes severe smearing and deformation during grinding and polishing. Abrasive particles embed readily in the surface. The low melting point requires cold mounting only. Lead is toxic, so all preparation debris must be handled as hazardous waste.

Extremely soft material. Use a low-speed diamond saw with continuous coolant at 100-200 RPM. Minimize feed rate to reduce deformation. The material is toxic; contain all cutting debris and coolant for proper disposal. Avoid generating fine particles or dust. For small or thin samples, sectioning with a razor blade or fine jeweler's saw is possible. Standard abrasive cut-off wheels can also be used with very low feed rates. Leave 2-3 mm allowance for grinding away deformation from cutting.

Cold mounting with castable epoxy is required. The low melting point (327°C) and extreme softness make compression mounting unsuitable; even moderate pressure will deform the sample. Use a low-shrinkage epoxy resin. Adding a mineral filler (glass beads or alumina powder) to the epoxy creates a harder mount that provides better support during grinding and reduces edge rounding. Ensure complete cure (typically 8-12 hours) before grinding.

The extreme softness (5 HB) requires very careful grinding. Start at 320 or 400 grit SiC; coarser grits will embed deep damage that is difficult to remove from such soft material. Use very light pressure (10-15 N per 30 mm sample). Disc speed: 150-250 RPM. Progress through 600, 800, 1200 grit. Fresh papers are essential at each step; worn papers cause more smearing than cutting. Thorough ultrasonic cleaning between steps is critical to remove embedded SiC particles. Diamond grinding discs are preferred over SiC paper to reduce embedding.

Grinding sequence:

• 320 grit: Remove sectioning damage (15-30 seconds). Very light pressure. Monitor constantly for smearing.
• 400 grit: Remove previous scratches (15-30 seconds). Fresh paper only.
• 600 grit: Further refinement (15-30 seconds). Continue very light pressure.
• 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° between steps. Use complementary rotation. Abundant water lubrication is critical. Grind only long enough to remove the previous scratch pattern; over-grinding causes work hardening artifacts.

The extreme softness requires very careful polishing with minimal pressure. Use napless or low-nap cloths throughout to avoid relief and orange peel.

Diamond polishing sequence:

• 6μm diamond: 2-3 minutes on a napless synthetic pad with very light pressure (10-15 N per 30 mm sample). Monitor constantly for smearing and orange peel.
• 3μm diamond: 2-3 minutes on a napless synthetic pad with very light pressure (10-12 N). Continue monitoring for smearing.
• 1μm diamond: 1-2 minutes on a napless pad with minimal pressure (8-12 N).

Final polishing:

• 0.05μm colloidal silica: 1-2 minutes on a soft final polishing pad with very light pressure. Or vibratory polishing with colloidal silica for 2-4 hours, which gives the best results for extremely soft lead by removing residual deformation without introducing new damage.

Lead is extremely prone to orange peel if over-polished. Check surface frequently under the microscope. If orange peel develops, reduce pressure or switch to vibratory polishing.

Pure lead responds to several standard etchants. The material etches quickly due to its softness, so start with short times and increase as needed.

10% Acetic Acid (Chemical Etching) - Primary choice:

• Composition: 10 ml glacial acetic acid, 90 ml water
• Application: Immerse for 5-15 seconds. Swab etching also effective.
• Reveals: Grain boundaries clearly. Good general-purpose etchant for lead.
• Rinse: Water, then ethanol. Dry with warm air.

5% Nital (Chemical Etching) - Alternative:

• Composition: 5 ml HNO₃, 95 ml ethanol
• Application: Immerse for 3-10 seconds. More aggressive than acetic acid.
• Reveals: Grain boundaries and substructure.
• Rinse: Ethanol, then dry with warm air.

Safety: Lead is toxic. All etching solutions containing dissolved lead must be collected and disposed of as hazardous waste. Work in a fume hood. Wear appropriate PPE including gloves and eye protection.