Darkening & Finishing Metal Parts
The engrave is half the product; the darkening and finishing pass is the other half. The single most useful fact in this module: laser darkening is nickel chemistry. Know which metals have the nickel to burn black, and reach for a chemical blackener on the ones that don't.
Laser darkening: it's the nickel
A darkening pass (ultra-dense line interval around 0.004mm / 6000 LPI, slow, single direction, see the Universal Depth recipe) works by burning the metal's nickel content black. No nickel, no black, no matter how you tune the settings.
| Metal | Laser-darkenable? | If not, use |
|---|---|---|
| Titanium | Yes | - |
| Brass | Yes | (Brass Black also works, see below) |
| Stainless steel | Yes | - |
| Silver | No | Liver of sulfur |
| Gold | No | Liver of sulfur |
| Aluminum | No | Aluminum Black |
| Copper | No | Brass Black |
Buying tip: if brass/copper work is a lane for you, buy Brass Black by the half-gallon. The small bottle runs out fast; the half-gallon is effectively a lifetime supply for a small shop.
The Brass Black dip + Scotch-Brite trick
The fix for "polished piece is too shiny to read": dip the whole polished piece in Brass Black, then hit just the top surface with a Scotch-Brite pad. The scuff pulls the black off the raised surfaces and leaves it in the recesses, dark background, bright raised detail, instantly legible. Works on brass and copper jewelry, charms, and cut pieces.
Finishing rules
- Everything gets polish. No metal part ships straight off the laser.
- Wet-sand, and stay at or under 800 grit. Flat-sand the top face to knock off slag, light-sand cut-out edges, Scotch-Brite for general cleanup. Coins: flat-sand only.
- Darkened parts: flash-sand + toothbrush only. NEVER tumble. A tumbler grinds a laser-darkened surface down to a gunmetal gray and the black is gone.
- Tumbler discipline (non-darkened parts): pointed media, lowest speed. Flat parts 15-20 minutes, 3D relief work ~30 minutes.
- Order of operations on relief coins: tumble first, THEN Brass Black. Never the reverse.