For anyone looking at this post that may be confused, the operation here isn’t engraving. The settings used in the Glowforge are labelled engraving, but the actual process the laser is doing is bonding a marking material to the underlying metal surface.
Since my original wording seems to have been confusing, here is the simplified version:
The Glowforge laser doesn’t have the power to remove material from most metals.
Copper alloys will reflect the laser beam back into the laser head. The more copper is included in the alloy the more reflection there will be. Nickel silver is the lowest commonly used percentage of copper at 60%. Other copper alloys include brass, bronze. Other metals may contain copper as well, though most are low percentage alloying amounts. Rose gold has a higher included copper percentage at 25% copper to 75% gold.
Reflecting the laser back into the head will cause damage.
A shiny surface will reflect to some degree, but the laser is high-band infrared so materials looking reflective isn’t a good indication of IR reflectivity.
Using a marking compound should be safe as the compound is the absorbing material, but any gaps in the compound may allow the laser to reflect from a laser-reflective surface.
Users should take care when using any copper alloy to ensure they are doing so safely.
Original:
The Glowforge doesn’t have enough power to effectively engrave, that is remove material, most metals and copper alloys will reflect the beam back into the machine and damage it.