Pennies?

[rpegg]

Have zero knowledge myself, just parroting. Support said at one point copper should not be used because it reflects the beam. Damage might result.

Edit: Actually I may be combining info from two different posts. Dan said that copper is an almost perfect reflector and not CO2 laser compatible. And Support (Rita) said the following.

@Rita

If the crumb tray is removed, use extreme caution to ensure that the laser only strikes laser-compatible material, and does not touch the metal bottom of the unit which could cause a reflection and damage the unit.