How can I avoid cracking glass ornaments when laser engraving?

Glass ball ornaments are usually made of thin glass. High heat will crack them. Fortunately, the paint is also thin, so it doesn’t take much laser power to vaporize it and expose the silvering. Power should be set relatively low. Heat can also be reduced by changing the color-fill from pure black to a shade of gray. This reduces the chance of cracking by keeping the laser pulses from overlapping too much.

   —COOLaserCraft