Since the composition and quality of glass varies widely, it's always best
to experiment with an unfamiliar glass source. But, generally speaking, flat
glass tends to have a very consistent hardness throughout. Engraved areas
will generally not yield lighter and darker areas.
on the other hand, tend to have soft and hard spots. This may cause one
engraved area to appear lightly frosted yet heavily frosted in another.
Engraving using medium and high power will compensate somewhat for this as
will doing two or more engraving passes. And although the laser beam itself
is very hot, the heat does not build up easily and should not prevent you
from engraving full bottles of wine, champagne or other filled glass
bottles. Laser engraving filled bottles is a very popular method of creating
custom presentations for special occasions and the laser will not damage the
liquid inside the bottle. Just remember, as long as you are not completely
engraving away large sections of the bottle, you have very little chance of
breaking the bottle. Enjoy!