typically you want to use as little power as possible to make a mark... kinda like when doing edge lit acrylic... 

the laser beam doesn't actually ablate the glass, it creates microfractures in the glass when the water molecules are converted to steam. the spawling usually occurs due to too much power.

Glass varies a lot (as do most substrates) so you may have to sacrifice a piece or 2 to get the settings dialed in right.  also, the cheaper the glass the better it engraves typically (cheap glass has a higher moisture content, among other 'things, which lends itself to higher contrasting engraves).

And there are lots of types of glass. Borosilicate glass, like Pyrex®, does not engrave well with a  co2 laser. 

Other engineered performance glass, and glass with different tempers, all have vastly different properties and may not lend themselves well to c02 laser engraving.

as far as masking, soap, etc... this is an attempt to mitigate the spawling and chipping that happens but,  spawling is a symptom of not using the correct settings for the material most times. 

Below is an example of severe spawling and delamination (the darker areas of the engraving).

And, if you have it dialed in right, none of that is needed. just a quick buff with fine steel wool knock down any 'burrs'.