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).

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 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'.