Regardless of what Mares says, unless your mask truly has a coating and a few do and it is usually evident or is plastic and again that should be readily apparent then otherwise the mask is tempered float glass. The tempering is a process that heats the glass to about 1200 degrees F and the quenches it at an accelerated rate that puts the glass under compression. That is what causes it to crumble instead of shatter. It is unlikely that using a BIC lighter to burn the mask could heat it to anything near 1200 degrees unless you really worked at it or used a torch. Fanning the lighter about the inside of the mask until it leaves a black smudge is all that is needed. Then rinse a few times with Dawn and hot water. Glass has a Mohs hardness of about 7, the same as steel. Feldspar and other mineral based abrasives are generally under 6 on the scale. If you insist on scrubbing with an abrasive then most products that do not contain quartz or corrundum or silica/aluminum oxides should not scratch the glass unless you use force or a steel dish pad!
Frameless masks seem particularly hard to clean, both my Artomic Frameless and my two Scubapro Frameless required bunring in addition to Dawn and hot water. I do not uses abrasives and have not found that useful. What is being removed is mostly a silicone or mold release contaminate from the manufacturing process.
Okay, I have made three posts in a row, time to bug out for another year or so. Y'all be well, dive safe or not, because well, safety is somewhat overrated.
N