JohnH writes:

What do you do that gave you that bit of knowledge?

Pyrotechnics.

Dross can interfere with the proper functioning of a lot of pyrotechnic
devices, like clogging up nozzles in a gerb or smoke signal.

Marine flares are designed to not slough dross. The Pains-Wessex
handheld type leave you holding an incandescent baton of solid dross.

Having fired quite a few expired P-Ws on land, illuminating acres of the
surroundings with a literally dazzling light, to celebrate Independence
Day, I don't want to ever hold one in hand on a pitching boat. It would
take some skill to avoid nasty burns.

One should practice with these things (not out on the water, of course).
Any occasion calling for their use would be enough worry in itself, to
which you should not add an initiation into how to hold a 3000 degree
gob of burning magnesium at arms length.

Light off a few of your expired boat flares at home next July 4 (or
whatever your appropriate local occasion is). You might find a dim
fizzle that calls for a different model, or be surprised at how scary
the "good" ones are.