You're hanging out with your fellow Romans, throwing back a few goblets of the good stuff. You're glad the wine is flowing, but from the taste of it, the vintner's focus was on quantity, not quality. Thank goodness for the burnt toast at the bottom of your chalice. The charcoal exterior tempers the bitterness of the beverage.
While adding burnt bread to wine is par for the course, breaking convention by toasting with an empty glass would be bad form. Throughout time, it's been bad luck to cheers with water or an empty glass. According to Roman etiquette, for example, there was little point in raising an empty glass to wish someone good cheer.