“Glaze covering an earthen vessel”
Like glaze covering an earthen vessel are burning lips and a wicked heart. (Proverbs 26:23 BSB)
What it sounds like today
A vague comparison to a shiny pot.
The cultural key
The picture depends on a potter's trick the first hearers knew well. A cheap earthenware vessel could be coated with a glossy glaze, made with lead, so that a plain clay pot gleamed as if it were something fine and valuable. The coating was all surface: underneath sat the same common, breakable clay. That, says the proverb, is what 'burning lips and a wicked heart' are like, speech that glows with warmth, eagerness, or affection, painted over a heart that is worthless and malicious. The next verses press the point: 'a hateful man disguises himself with his speech,' so 'when he speaks graciously, do not believe him, for seven abominations fill his heart.' It is the same insight Jesus captures with whitewashed tombs, beautiful outside, full of death within (Matthew 23:27). The glaze is a warning to distrust a shine that is only skin-deep.
Recovered meaning
A lead glaze made a cheap clay pot look precious while hiding common, brittle earthenware. So warm, glowing words ('burning lips') can be a thin coating over a worthless, wicked heart, a shine not to be trusted.