According to a note written by Armfield in 1971 (note book no VIII p 695) the title comes from Ecclesiastes 7.6 For as the crackling of thorn bushes under a pot, So is the laughter of the fool; And this too is futility.
Armfield might also have had in mind Psalm 58:9
Before your pots can feel the fire of thorns He will sweep them away with a whirlwind, the green and the burning alike.
As crackling nettles under kettles, As crackling stubble makes the pot bubble are the nearest English equivalents. The image is drawn from the Eastern use of hay, stubble, and thorns for fuel (Matthew 6:30; Psalm 118:12). A fire of such material, burnt up more quickly than the charcoal embers (Jeremiah 26:22; John 18:18), which were also in common use, but then it also died out quickly and left nothing but cold dead ashes. So it would be with the mirth which was merely frivolous or foul. That also would take its place in the catalogue of vanities.