Cocoa butter is a fussy fat. Left to its own devices it will set into any one of six different crystal structures, and five of them produce chocolate that nobody particularly wants to eat.
This is not a figure of speech. The forms were catalogued properly by food scientists in the 1960s and they are still numbered I to VI, which tells you something about how little the problem has changed since. Each has its own melting point. Form I gives up at around 17 degrees, roughly the temperature of a cool British kitchen, so a bar made of it would slump on the plate before anyone reached for it. Form IV melts near 28 and feels waxy, faintly greasy, and disappointing in a way most people can taste but not name. Form VI is hard, crumbly and dull.
Form V melts at about 34 degrees. Body temperature is 37. That gap of three degrees is the whole business.
Everything you like is Form V
The gloss on a good bar is Form V crystals packed tightly enough to reflect light evenly rather than scatter it. The snap is those same crystals resisting a bend and then failing all at once, instead of yielding slowly like a candle. The clean release from the mould is the bar contracting very slightly as it sets and pulling itself away from the polycarbonate. And the melt, which is the part that actually matters, is Form V sitting just below body temperature and then going entirely liquid in the mouth without needing to be asked twice.
Chocolate that has not been tempered does all of this badly. It sets pale and streaked, it smears rather than snaps, and it leaves a film on the tongue that lingers, because some of it is still solid at 37 degrees and simply declines to leave.
Persuasion, conducted at half a degree
Tempering is the business of making Form V the only reasonable option available. The chocolate is melted fully, to around 45 degrees, which destroys every existing crystal without prejudice and gives a clean start. It is then cooled to roughly 27, at which point crystals begin forming enthusiastically and indiscriminately, including several we have no use for. Then it is warmed back to about 31. The unstable forms, being unstable, melt out. Form V, being the stubborn one, stays.
What is left is a liquid seeded with a small population of correct crystals, and the rest of the batch copies them as it sets. It is less like cooking and more like a very small act of persuasion, held to within about half a degree throughout. Miss the window and the chocolate tells on you within the hour.
Anyone who has left a bar in a warm car has seen the failure mode. The pale grey film that appears is not mould and it is not dust; it is cocoa butter that got warm enough to move, migrated to the surface, and recrystallised into Form VI, which is stable, ugly, and irreversible without melting the whole thing down and starting again. It is still perfectly safe to eat. It just no longer tastes like anything anybody worked on.
August is the month this happens most. Chocolate is not fond of British summer, or of the twenty minutes it spends on a doorstep in the sun, or of the fruit bowl next to the window. Cool, dark, dry, around 16 to 18 degrees, and not the fridge, where condensation causes a separate and equally tedious problem involving sugar.
All of which is a great deal of fuss over a fat that most people never think about. But a bar should be solid in the hand and liquid in the mouth, with nothing in between. Everything that happens between those two states is our problem, and we would rather keep it.