Two bars of dark chocolate. Same recipe, same cocoa, same factory. One is glossy, breaks with a clean snap and stays firm on the kitchen counter, then melts in your mouth. The other is dull, soft or crumbly, and wears a grey-white film. The ingredients did not change. What changed is how the fat set.

One fat, six ways to stack

The fat in chocolate is cocoa butter, built from a handful of closely related triglyceride molecules, chiefly the ones food scientists call POP, POS and SOS. When cocoa butter freezes, those molecules lock into a regular lattice. The catch is that they can lock together in more than one pattern. The patterns are numbered I to VI (the Wille–Lutton scheme), and each has its own melting point: roughly 17, 23, 25.5, 27.5, 34 and 36 °C.

Think of packing a suitcase. Toss things in and they fit loosely; fold and stack them and they take less room and stay put. The loose lattices form quickly and fall apart at low temperatures. The tight ones take more organisation and hold together until it is much warmer.

Why form V is the one you want

Form V melts at about 34 °C. That is a sweet spot: well above a warm kitchen, so the bar stays solid and glossy, but below body temperature (37 °C), so it melts on your tongue. It is also the form that gives chocolate its shine and its clean snap. Form VI is more stable still, but it takes weeks to appear, so nobody aims for it. Forms I to IV are the other kind: softer or less glossy, and they melt too easily.

The problem: the wrong crystals win the race

Melted cocoa butter does not jump straight to form V. As it cools, the loose forms start first and fastest, so a quick chill fills the chocolate with a messy mix of low-melting crystals. Tempering is the craft of steering the fat towards form V. Exact temperatures differ by a few degrees between makers and between dark, milk and white chocolate, but the shape of the method is always the same:

1. Melt everything, to roughly 40–45 °C, so no crystal of any form survives.
2. Cool to about 27 °C while stirring. Crystals begin to form, and stirring helps them start.
3. Warm gently to about 31 °C. Forms I to IV melt away; form V, which holds out to 34 °C, stays.
4. Work and pour in that window. The remaining liquid sets around the form V crystals and copies them.

Step 3 is a filter. Between about 28 and 34 °C, every crystal that is not form V is above its own melting point, so only form V can survive there. A shortcut is seeding: stir in finely chopped chocolate that is already tempered. Its form V crystals are ready-made nuclei that guide the rest of the fat into the same structure.

Temper it yourself

The grid is a thin slice of melted chocolate. Dark cells are liquid; coloured cells are crystals, lime for form V and warm colours for forms I to IV. Drag the water bath to change the temperature, switch stirring on, add a seed, and when you are ready, pour. Try the guided plan first, then try the fridge shortcut.

What goes wrong

Pour without any seeds and the fat sets with whatever crystals happen to appear first, which are mostly the unstable ones. Overheat tempered chocolate past 34 °C and you melt the form V crystals you worked for, and must start again. Chocolate tempered badly can also develop bloom, a white powdery film on the surface. It is why a bar that melted in a hot car and re-set in the fridge so often comes out looking different.

Same ingredients, different crystals. Tempering is the art of making cocoa butter freeze in the one pattern that melts at 34 °C.