Why chocolate sets dull instead of glossy

A glossy surface is a consequence of dense, uniform Form V crystals forming as the chocolate sets. Untempered chocolate crystallises into a mixture of unstable forms with irregular surfaces, and irregular surfaces scatter light rather than reflecting it.

So dullness and streaking are the visible signature of a failed temper, and they arrive with two companions: a soft set and a break that crumbles rather than snapping. If all three are present, the diagnosis is certain.

For moulded work there is a second cause that has nothing to do with crystallisation. The gloss on a moulded bonbon is a cast of the mould's own surface — polished polycarbonate gives a mirror finish, and a scratched or dusty mould gives a matt one however well the chocolate was tempered.

This is why professional moulds are cleaned with cotton wool rather than washed, and never scrubbed. A dishwasher will destroy a polycarbonate mould's finish permanently in one cycle.

Condensation, from setting in a fridge or a humid room, leaves a matt bloomed surface even on well-tempered chocolate.

And handling: touching the surface while it is setting, or spreading a glaze with a palette knife after pouring, destroys gloss immediately. A glaze is poured in one movement and then left alone for exactly this reason.