Tempered chocolate that blooms — develops white streaks or a dull, soft surface — has gone through an incomplete crystallisation process. The cocoa butter formed the wrong crystal structure, or the right one collapsed before setting.
The temperature window is narrower than most guides admit
Dark chocolate needs to be worked at 31 to 32 degrees Celsius during the tabling or seeding stage. At 33 degrees, the stable Form V crystals melt back into the mix. At 29 degrees, the chocolate thickens too fast to coat evenly. Most kitchen thermometers are not accurate enough for this range. An infrared thermometer aimed at the marble slab or the bowl surface reads faster and more reliably than a probe submerged in the mass.
Seeding versus tabling
Seeding — adding finely chopped tempered chocolate to melted chocolate — is more consistent for home and small-batch work than tabling on marble. The seed chocolate introduces pre-formed crystals directly. The ratio is roughly 20 to 25 per cent seed chocolate by weight of the total mass.
Where seeding fails is when the base chocolate is too hot when the seed goes in. Above 35 degrees, the seed melts completely and contributes nothing. Let the melted chocolate cool to 34 degrees before adding seed, then stir continuously until the working temperature is reached.
Testing before committing
Spread a small amount on baking paper and set it aside at room temperature. After 3 to 4 minutes it should be firm, glossy, and snap cleanly when bent. If it does not, the batch needs to be retempered.