The thing that separates true tempering from the rest of the so-called “tempering” that happens in kitchens is that it involves both controlled heating and cooling. Just like in metal casting, chocolate tempering means manipulating the liquid goods so that specific types of crystallization can occur. Different types of cocoa butter crystals form at different temperatures, you see. But as fate would have it, the temperature range where the desirable crystals form is a lot narrower than the one where the undesirable ones form.
The stable cocoa butter crystals that give chocolate its lovely sheen and bite only happen between about 86 and 90 degrees Fahrenheit (86-88 degrees for milk chocolate, 88-90 degrees for dark chocolate). Given that razor-sharp temperature range, it’s easy to see why unstable cocoa butter crystals are generally the rule with chocolate that’s been melted. As chocolate cools from its melting temperature (say, 115 degrees or so) it passes through that three-degree temperature window pretty fast. Any stable crystals that do manage to form are soon vastly outnumbered by the unstable crystals that have from 85 degrees all the way down to 60-or-so degrees to take up real estate.
So the main task of tempering is finding a way to grow more of those pretty, shiny, stable crystals. We start by wiping the slate clean. That is, heating up the chocolate so all the fat crystals melt: 115 to 120 degrees (much higher than that and you risk breaking the chocolate). Then we allow the melted chocolate to slowly cool down to the temperature range where stable fat crystals form. We’ll pretend we’re making dark-chocolate-coated strawberries here, so we’re shooting for 89 degrees on the nose. At this point we can either wait for stable crystals to form on their own, or we can “seed” the chocolate with the kind of fat crystals we want, letting them serve as a catalyst for crystal growth through the entire melted mass. Sort of like setting a pattern. We simply take a few bits of unmelted bar chocolate (most good bar chocolate is tempered) and add it to the pot where it will slowly melt and spread its stable crystal goodness. You’ll be able to tell when crystals are forming because the liquid chocolate will really start to thicken up. A couple of minutes later you can start dipping your berries.
“But wait!” you say, “Once the berries have been dipped the chocolate will cool below the stable crystal range and all our work will be undone!” Oh no it won’t. Because the stable crystals will have gobbled up most of the available fat by then, leaving few raw materials for unstable crystals to use. Every berry dipped will end up shiny and lustrous. Pretty neat, eh?
Just so long as you can maintain a rock-steady temperature of 89 degrees throughout the dipping process. Which is of course a lot easier said than done. The other down side is that if you hold chocolate in the tempering range for too long it can over-temper, which means too many stable fat crystals will form and the chocolate will go from firm and pretty to dry-looking and crumbly. Indeed, it’s a true rock-and-a-hard-place type scenario, and one you really have to try a few times to get right. The good news is that if you goof, you can always heat the chocolate back up and start again. And anyway, it’s not exactly torture fiddling around with chocolate all afternoon.