Many cooks stay away from candy making because they are worried about crystallization, an event that can ruin a batch of sugar within minutes. If you understand a little about the process, it’s not that hard.
Sugar starts out in crystalline form. Once liquefied the two single sugar molecules, glucose and fructose, separate. During cooking they naturally try to rejoin with each other. If a stray crystal, or a speck of anything foreign, enters the pot, the molecules will gravitate towards it, grab hold, and start forming a gigantic crystal. (It’s the phenomenon demonstrated when you made rock candy in third grade science class.)
To prevent those pesky molecules from taking over, recipes contain extra glucose (in the form of corn syrup) or acid. These ingredients discourage the re-forming of crystals. There are also some precautions you can take while the sugar is cooking. Never stir a pot of sugar unless a recipe tells you to. (It may if there are other ingredients in the pot, like butter). The agitation will bring any foreign particles into play. If crystals begin to accumulate on the side of the pan as the sugar cooks, they can be dissolved and wiped away with a moist, clean pastry brush. Be careful not to simply wash the crystals down into the pot.
The best defense against crystallization is to simply use a clean pan, clean sugar, and wipe the sides of the pot clean of any stray grains of sugar before the pot hits the heat. Set the pot over high heat and do not touch it until it is done. The more you wiggle, stir, shake, and jostle the syrup, the closer to the edge of crystallization you push it.
When cooking sugar on top of the stove a candy thermometer makes the task super simple, provided that the thermometer is accurate. The old-fashioned method of testing sugar stages with ice water is easy too, and in most cases more reliable. Have a bowl of ice at the ready as the sugar cooks. As the bubbles get larger and the mixture starts to thicken, spoon out a small amount into the ice water. Immediately feel its consistency. If the recipe calls for soft-ball sugar, you should easily be able to form the sugar into a ball. Hard-ball sugar will keep the ball shape once formed. Crack stage will harden immediately once it hits the water, and will crack easily. Hard crack will not stick in your teeth when you chew it.
Sugar boiled on its own will be clear until it reaches the hard-crack stage. There it begins turning amber as it hits caramel, and on to black as it starts to burn. If the recipe has other ingredients, like butter or milk, it will begin to darken sooner, and hard-crack will be rich caramel color.
One final note of caution: It’s hot! Be careful, pay attention, and don’t screw around near the stove. The worst kitchen injuries of my past all revolve around sugar and stupid behavior.