Hard crack stage is the hottest stage in candy making before you begin making caramels, which are heated to caramel stages. Hard crack is used for candies like English toffees and some nut brittles. When you cook sugar with other ingredients together to make candy, the heat temperature rises, reducing the moisture in the candy significantly. During this stage, all but 1% of the moisture in the original ingredients has been evaporated.
On a candy thermometer, hard crack stage is defined as between 300-310 degrees F (148.89-154.44 degrees C). It is highly recommended that you use a candy thermometer to reach this stage, since you can easily miss it by a few degrees and produce candy in soft crack stage, or you may caramelize or burn your sugar. If the candy becomes a deep, dark brown you’ve probably passed hard crack, and if it begins to smoke, you’ve very likely burned the candy.
You can also tell when you’ve reached this stage by the way the candy drips off a spoon—it will have strands that are very stiff when they are dropped in water. When you remove these strands from the water, you’ll note that they break very easily, representing the “crack” of hard crack stage.
If you test your candy in this manner, you should do it right next to your stove, because it’s important to keep an eye on the candy. You should also exercise extreme care when performing a water test with candy. The high temperature can cause severe burns. Use a long spoon to remove some of the heated sugar, and use oven mitts for extra safety. Though it can be fun to having children help you prepare goodies, you might want to keep them out of the cooking process when you must heat sugar to this high of a temperature to protect little hands from burns.
As your sugar syrup reaches higher temperatures, even before it hits the hard crack stage, you may want to brush down the sides of the pot, because sugar can stick to the sides and granulate. Most recipes recommend that you don’t stir the sugar once it is boiling unless you are adding nuts. If you perform the spoon test to determine this stage, many chefs recommend using a warm metal or wooden spoon, because the test can be corrupted if you use a cold metal spoon.
Since hard crack stage cools so quickly, it is very important to pour the candy out immediately into a waiting pan or onto a marble slab. Simply smooth the candy out and allow it to set and cool. If you’re making recipes like English toffee, you need to spread chocolate chunks or chips onto the candy while it is still warm so the chocolate will melt.