Old cookbooks and magazine recipes can be a lot of fun: For instance, you won't find many modern chicken recipes that start with plucking the chicken. Actually cooking from an old recipe can sometimes be a bit of a challenge, though, either because you don't recognize the name of an ingredient – internet search is your friend here – or because the recipe assumes you know a method that's not used much anymore. A classic example is candymaking, which requires you to cook the candy to a specific temperature. Modern recipes take for granted that you'll use a thermometer and give you a set temperature, whereas older recipes rely on you to test the candy as you go.
It's Surprisingly Scientific
The basic technique is pretty simple. When you're ready to test your candy, scoop up a small amount on a spoon and drop it in a glass of cold water. Then pick up the resulting blob and see what it looks and feels like. This can seem kind of slapdash and haphazard, but it's surprisingly reliable. That's because sugar behaves in specific ways at specific temperatures. When you're using a thermometer, you know it'll show a certain consistency, because it's reached a given temperature. When you're using the cold-water test, you'll know you've reached the right temperature when the candy reaches a given consistency.
Make It Roll: Soft, Firm and Hard Ball Stages
When you first start cooking up your sugar, essentially all you've got is a syrup. If you drop that into a glass of cold water, it'll just sweeten the water. Once your syrup gets above 234 degrees Fahrenheit, however, it starts to hold together and make visible threads in the water. You'll be able to gather them together and roll them into a ball that holds its shape in the water, but flattens easily between your fingers. Recipes call this the soft-ball stage, and it lasts until you get up to about 240 F. That's how far fudge is usually cooked.
Between 245 F and 250 F, you'll get a ball that's still a bit soft but needs a good squeeze to flatten it. That's the firm-ball stage, which is what you want for caramels and other soft-but-chewy candies. At 250 F to 265 F, the ball won't flatten at all. That's the hard-ball stage, and it's what you want for homemade marshmallows or a really chewy toffee.
Make It Crack
Some candies require you to cook the sugar to an even higher temperature. At this point there's nothing more to learn from rolling the sugar into a ball, partly because it won't squish and partly because you'd have trouble actually forming a ball. Instead, you'll find out what you need to know from the threads themselves. If your sugar is between 270 F and 290 F, the threads will bend before they break. That's the soft-crack stage, which is what you want for popcorn balls and taffy apples. If the threads break without bending, you've reached the hard-crack stage at 300 F to 310 F. That's how high you need to go for brittles or hard candies.
Use a Thermometer When Possible
Once you've got the hang of it, the cold-water test works really well, but it's not perfect. Its biggest drawback is that the candy keeps cooking while you're messing around with the cold water, so you may go past the temperature you want while you're testing to see what stage you've reached. You can deal with that by working with a lower burner, but it's still a risk. All in all, if you decide to make your own treats at home on a regular basis, it's best to buy a decent candy thermometer and use it. It saves time, you don't have to handle the hot syrup as much – which means fewer chances to burn yourself – and you'll get finer temperature control for the recipes that need it.