When making candy, using a candy thermometer is the best way to get consistent results. When a candy thermometer appears to be misreading temperatures, or is clearly broken, there are several ways to deal with the situation. In some cases, the thermometer can be salvaged. However, if your candy thermometer truly is broken, there is another way to gauge the temperature of sweet treats while they're cooking.
Troubleshooting Glass Thermometers
Candy thermometers come in a couple of different designs. The most common is a thermometer that is either entirely made out of glass, or has a glass bulb. When a thermometer like this breaks, it is obvious, as it will either shatter or crack. Throw away a broken thermometer, even if it just has a crack. Additionally, if the thermometer breaks while it is submerged in cooking candy, throw away the candy as well. It could contain bits of broken glass, as well as some of the liquid from the inside of the thermometer. This liquid may be toxic, especially if the thermometer is an older model.
Troubleshooting Digital Thermometers
Digital candy thermometers are also available. This type of thermometer usually has a metal probe that's attached to a readings box with a wire. When this type of thermometer breaks, it will generally give incorrect temperature readings, or the screen will go blank. If the readings seem off, try calibrating the thermometer. Submerge the clean probe in boiling water and take note of the reading. Water boils at 212 degrees F; note the difference between this boiling temperature and the thermometer's temperature reading and adjust your recipe accordingly. For instance, if the thermometer registers the temperature of the boiling water as 207 degrees F, you'll know the thermometer is off 5 degrees.
If a digital thermometer fails to give a reading, try changing the batteries. If changing the batteries doesn't work, the thermometer cannot be used.
Even if your thermometer breaks and you don't have a replacement readily available, you can still make candy. It's possible to gauge the approximate temperature of candy by dropping a small amount of the concoction into cold water while it's cooking. Most recipes include a stage that the candy should be cooked to, as well as a temperature. The shape that the candy forms when it hits the water will show you what stage the candy is at in the cooking process. To start the test, fill a bowl with cold tap water and dribble some of the liquid candy into the bowl.
The first stage is the soft-ball stage, which means that the liquid candy forms into a very soft, pliable sphere when dropped in the water. After it's removed from the water, the heat from your hand will cause it to flatten out.
The firm-ball stage means that the candy forms a slightly firmer ball when immersed in the water. It will retain its shape when removed from the liquid, but you will still be able to easily squish it with your fingers.
The hard-ball stage yields a stiff ball of sugar. It can still be smashed, but it is more difficult to do so than in the firm-ball stage.
When the sugar reaches the soft-crack stage, the liquid candy will form into threads in the water. These threads can still be bent, but will break when pushed too far.
The final stage is the hard-crack stage. At this stage, candy will form hard threads that cannot be bent. This candy will be very hot and sugary, so wait until it cools before scooping it out of the water. It can cause severe burns.