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You can purchase candy or candle thermometers at a cooking or craft store. If you do not have a candy thermometer, you can also use a meat thermometer. Just remember that wax can be hard to remove.  Paraffin wax should be melted until it reaches between 122 and 140°F (50 and 60°C).  Soy wax should be melted until it reaches between 170 and 180 degrees (76.6 and 82.2°C).  Beeswax should be melted until it reaches roughly 145 degrees (62.7°C). You can go a little bit higher but try not to exceed 175 degrees (79.4°C).  Old candles should be melted at around 185 degrees (85°C). Remove the old wicks with tongs.

Summary:
Use a thermometer to keep an eye on the temperature of the wax.