Do you have and use a Candy Thermometer? I always use one. I read the info on all three of the links Corazon provided, and there are some good tips there!
The allrecipes site had probably the most important tip of all. WEATHER plays a most important part. The best time to be cooking candy is on a cold, crisp, day. If you notice that you are generating a lot of static electric as you walk around, then it's a good day to make candy.
I really like the tips about checking your candy thermometer for accuracy. That's something I've never done. I also liked the last tip, the one about lowering the temperature you need to cook candy to by 1°F for every 500' above sea level. Also, remember that when you boil water, as you go up in altitude (lowering the air pressure), water boils at a cooler temperature, so if you are calibrating your candy thermometer, you need to take that difference into mind.
A really big problem when making candy is pure physics. If you have a mass of molten sugar in a pan, and JUST ONE CRYSTAL of sugar enters the molten mass, the entire mass will crystalize.
Therefore, you always want to have a pastry brush (dedicated to making candy) that you can moisten, and use that to push sugar crystals down into the mix from the sides of the pan. The only time these crystals will get there is when you first add the ingredients, and, if you stir the mix.
Putting a lid on until it reaches a boil also helps, as the steam generated will help to wash the sugar crystals down into the mix.
You will know when you've cooked off all the water, as the bubbles will change size, frequency, and pitch, going from large, slow, and low-pitched bubbles to smaller, fast-forming, and higher-pitched. When you not this change, keep all crystaline sugar away from your mix.
While I was in college, I also learned that adding a little bit of corn syrup to the sugar before cooking will help keep the sugar from crystaling (this is why, when you read the ingredients for many candies, you'll notice corn syrup is one of them). Also, a little lime juice prevents crystalization, but I doubt you want a lime flavor in chocolate fudge.
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