The most common crystal, table sugar, is a component in many candies because so many things can be done with it. Sugars (sucrose) are composed of simple molecules of one glucose and one fructose molecule bonded together. Identical sugar molecules arrange themselves in orderly geometric patterns repeated over and over again which can't be seen with the naked eye. But the underlying atomic nature of sugar results in its crystalline form. 

Candies are formed by adding sugar (a solute) to water (a solvent), called a sugar solution, and boiling the mixture to a codified temperature or density that ranges from 215 to 356 degrees F, measured with a Candy Thermometer. This high heat dissolves the sugar, evaporates the water and breaks apart the sugar's molecules. As a result, you get a greater concentration of the sugar, eventually reaching a supersaturation of the sugar molecules in the water.  

When broken apart and concentrated in a supersaturated solution, sugar molecules are unstable. They want to come back together again at any chance to return to their previous crystalline structure. An unclean pot, any jarring or stirring of the supersaturated solution at the wrong time, can send them back to their original crystalline pattern and dry state, crystallizing the mixture and ruining the whole candy batch. There are many ways to control unwanted crystallization, depending on the recipe.

The secret to making different types of candy lies in attaining the correct concentration of the supersaturated solution through heat and then controlling the recrystallization of the sugar crystals. This results in two categories of candy: Crystalline and Non-crystalline. Here candy can range from the soft textures of caramels and fudges, where crystallization is minimized, to hard candies where crystallization results in a desired grainy or crystalline structure. 

Recrystallization of the sugar's molecules are influenced by the temperature and density to which the sugar solution is boiled to, whether or not other ingredients are added, such as cream, butter, corn syrup, etc. and when and how fast the solution is cooled, or whether or not it is stirred or kneaded. As a result, you can create a wide variety of candies, that all start with a simple, boiled sugar and water solution, that will satisfy the particular cravings of anyone with a sweet tooth. 

For example, for a more crystalline candy like Fudge or Fondant, the mixture is boiled to 234 - 240 degrees F and then set aside to cool slowly. Then it is stirred again to break crystals into smaller pieces, making the fudge smooth and creamy. When making rock candy, the solution is cooked to 247 - 252 degrees F and not stirred during cooling, allowing large sugar crystals to form. To make hard candy like lollipops, the solution is heated to 300 - 310 degrees F and cooled quickly. 

Unwanted crystallization can occur at any time during the making of sugar candy, ruining the recipe. That's because sugar crystals look for any excuse during the candy making process to return to their original, dry and stable state. Crystallization at the wrong time can be obvious early on when the sugar solution becomes cloudy or the whole mixture will literally freeze turning into a crackled mess. Sometimes crystallization is found when it's too late, such as when biting into fudge or caramel and you find its texture is like sand.  

However, sugar cannot start to crystallize at the wrong time without something to serve as a seed, pattern, or nucleus. It takes only ONE little, tiny crystal at the side of the pan, introduced from an outside source, to act as a seed, causing nearby crystals to follow the leader, ruining the whole batch. Here are some ways to prevent it: 

Interfering Agents: Ordered crystallization in non-crystalline candy or an interference with crystal growth in crystalline candy is prevented by ingredients. These ingredients are called interfering agents. 

You'll sometimes see an interferring agent(s) used in candy recipes to help stop premature crystallization and to affect the size and amount of crystals that form when the sugar recrystallizes. One way to prevent the crystallization of sucrose in candy is to make sure that there are other types of sugar—usually, fructose and glucose—to get in the way. Large crystals of sucrose have a harder time forming when molecules of fructose and glucose are around. Crystals form something like Legos locking together, except that instead of Lego pieces, there are molecules. If some of the molecules are a different size and shape, they won’t fit together, and a crystal doesn’t form.

A simple way to get other types of sugar into the mix is to "invert" the sucrose (the basic white sugar you know well) by adding an acid to the recipe. Acids such as lemon juice or cream of tartar cause sucrose to break up (or invert) into its two simpler components, fructose and glucose. For example, most fudge recipes contain either corn syrup (which contains glucose instead of the sucrose of table sugar) or cream of tartar (which breaks sucrose into glucose and fructose). 

Another way is to add a nonsucrose sugar to a candy recipe, such as corn syrup, which is mainly glucose. Some lollipop recipes use as much as 50% corn syrup; this is to prevent sugar crystals from ruining the texture.

Fats in candy serve a similar purpose. Fatty ingredients such as butter help interfere with crystallization — again, by getting in the way of the sucrose molecules that are trying to lock together into crystals. Toffee owes its smooth texture and easy breakability to an absence of sugar crystals, thanks to a large amount of butter in the mix.

Other interfering agents such as cream, milk, or cocoa powder also help prevent unwanted crystallization and also add flavor and interest. But, remember these ingredients do not entirely stop the unwanted crystallization process.

Check candy formulas to see what types of interfering agents are used. For example:

Start with clean bowls, pans and utensils: The first step to keeping any candy from crystallizing is to start with clean, dry bowls, pans and utensils. Having a foreign object in the pan, like old, dried caramel or dirt, will cause crystals to congregate and grow.  

In addition, introducing the mixture with sugar crystals (seeding) from a thermometer or spoon can start the crystallization process. These utensils should be cleaned and dried between uses and the pan should not be scraped. The thermometer should only be placed in the pot at the appropriate time.

Dissolve the sugar completely. If one tiny speck of a sugar crystal that hasn't been dissolved falls into the mixture during cooking, the whole batch will return to a solid state, ruining it. 

Place the sugar in the pot by pouring it in the center. You don't want a lot of unwanted crystals to adhere to the sides. Pour in the water, washing the sides of the pan as you do. Mix the sugar and water together with your finger, by drawing an "X" across the sugar. You can also combine further, but avoid touching the sides with the sugar.

Never stir your candy after the sugar crystals are completely dissolved in the water and when it has started boiling - this will incite the formation of big crystals that will make your candy grainy when it cools.

Keep the sides of the pan clean at all times: To ensure all crystals are off the side of the pan, oil the sides of your pan right up to sugar syrup's water line before you begin cooking. Because the fat is slippery, the crystals won't cling to the sides of the pan. 

After dissolving the sugar crystals and before boiling, you can also get rid of unwanted crystals on the side of the pan by: brushing the sides of the pan with a heatproof pastry brush dipped in water or by leaving the cover on the pan so steam forms, condenses and then washes off the side of the pan.  

Concentrating the solution to the appropriate degree. Candy solutions are boiled to a particular temperature and color, which are recorded on a Sugar Syrup Chart. During this process, the water evaporates and sugar in the solution becomes more concentrated. When the concentration of sugar increases, the boiling point of the solution also increases. As the temperature is raised, the solubility of the sugar increases and more can be held in solution making the mixture unstable and more prone to crystallizing at the wrong time. 

Supersaturating the syrup to the appropriate degree. During cooking, the sugar solution is becoming supersaturated or it holds more sugar in the solution than normal, with the molecules pressing against one another, making the solution unstable. Because of their tight proximity to one another, the least agitation will make them return to their original crystalline structure. 

Recipes will point out that the candy should not be disturbed; agitation may initiate the crystallization process. Large crystals would form and the candy would look and feel gritty and sandy. Sometimes this is referred to as "sugaring." However, bumping into the pan, picking it up and moving it or stirring at the wrong time cause unwanted crystals.

Stirring or Agitation: crystallizing gives the desired size of sugar crystals, unless done at the wrong time. Whether fudge has a grainy or smooth texture is determined by controlling the sugar crystallization through stirring, beating or kneading. 

Whether you stir the sugar syrup or not during cooking or afterwards is determined by the type of candy being made. Each recipe has its own specific mixing instructions that are important to follow carefully. If candy is agitated at temperatures higher than recommended, the crystals formed will be very large, adversely affecting the texture of the finished recipe. 

If the syrup is cooled and not stirred, it is a saturated solution. Then when you stir it, millions of tiny crystals form resulting in a smooth texture. With rapid stirring, more baby crystals are formed which is perfect when making Fudge or other creamy candies.    

As you stir, the candy will begin to lose its glossy appearance, becoming somewhat opaque and lighter in color. As crystallization proceeds, the candy will become much thicker and cease to flow. At this point, crystallization is complete and candy should be removed from the pan as quickly as possible.

When the temperature and color of the sugar solution is reached, as specified in the recipe, the pan is immediately removed from the heat. (Note that the temperature of the mixture continues to rise even after it has been removed, so some recipes have you quickly cool the mixture by placing the pot in ice water). 

Optionally, cream, nuts, etc. can be added all at once and quickly combined with a clean and dry wooden spoon. With the addition of a liquid, such as cream, which causes the darkening sugar to bubble up violently for a few seconds. Stir madly and it will settle down into a creamy, golden syrup that can be made even more tasty with the addition of vanilla or other flavorings. 

When done, the candy mixture is cooled by pouring its contents onto a marble slab, silpat mat or into a glass bowl or into a pan or mold to harden. (Do not scrape the bottom of the pan while removing its contents, unless specified.) Cooling is important in determining the size of the crystals. If you cool the sugar syrup quickly after you boil it to a known heat, the candy forms as a crystalline or brittle type such as rock candy. At a bit slower cooling after boiling at the same temperature, the candy forms a non-crystalline structure known as a taffy or caramel.  If you stir a concentrated sugar syrup only occasionally, while hot, few crystals form. They become bigger as the mixture cools. Upon cooling, the sugar can crystallize into several small crystals or one large mass, depending upon what's being made. This is not desirable for fudge, because it will have a gritty texture.