Glass or Crystal?

Natural glass can be formed when volcanoes melt minerals, then force them to Earth's surface where they cool rapidly. Crystals, on the other hand, form underground when minerals are dissolved in water and left to grow over much longer periods of time like thousands or millions of years.

The material we commonly call "glass" is made of silicon dioxide (the most abundant mineral in sand) that's heated until it starts to melt, then super-cooled; this "glass" is really many, many molecules of silicon dioxide that have no repeating pattern.

Now, you may recall that the molecules in a solid aren't free to move around. They are usually locked into a repeating pattern. Not so with glass. Glass feels solid because its molecules aren't free to move around. However, the molecules in glass are not locked into a pattern. Instead, glass's molecules are just a wee bit out of line. This means glass is not a crystal.

But what about a common household item that is a crystal? Let's think about sugar. If you look closely, you will see that granulated sugar is made up of tiny crystalline cubes. The sugar molecules in each tiny crystal are arranged in a regular pattern that forms a cube. You can change these crystals by melting them and by dissolving them in water. When you melt sugar and quickly cool it, you make sugar glass. When you dissolve sugar in water and let the crystals reform over time, you make new, larger sugar crystals.

Here are two yummy ways to transform that sugar sitting in your sugar bowl. Try them!

Cook Up A Batch Of Glass

Materials You Will Need:
Two cups of sugar
One cup of water
2/3 cup of light corn syrup
1/4 teaspoon of cream of tartar
Two-quart cooking pot
Candy thermometer
Wooden spoon
11 x 17 inch cookie sheet with raised edges
Aluminum foil
Non-stick cooking spray
Hot mats
Your Procedure:
1. Place the cookie sheet on top of the hot mats. Make sure the cookie sheet lies flat. Line the cookie sheet with aluminum foil and spray it thoroughly with the non-stick cooking spray.
2. Mix the sugar, water, corn syrup, and cream of tartar in the cooking pot over medium heat. Stir until the mixture begins to boil, about 15 minutes.
3. Stop stirring and use the candy thermometer to take the mixture's temperature. It should be between 200 and 220 degrees Fahrenheit (F).
4. Let the mixture continue to boil, checking the temperature every five minutes. When the temperature reachers 200 degree F, turn off the heat. It should take about 30 to 40 minutes of boiling to reach this temperature.
5. Pour the mixture onto the cookie sheet and allow it to cool.
6. The glass should be cool to the touch after one hour. Test the edges to see if the glass is firm. If it is hard, slowly peel it away from the aluminum foil.
The Result:
You have your very own pane of sugar glass.
Your Conclusion:
When heat is applied to sugar, the sugar melts--the sugar crystals are changed. If the hot mixture is cooled quickly, the sugar crystals do not have enough time to reform and instead, form a substance that looks like "glass."

Why do we add the extra ingredients?
Even "real" glass doesn't contain just one ingredient. The water is your sugar glass helps the sugar to dissolve and prevents it from burning in your pot. The corn syrup and cream of tartar are known as interfering agents, because they prevent the sugar from reforming into crystals.

Rocks Good Enough To Eat

Materials You Will Need:
A piece of cotton string about 12 inches long
One pencil
One paper clip
One heat-proof glass jar approximately one-pint size
One-quart cooking pot
One cup of water
Three cups of sugar
Wooden spoon
Your Procedure:
1. Tie one end of the string to the middle of a pencil.
2. Place the pencil across the top of the jar so that the string hangs down into the jar.
3. Cut the string so that it's long enough to hang from the top to the bottom of the glass jar. Tie the paper clip to the hanging end of the string. The paper clip acts as a weight to help hold the string in place. It's "ok" for the paper clip to touch the bottom of the jar. Put the pencil, string, and paper clip aside.
4. Pour the water into the cooking pot and heat it over a medium-high heat until it boils.
5. Very slowly pour the sugar into the boiling water, stirring constantly as it dissolves. Keep adding sugar while stirring the mixture until you notice that no more sugar will dissolve. You may not need all three cups of sugar. Turn the heat off.
6. Carefully pour the mixture into the jar.
7. Rub a pinch of sugar crystals into the string. Then place the string into the jar, making sure that it hangs straight down.
8. Place the jar in a location where it will not be disturbed. Check it every day and notice what forms on the string. It will probably take at least a week for a sizeable amount of rock candy to appear.
The Result:
Large crystals of sugar form on the submerged string.
Your Conclusion:
When sugar is dissolved in water and left alone with a string (or support), sugar crystals reform over time, making new, larger sugar crystals.

Cox, Mary Beth. "I Know What Glass Is, Thank You." Odyssey: Adventures in Science. November/December (2008): 6-8.
Artinian, Zareh M. "Edible Glass and Rocks!?" Odyssey: Adventures in Science. November/December (2008): 31 - 33.