Things You'll Need
Old skillet or disposable aluminum pan
Cracked marbles are manufactured out of glass, which is cracked through a process known as thermal shock. The glass is first heated, forcing it to expand unevenly, and then cooled rapidly, forcing it to shrink unevenly. The resulting stress on the glass causes it to crack. Cracked marbles therefore serve as an excellent classroom experiment, but also have a crafty, decorative appeal since the resulting cracked surfaces sparkle and glimmer like gemstones.
Fill a bucket with two parts water and one part ice.
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Put on your safety goggles.
Pour your marbles in a single layer into an old skillet or disposable aluminum pan.
Heat the marbles on your stovetop, between low and medium heat, for approximately 10 to 15 minutes; or preheat your oven to 350 degrees and heat the marbles for 10 minutes.
Turn off the oven or stovetop and remove the skillet or pan from the heat source with a potholder. Pour the marbles into your bucket of ice water.
Wait for the ice to melt. When melted completely, scoop the cracked marbles from the bucket with your slotted ladle and lay the marbles on a soft cloth to dry.
Clear and light-colored marbles show cracks and sparkle better than darker colors.
Always wear safety goggles to protect your eyes, because the process of thermal shock can result in glass splintering and shattering.
Never microwave, boil, or fry (with vegetable oil) your marbles. Although heating marbles in a microwave results in rapid internal expansion, immediate exposure to the air can cause the marbles to shatter outward -- splinters of glass flying into your face and skin. Attempting to ladle boiled marbles from a pot to your bucket can result in the same outcome. The boiling method to crack marbles also poses a burn hazard -- as does frying with oil -- since either method can result in accidental splashing or splattering.
Always dispose of the used ice water down a drain when finished -- never reuse to water plants or drink, because the water may contain glass slivers and splinters.