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You usually can get a nice alum crystal overnight.

You usually can get a nice alum crystal overnight (shown here). If you let the crystal grow for a day or more, you can get larger crystals.

Anne Helmenstine
Alum is an ingredient you can find on the spice aisle of any grocery story. If you mix alum with water, you can grow impressive crystals. Because it is so safe, alum is the chemical found in many commercial crystal growing kits. The 'white diamonds' in the Smithsonian Crystal Growing Kits are made from alum. This is good to know because it means you can get a refill for those kits at any store or if you have the chemical but have lost the instructions, you can use the do-it-yourself instructions.

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