If your kids locate a strange footprint in your yard someday, they'll surely want to preserve it carefully as evidence. You can easily show them how to prepare a cast of a shoe print should that event occur in the future. They'll appreciate that you're helping them hone this useful skill but most of all they'll have a great time with a fun and fascinating activity. All you'll need is some plaster of Paris and a few simple materials that you probably already have around the house.
- Skill level:
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Things you need
- Old newspapers
- Petroleum jelly or cooking oil
- Empty plastic gallon milk jug
- Plastic dishpan
- Sand or garden soil
- Powdered plaster of Paris
- Disposable plastic spoon
- Paper cup
- Old toothbrush
Cover your work area with several layers of old newspapers. Cut four pieces of cardboard to create a casting frame or mould. You'll need two pieces about 9 or 10 inches long and two that are about 4 or 5 inches long. Make them all about 4 or 5 inches wide. Tape the ends together to form a rectangular frame. Seal the corners of the mould with tape.
Cut the top half off of an empty plastic gallon milk jug at the bottom of the handle. Discard the top. Wash the bottom half in warm soapy water and rinse thoroughly. Pat dry and set aside. You'll use this as a disposable bowl for mixing your plaster.
Fill a plastic dishpan about 2 inches deep with damp sand or garden soil. The print medium should be thoroughly moistened but not soggy, wet or muddy. Press your fingers into the surface. The impression should be sharp and clear. Level the surface.
Press the sole of a shoe slowly and firmly into the centre of the print medium. Make the impression about ½ to 1 inch deep. Carefully lift the shoe directly upward out of the medium. Examine the print, which should be clear and sharp. If it looks messy or fuzzy, smooth the medium and try again.
Coat the inner surfaces of the cardboard mould thinly with petroleum jelly or cooking oil. This will prevent the plaster cast from sticking to it after the material has dried. Encircle the shoe print with the mould and press it about 1 inch deep into the medium. This will prevent the wet plaster from running out of the mould when you pour it.
Pour 2 cups of warm water into the disposable mixing bowl. Add 4 cups of powdered plaster of Paris and begin stirring immediately with a disposable plastic spoon. Stir the mixture for about 3 to 5 minutes, until it has the consistency of pancake batter. Work quickly because plaster begins to set as soon as it's exposed to water.
Pick the mixing bowl up and tap it lightly on the tabletop a couple of times to eliminate any air bubbles that have formed while you were stirring. Dip out lumps in the plaster mixture with the plastic spoon. Put them in a paper cup to dry for about an hour so that you can dispose of them in the trash later.
Pour the plaster slowly and carefully into the mould next to the shoe print. Don't pour it directly into the print, or you risk ruining it. Let the mixture run into the print on its own. Keep pouring steadily after the print is completely filled to add another ½ to 1 inch of material to the mould. Pour any excess plaster into the paper cup with the plaster lumps. Let it dry completely for about an hour and dispose of it in the trash.
Scribe your name and the date into the stiffening plaster with a toothpick after it has set for about 10 minutes. Let it continue to cure and dry for about 30 minutes. Don't touch the wet plaster with your hands. Powdered plaster of Paris reacts with water to produce heat and becomes progressively hotter as it sets until it hardens.
Touch the surface of the plaster cast gently about an hour after you have poured it. If it feels dry, try to press your fingertip into it. If you can't make a dent in it, rap it with your fingernails. The plaster cast is ready when it's very hard and you can no longer make marks or impressions in it.
Poke the fingers of both hands into the casting medium just outside of the mould. Dig under the edges of the plaster cast and slowly lift it upward. Shake off the excess medium and set the cast on newspapers to air dry uncovered at room temperature for 3 days. Don't try to clean the casting medium from it.
Hold the plaster shoe print cast under cool, slowly running water. Gently whisk excess casting medium away with an old toothbrush. Don't rub the cast. Carefully pat it dry. Set it on several layers of clean newspapers to air dry completely.
Tips and warnings
- This project is a messy one, so have plenty of old newspapers and paper towels on hand.
- Don't pour plaster down a household drain. The material will harden and clog the pipe.
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