A negative cast is a three-dimensional model taken from a depression left in a soft surface. Naturalists make negative casts to record animal tracks left in mud, while parents often make a negative cast of their baby's handprints or footprints, as a memento. Any object that leaves a dent or impression in a surface can be negatively cast using plaster of Paris. With a little practice, you can create negative plaster casts of virtually anything by following some straightforward steps.
- Skill level:
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Things you need
- Wide deep dish, such as a roasting dish
- Sticky tape
- Petroleum jelly
- Plaster of Paris
- Plastic bowl
- Large spoon
- Sheets of newspaper
Fill a wide, deep dish with sand to within 1 inch of the top. Ideally, this should produce a layer of sand 2 to 3 inches deep.
Tip the sand from the dish into a bucket. Add water from a jug, little by little, mixing it well into the sand with a stick. Stop adding water when the sand is damp enough to take and retain the impression of your thumb.
Tip the damp sand back into the dish. Pat it down, and smooth it over carefully with a ruler, until it presents a flat, level, even surface or "bed," ready to take an impression.
Take an object you wish to cast and press it down firmly into the damp sand. Natural objects such as shells, pine cones and twigs make good subjects, as do the prints of hands or feet, but any object that takes your fancy will do. Lift the object from the sand, leaving its impression in the surface.
Cut, with scissors, a strip of cardboard, 3 inches wide and long enough to encircle the imprint you have made. Form a circular wall around the imprint using this cardboard strip. Press the bottom of the strip 1/2 inch down into the damp sand to hold it in place. Fix the ends of the strip firmly together with sticky tape.
Rub the inside walls of the cardboard strip all over with petroleum jelly. This will act as a release agent and make it easier to remove the cardboard when the negative cast is finished.
Measure 4 cups of plaster of Paris into a plastic bowl. Add 2 1/2 cups of water. Mix the plaster and water together for 2 minutes using a spoon. Do not plunge your fingers into the mixture, as it will get very hot. Stir slowly and carefully to avoid getting air bubbles in the mix. Gently shake the bowl to release any trapped bubbles of air.
Pour the mixture into the well around your sand imprint made by the cardboard walls. Spread the liquid evenly and let it build up to fill the well. Gently tap the dish of sand to release any trapped air bubbles and to encourage the plaster to settle. Leave the plaster to set hard overnight.
Lay sheets of newspaper over your work surface. Put the dish containing the sand and the set plaster cast on top of the newspaper.
Place one hand firmly on top of the plaster cast, and with the other hand, invert the dish, allowing the sand to spill out onto the newspaper. Remove the dish and free the finished cast from the sand pile. Gently pull the cardboard strip away from the set plaster.
Whisk away any remaining grains of sand from your negative cast using a paintbrush.
Tips and warnings
- Plaster produces heat as it sets. To test whether a cast is fully set, touch the surface briefly with a fingertip. If the plaster still feels hot, it has not yet set properly.
- Waxed cardboard strips, cut from a milk carton, are ideal for making the walls of the well.
- When casting an animal print outdoors, make a cardboard well around it and pour in the plaster. If you cannot leave the plaster to set overnight, gently dig up the cast using a trowel after 10 to 20 minutes, keeping as much earth around it as possible.
- Never put body parts directly into liquid plaster. The heat produced as the liquid sets can cause severe burns.
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