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Plaster Cast Footprints

Develop your group's fieldwork skills by taking them plaster casting. This activity teaches you how to make casts of animal footprints using plaster of paris.

You will need:
  • strips of card approx 50mm wide and long enough to make a collar around the print you want to cast, plus 40mm overlap OR circular sections cut from large plastic drinking bottles
  • two paper clips
  • petroleum jelly
  • empty 500g margarine tub for mixing plaster in
  • spoon
  • water
  • plaster of paris - about three heaped tablespoons for a small cast




Find a clear animal print (possibley one 'made earlier'), preferably in mud. Clean away debris such as fallen leaves ot twigs.


Then you need to construct a water-tight collar around the print you want to cast. This can be made either from card, made into a ring, fixed with paper clips and pushed into the mud around the rpint, or from a section of plastic bottle (these are best for small prints).


The inside surfaces of the collar should be lightly smeared with petroleum jelly and the sides of the collar should remain at least 10mm higher than the most sticky-up part of the enclosed area. Once you are satisfied that you have a water-tight colar you are ready to mix the plaster.


Judging the right amount of water and plaster is something you can easily experiment with and quickly learn. Too thin and your cast will take ages to dry and be prone to breakage, too thick and it will set before you are ready! Start by pouring water into your mixing tub. Use as much as you judge would be needed to cover the print to a depth of about 10mm over the highest point. Then add the plaster, Sprinkle it in, stirring constantly. It will bubble and get hot. When it reaches the consistency of double (un-whipped) cream, stop stirring for a few seconds to let the bubbles escape, then carefully pour the plaster into the collar. If it is too thick to spread out properly, use the spoon to make sure it reaches into the whole collar.


It should set in a few minutes, but will take longer to go really hard. Avoid putting the wet spoon back into the dry plaster as it will make it go lumpy. With care you can move the cast soon after it sets, but the longer you wait the stronger it will be. Your cast will show the reversed-out print.


If you need to use the collar for another cast, start by removing it carefully and then prising the cast from the mud. If you have plenty of collars availabel then removing the cast with its collar will provide some extra protection whilst it is transported.


Once the cast has set really hard you can rinse it clean with water and paint or varnish it. You may prefer just to leave it in the raw authentic state to remind you of your day. Label the cast with the name of the species, the date and the location you made it.


Hardened casts can be used on their own to show what different prints look like or to make decorative display items by pressing them into soft clay or air drying modelling mediums. Small tiles made this way are especially popular.


Once you become a dab hand with plaster of paris, there are all sorts of possibilities - making 'fossil' casts from shells or other fossils, or even making bark prints from trees (these need soft clay to press onto the tree first so you can make a mould you can lay horizontally).


Additional info

Leader - outside

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