Keeping Tracks

Making a Plaster Cast of a Footprint

If you are fortunate enough to find otter tracks when you are out monitoring you might want to make a permanent record of your discovery. A measured pencil sketch or photograph is always useful but a plaster cast will provide an accurate, three-dimensional copy of a track, which you can take home and study at your leisure. Plaster casts are very easy to make and require only simple and inexpensive equipment.

Equipment

Plaster of Paris
Plastic measuring cup
Cardboard or plastic strips about 4 cm wide and 30 cm long
Paper clips or sticky tape
Plastic water container
Plastic mixing bowl
Small Spoon

You can obtain Plaster of Paris from a chemist or a supplier of artist's materials. Herring's of Dorchester sell a 2kg pack of a Fine Casting Plaster for £1.35, which will be enough for about 10 casts. Make sure you store it in an airtight container.

Nearly everything else you need can be recycled from plastic containers. The plastic strips can be cut from large mineral water bottles and a cottage cheese container makes an ideal mixing bowl. It is best to make the equipment as lightweight and as compact as possible so that you can always take it with you when you go monitoring.

Method

First find a suitable track. It is a good idea to do a practice on the tracks of domestic animals first so that you can get your technique right. Carefully clear away any loose debris, such as leaves or twigs. Form the cardboard or plastic strip into a circle with the paper clip or tape and press this frame gently into the ground round the track. If the ground is hard or stony you may need to push some soil up round the outside of the container.

Pour a measured amount of water into the mixing bowl. The suggested proportions for Fine Mixing Plaster are 10 parts of plaster to 7 parts of water by weight. Carefully and slowly tip the plaster into the water until there is a little summit just above the water surface. Leave for about thirty seconds and then slowly stir the mixture. It should have the consistency of thin cream. Pour the mixture slowly and carefully into the frame so that it forms an even layer. Tap the outside of the frame gently to help the plaster settle in the track and to encourage any air bubbles to rise to the surface.

Leave the cast for at least twenty minutes until it has set. While you are waiting you can look around for any other tracks that you have missed! When the cast has hardened lift it carefully with its frame, remove any large lumps of soil and take it home wrapped in newspaper or a piece of paper towel.

You can remove the frame when you get home but it is best to wait until the following day before carefully cleaning the cast with an old tooth brush and a little water, taking care not to soak it or damage the surface.

Further Ideas.

Once you have worked out how much plaster is required for a typical track you can mark your measuring cup at the required level and also mark off your water container at measured intervals to make mixing easier when you are 'out in the field'.

The cast you make will be a 'negative' of the original track. To make a positive impression coat the surface with soap or Vaseline, place a frame round it and make a cast as before. When set carefully separate the two casts and you will have a copy of the original track, which you can then varnish or paint an appropriate colour.

It is a good idea to mark the back of your cast with the date, location and species because you can of course use the same method to preserve the tracks of any other animals you may find and gradually build up a useful reference collection.

Try making casts not only of 'perfect' tracks but also of partial tracks and tracks which you are not quite sure about. In this way you can always get a second opinion and help others to improve their tracking skills. The size of tracks you find can help to establish the presence of young otters.

By Peter Irvine (with credit to Stephen Carroll).