Making mould, cast and trace fossils in clay and plaster · Concept Studio

The idea

A fossil is usually not the organism but the shape it left, a mould, the filling of that shape, a cast, or the mark of its activity, a trace.

Safety card

Low riskAn adult supervises

Hazards

plaster sets hot and hardens; never enclose hands or fingers in setting plaster
plaster dust

Controls

mix in a cup, not against skin
do not wash plaster down the sink
wipe up dust damp

Note

Plaster of Paris is a chemical product that sets warm and gives off dust: read its Safety Data Sheet, follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1, and record a RiskAssess risk assessment before the lesson.

What you need

modelling clay or plasticine slab 2 cm thick per learner; shells, gum foliage, a small bone or a plastic model
plaster of Paris, water, mixing cup and spoon, petroleum jelly
a tray of damp fine sand for trace fossils (footprints, burrows made with a pencil)
hand lens, ruler

How to do it

Press the shell into the clay and lift it out: the impression is the mould; measure its length.
Grease the mould lightly, mix plaster two parts to one of water and pour it in; leave 30 minutes to set.
Peel off the clay: the plaster is the cast; compare its dimensions with the shell.
In the sand tray make a trail and a burrow, then gently pour plaster over the surface to preserve the trace.
Label each product mould, cast or trace and state what each tells about the organism and what it cannot.

What you should see

The cast reproduces the shell’s outer shape and size in the reverse relief of the mould; the trace preserves the trail but nothing of the animal’s body; the plaster warms as it sets. The learner states that none of the three models preserves original material and matches each to a rock example.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Fossils are bones turned to stone.
Every organism that dies becomes a fossil.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES12-12 EES11/12-5 EES11/12-6 EES11/12-7
Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-12-01
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.