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White-glue fossils: Casts, molds, and impressions

Stephen F. Greb Kentucky Geological Survey


You don't want to mess with plaster to make fossil molds? Here's a simple solution. Students make impressions and molds in clay and then make casts with glue. After the glue dries, students pull back their “fossils.” You can see examples of fossil impressions on-line at the Kentucky Geological Survey's pictures of different fossil types.

Grade Level : K-8

Time : 15-20 minutes preparation in class, and then glue has to dry overnight

Materials :


Fossils are any evidence of ancient life preserved (usually) in stone. There are many types of fossils and many different ways that fossils form. Most fossils are not the actual body parts of the original organisms. Rather they are altered remains, impressions, molds and casts of parts of the organisms. A mold is the impression and void (space, hole) that an organism or organism's body or body part leaves in the sediment. An impression of the outside of the object can leave an external mold in the sediment.  If the object is filled with the same sediment that surrounds the object, an internal mold can be left on the sediment inside the object. If the buried object dissolves and leaves a space or void and  the void is filled with minerals carried in groundwater through the sediment, then a cast is formed.  A cast is the material that fills the void. A cast is made of different material than a mold. Both casts and molds are types of fossils. Sometimes the mold and cast are found together, although molds are more than casts.


Repeat this process at least five times. If working in a group, have each member of the group fill out one line of the chart on the worksheet . Compare different objects to see which objects make the best impressions (hard vs. soft). Answer the questions on the worksheet.