Casts, Molds, and Impressions

Can we have a bit of fun here? I thought my rock and mineral backgrounds were boring so I went retro to 1964. This is from my Instagram account which I know many of you do not follow.

Casts, molds, and impressions are confusing terms. Molds are impressions. Casts are relief. To explain, let’s say you wanted to make a beaver footprint cast down by a creek. With some plaster of Paris, you would fill in the footprint the animal left behind. This is an impression or mold in preferably moist soil. Once dry, you would pop out the hardened compound. You now have a cast. A mold needn’t be a track left by a live creature, it can also form when a body fossil decays or dissolves, leaving an impression.

In this video we have a puma track cast. As Authentic Wild relates, “This cast was made from a captive lion taken in as an orphan by the caretakers at the famous Sonoran Desert Museum in the Sonoran Desert in Arizona. An impressive cat.” The simulated impression of a dinosaur track comes from the museum store at the St. George Dinosaur Discovery Site in St. George, Utah. Artist David Slauf replicated the track of a small Grallator, also known as a Katiebell track. SGDS is the place to see all sorts of real casts and impressions. Even the impressions of dinosaur tail dragging!


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