March 11, 2011

How to Make a Fossil Cast: A 10-step Guide

If you came to Dino Day this past weekend, especially if you came with a kid, you probably saw the Burke’s brand new fossil dig pit.  In the dig pit was a cast of an ichthyosaur fossil from the museum’s paleontology collection.

And you may have wondered: What steps go into creating a cast of a fossil?

Creating a fossil cast is a multi-step process (I’ve reduced it here to 10 steps). Over a few days in January, I followed around fossil preparator Bruce Crowley and his team of volunteers as they worked on creating the mold and pouring the cast of the ichthyosaur.

Step 1: Get the original fossil out of storage and uncrate it.

Step 2: Identify the area around the fossil that is going to be cast; build a barrier and apply a layer of silicone to the fossil -- this will easily come off later (the silicone in this picture is purple).

Step 3: After covering the rest of the exposed fossil with plastic (gotta protect it!), begin to spread on a layer of plaster (in this case, a material known as Hyrdocal). While spreading the plaster, add criss-crossing layers of fiberglass sheeting—this will reinforce the Hyrdrocal and make a nice plaster jacket for the fossil. Be sure to work fast and don’t forget to wear gloves and a mask.

Step 4: Come back the next day and the plaster jacket is complete!

Step 5: Very carefully, remove the plaster jacket from the silicone layer.

Step 6: Remove the silicone layer from the fossil. See, I told you it would come off easily!

Step 7: Back in the lab, place the silicone layer (aka the mold) on top of the plaster jacket (for reinforcement) and build a wall around the silicone. If this step is overlooked, the material used to pour the cast will spill over and onto the floor!

Step 8: Pour the cast into the mold! (Again, using a brand of plaster known as Hydrocal) When done, the cast will look like this:

Step 9: Remove the cast from the mold.

Step 10: Paint the cast…it helps accentuate the different parts of the fossil!

There you have it! It’s a perfect cast of a real ichthyosaur fossil! And the kids at Dino Day loved uncovering it in the dig pit.

Posted by: Julia Swan