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


Samantha said...

Wow. I love this! As a museum studies student, I enjoy seeing the behind-the-scenes work when preparing for exhibition and/or educational program. I'm sure visitors of all ages enjoyed being a paleontologist for the day!


Nikki Nasvytis said...

Could you tell me what material you used to create the barrier? I see that you lined the fossil in some sort of sheet metal, then were able to cleanly pour the silicone and plaster.

I'd absolutely love to know about the barrier!

Burke Museum said...

Hi Nikki - I'll ask what was used and get back to you soon!

Nikki Nasvytis said...

Thanks, I'll be waiting anxiously!

Burke Museum said...

Hi again - Bruce (our fossil preparator) says he used some really basic aluminum flashing from the hardware store but you can use just about anything! Did you happen to see our blog post where we used legos to help make fossil casts?

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