Modeling clay casts of big fossil molds...

[Arizona Chris]

ItHi all,

Well, we've got large coiled fossil we were not quite sure of what it was found recently in the Permian on the Mogollon Rim.  I have  had a hard time seeing the right shapes with a concave mold in the coarse limestone, so we decided a final look at the cast would confirm what we suspected as for its identity.  It worked!  Ill tell you at the end what it was but first I documented the process of filling in the mold in the limestone with white modeling clay as to get a better look at what we had.  Here is the huge spiral fossil we wished to cast:

 

First, I wanted to try it on a smaller planospiral gastropod, similar to the one in question.  Details are coarse in limestones, and trying to make a latex cast would be a nightmare to remove because of all the fine pits and depressions would make the latex impossible to remove later when it dried.  So we went to the craft store and got some pure white modeling clay.  If your not familiar with this stuff, it is not the stuff you played with as a kid.  Modeling clay feels the same, but is water soluble and will harden rock hard when it dries out. So here is what we started with:

 

The small test fossil was first sprinkled with talcum powder as a release agent.  This works very well and is white like the clay.

 

ThisA round clay ball was tore off the big brick of clay in the package ($8 at Micheals) 

 

This was pressed into the fossil as best as possible to get the basic gastropod shape:

 

When removedhere - which is very easy with the powder, you get a perfect shape:

 

Out in the sun, here is the result when dry:

 

Well,since that worked pretty well, we started on the big fossil.  First the powder:

 

Covered:

 

Then started packing in clay balls all over the fossil:

 

 

The final load of clay was packed on top to give it strength.

 

It then easily lifts right off, and the cast is carefully laid in a sunny window to dry:

 

And the final result with the proper sun angle two days later was this:

 

So we were able now to confirm what we had suspected all along, this was a huge Straparollus kaibabensis with the nodal bumps on the sides of the adult whorls.  The total size was about six inches.  

 

So thats it, it works well for fossils in limestone which have little detail and gives you the basic shape.  For highly detailed molds in cherts like the stunning crinoids we find in the Redwall, we still use latex.  

 

Thanks for looking!

 

 

[Macrophyseter]

Nice molds! 

 

But does the powder come off the original fossil? It would suck it it doesn't.

[Fossildude19]

5 hours ago, Macrophyseter said:

Nice molds! 

 

But does the powder come off the original fossil? It would suck it it doesn't.

The powder should come right off. It's water soluble. 

 

Nice technique for figuring out those hard to see/place fossils. 

Thanks for posting. 

Regards,

[Arizona Chris]

Yes, the talcum comes right off in water.  

[jpc]

cool, and thanks for sharing.