How do I stop destroying the center of my ammonites?

[MrFrog]

Hi, first post to the forum. 

 

I have been practicing prepping with my Dremel 290 on my charmouth ammonites.

 

The outer keel always works alright but I loose it when I get into the center and I keep ruining them.

 

Anyone have any tips? Any other methods I can use when it comes to the center. I've tried using hand picks but the stone seems too hard.

 

I'm going to try dripping ascorbic acid on it to loosen it but in past experience it dulls the detail and colour.

 

Any tips much appreciated.

 

Many thanks.

[Ludwigia]

I could imagine that by the looks of the photo the problem with your ammonites might be that the centers are not preserved, which makes it difficult to know when to stop.