How do you clean limestone stones containing nice fossils?

[ntloux]

I have a nice piece of limestone shale with some gomphoceras fossils on it.  It was apparently on the surface and there are portions with unsightly white deposits that are presumably due to abrasion.  I have read that some soak a specimen in 5% acetic acid overnight; however, I am concerned about damaging the nearby fossils.  I have tried to use an artist's paint brush with white vinegar containing 5% acetic acid and even with scrubbing with a toothbrush, the white abraded portions remain.  Should I try soaking the entire specimen over night?  Any suggestions/recommendations would be appreciated.

[DPS Ammonite]

Please show us a picture.

[Ptychodus04]

Also, is it limestone or shale? Each requires a significantly different approach to prep. Limestone is calcium carbonate based and shale is clay based.

[ntloux]

This is the Kentucky gomphoceras specimen where you can see the white abrasion wounds because the specimen was apparently on the surface.  The photo does not display all of the damage.  From what I can tell, much of the "limestone" in the Cincinnatian area is composed of both calcium carbonate and clay particles.  Sometimes rocks of this nature will fizz when you add a strong acid to it.  Any suggestions would be greatly appreciated.

[DPS Ammonite]

Selectively add acid to the abraded areas until they are gone. Then put whole rock into water for a while. You could dip the whole thing in acid until the abrasions disappear while carefully monitoring progress. Again soak in water.

[izak_]

If you decide to soak the whole rock in vinegar I would make sure to keep the fossil bearing surface facing upwards. The surface of limestone which faces downwards during acid preparation can dissolve away unevenly, leaving furrows and gouges. I assume this is caused by the bubbles which form as a result of the reaction. Your soak may not be long enough for these to appear, but worth keeping in mind. 

Do you have any smaller pieces to test? And do you know what colour the limestone is when it is unweathered? Sometimes dark limestone can turn a much paler colour when it weathers (especially if it has been weathering in soil), so it might be worth seeing if the acid will strip away the weathered surface and reduce the contrast between the fossils and the surrounding limestone. If the limestone is always this pale this won't be an issue, but I'm not familiar with the site.

 

Either way, I doubt you'll be soaking it for long enough for these issues to arise, but do keep it in mind! Good luck  

[minnbuckeye]

Very watered down acrylic paint applied to the scuff marks will not have the negative consequences that acid does.

[ntloux]

Thank you all for the suggestions.  The specimen is pretty much what it was when it was found.  I do not have a smaller specimen.  As the gomphoceras seemed to do well with natural weathering, I have considered placing it outside and let it weather naturally.  I used the white vinegar only on those portions where the abrasion exists and was hopeful that the white was do to powdering and hence was more susceptible to the acetic acid; unfortunately the white remained.  I guess I have to decide whether I want to live with it, paint the scuff marks with a watered down acrylic paint, or try a more extended vinegar soak and risk compromising the specimen.  Thank you all for the advice.