Need some advice on vinegar solution

[bamontgomery]

Hi all.  I looked through a few pages of this forum, but couldn't find an answer to this question.  I hope I'm not duplicating something asked and answered elsewhere.  My family came across this mortality plate in Carfordsville, IL earlier this summer.  We've already cleaned it off with water and a toothbrush, but I reached out to a geology expert at my state's museum to ask her what else we could do.  She told me: "[T]he fossil is sitting in a rock that will dissolve in acid. If you want, you can try letting it soak for a bit in some dilute vinegar and water for a few hours at a time and then brushing at it as you have if you want to try to clear a little more of the rock away. The fossil itself is harder but is also made of a material that will dissolve, so that's why I recommend doing that slowly if you choose to go that route."  She mentioned elsewhere in the email how understaffed and over worked the museum employees are right now, so I don't want to bother her anymore.  I wonder if anyone on here knows 1) what ratio I should use in preparing a vinegar and water solution, and 2) how long I should soak it for and/or how long might be too long?  Any advice would be sincerely appreciated!

[Mainefossils]

@bamontgomery, very nice piece of crinoidal limestone! I think that by dissolving the limestone, you would also dissolve the crinoid stems themselves. The reason the limestone dissolves is because of the presence of calcium carbonate. The crinoid stems themselves also contain this material, so the vinegar would dissolve them as well. 

 

I think that there are other ways you could prep it though. You could take this piece along with a whole lot of patience, a pin-vise with a few needles, and a set of dental tools. Of course, you could still dissolve the limestone and the crinoids - there is always a chance that something else is hiding in the rock. 

 

Generally, acid preparation is reserved for non-calcium based fossils, such as conodonts and some brachiopods. Also, many fossils become replaced by silica, a material that will not dissolve in the acid. In these cases, a 5-10 percent solution of acetic acid (basically household vinegar) often will suffice. 

 

Hope this helps! 

[Fossildude19]

You can start with  a 50/50 split. (1/1) Conversely, if after a few hours,  you are not seeing much change, and you feel that it is too slow, you can up the amount of vinegar to the solution. Water 1, vinegar2, and so on. .

 

Or you can directly add vinegar to the matrix, and see if it is too aggressive.

[minnbuckeye]

If you are looking to extract the crinoids with vinegar, the acid will wreck the specimens by the time you are done. If the purpose is to "clean " the crinoids a bit so they stand out better, a short bath in vinegar should suffice. Afterwards, soak the specimen in a bucket of clean water overnight. I add a touch of baking soda. Not sure if this helps, but it makes me feel better that I have neutralized the acid.

[bamontgomery]

20 minutes ago, minnbuckeye said:

If you are looking to extract the crinoids with vinegar, the acid will wreck the specimens by the time you are done. If the purpose is to "clean " the crinoids a bit so they stand out better, a short bath in vinegar should suffice. Afterwards, soak the specimen in a bucket of clean water overnight. I add a touch of baking soda. Not sure if this helps, but it makes me feel better that I have neutralized the acid.

Thanks.  I absolutely don't want to extract the crinoids.  We found plenty of loose crinoid pieces there, and I love the piece as is.  However, I do want to expose more of it and clean as much of it as possible, but am scared of destroying the specimens if I go this route.

[Bobby Rico]

I think it would look really great prep . You could  look to get it done professionally .  I got small piece prepared with an air scribe and it really nice.Just a thought 

[minnbuckeye]

Buy yourself a $20.00 dremel engraver. Work some of the matrix away. Then a quick vinegar bath as mentioned earlier. This is easy to do with crinoid plates like this, even for beginners!!!!!!

[Kane]

2 hours ago, minnbuckeye said:

I add a touch of baking soda. Not sure if this helps, but it makes me feel better that I have neutralized the acid.

That is key advice!

Any lingering acids will continue to do its work even after many rinses. Since water has a pH of 7 (middle of the scale) and vinegar is around pH value of 2 or 3, water alone will not work to neutralize any remaining acid. Baking soda has a pH of 9, so it won't completely neutralize the vinegar as much as, say, ammonia solution or soap. 

[bamontgomery]

6 hours ago, minnbuckeye said:

Buy yourself a $20.00 dremel engraver. Work some of the matrix away. Then a quick vinegar bath as mentioned earlier. This is easy to do with crinoid plates like this, even for beginners!!!!!!

Thanks.  I've seen people using them and air scribes in YouTube videos, but I'm kind of afraid to try it without a professional walking me through it.  After all, you can't learn to drive just by watching videos of people driving, right?

[Ludwigia]

If you are wanting to remove the brown and grey colored substance from the block, then you could try using caustic potash(potassium hydroxide) on it, since that does not dissolve calcite. This article would give you the necessary advice on the procedure.

[bamontgomery]

On 9/18/2021 at 2:21 AM, Ludwigia said:

If you are wanting to remove the brown and grey colored substance from the block, then you could try using caustic potash(potassium hydroxide) on it, since that does not dissolve calcite. This article would give you the necessary advice on the procedure.

Thanks for the advice!

[bamontgomery]

On 9/17/2021 at 11:22 AM, Mainefossils said:

Generally, acid preparation is reserved for non-calcium based fossils, such as conodonts and some brachiopods.

Thanks again for the advice!  My son got this as a gift on Monday from a more seasoned rockhound in our area.  Since it's all brachiopods, would it be a better candidate for a slightly acidic soaking?

[Ludwigia]

3 hours ago, bamontgomery said:

Since it's all brachiopods, would it be a better candidate for a slightly acidic soaking?

 

 Not if they're preserved in calcite, which they probably are. You'd need to either get them abraded or give them the caustic potash treatment. It's a nice piece, so I wouldn't want to ruin it.

[bamontgomery]

Thanks Luwigia.  I'm glad I asked before I tried anything.  Yes, it's a very nice piece, and was gifted to my boy by a very kind and generous individual.  She also gave him a number of geodes, some slag, druzy quartz, and this amazing crinoid piece.  We're very lucky to have people like her helping out novices like us.