Limestone & Vinegar Prep

[Tachikoma_kun]

Hello!

I have a lovely fist-sized lump of limestone that is packed with sea urchins from Lebanon. I have already removed some sort of oyster from another lump of limestone with this method, but obviously that was just one large fossil, this is dozens of dime-size fossils sat on top of each other.

I have it bubbling away now to remove the top surface, but was wondering if there was a way to selectively dissolve the limestone around the urchins so I can keep them in place and not dissolve the limestone inside them so they are supported.

Would selectively painting on vinegar or a stronger acid work, or would this just take months due to the small amount applied?

I don't have an air scribe or anything more exotic than dental tools and a supermarket, so any advice based around those limits are most welcome.

Thanks

[sward]

Would it be possible to coat your find in the areas you don't want affected with something that would be impervious to the vinegar? I'm not a chemist, but could you use something like fingernail polish to coat those areas, soak it in vinegar to remove what you want, then remove the fingernail polish afterwards?

BRGDS

sward

DFW, TX

[Uncle Siphuncle]

echies dont like acid as it will attack calcite and erase surface detail. work on the alkaline end of the pH scale. i alternate between air scribe and potassium hydroxide to expose echies in calcareous matrix. KOH however is pretty caustic stuff so i use gloves and goggles and dont breathe the fumes. it isnt cheap either and must be purchased in bulk from a chemical supply house. sounds like a pain but its the best method i know.

[Tachikoma_kun]

Thanks danwoehr

I'm only dissolving away the outer weathered area at the moment with all the broken cross-sections so there's no worry about damaging them yet, but will look at a strong alkali online tonight. They all seem mostly crushed or damaged so I'm not too worried about experimenting on them. I doubt there will be many that will make it a nice show piece, I mostly just wanted to expose them as my first death assemblage. I have a few relatively undamaged single specimens already that I will post at some point to ask for an ID.

[Tachikoma_kun]

I see what you mean about the acid attacking the fossils, by the time I came home from work the vinegar had dissolved more limestone than anticipated and was attacking the broken sections already exposed Although I found an interesting echinoid specimen in there, a fossil with split-fossilisation, part calcite and part something else, much darker and unaffected by the vinegar

Think I will dry it out and try some manual preparation after I have neutralised the acid present, I have some dental tools (picks) and a Dremel so will try with that for a while before ordering some KOH as I enjoy the manual preparation, just normally don't have the time to do it. I'm in no rush, so happy to spend a few weeks chipping away slowly.

Just a quick question though, if KOH will eat away limestone, why doesn't it affect the calcites in the fossil? or is it just a case of the KOH being so caustic it attacks the rock quickly enough that you don't need to expose the fossil to it for long?

Thanks again danwoehr and sward, and if anyone has any opinions around swards idea, would be glad to hear them.

[jpc]

Maybe some good thick vinac (polyvinyl acetate) which you could use to make a nice wall around the area you want to use the acid or KOH on. Vinac is what we use a lot on vertebrate fossils out west. You can get it from PaleoTools, I think.

Dan... Now that I have a bunch of echies from Europe, I would love a quick lesson in KOH use. I have a small sample and a paper on its use in German, but if I can get some directions in English from someone who has used it, that would be great.

Thanks

jpc

[Tachikoma_kun]

Thanks jpc, I can get that easily enough, will try exposing some of them manually and coating the exposed ones before eating away more limestone with acid.

[Coco]

JPC, you have to pay very attention if you use KOH, it is very dangerous !

You wet a sea urchin with a little water, and you deposit on the zone that you want to clean glitter of KOH, with a crowbar (pliers).

You wait that the glitter is dissolved completely. The rock is going to split. You rinse well, you rub a little if you want with a brush, and you begin again. But after the complete cleaning, you have to let soak for a long time into some water by changing it every day to eliminate any track of KOH.

I know nothing in chemistry, but I think that it can be necessary to neutralize. I do not know with what (vinegar ? Bicarbonate ?).

JUST BE CAREFUL, my friend !

Coco

[ashcraft]

A short teacher story from the "Why your kid is an idiot" department. In Chem I we make soap as a lab dealing with polarity. I had a student come up to me after the lab and remark that "that was the worst tasting rock salt I have ever tried", I gave her the incredulous evil eye and asked her what she was talking about, as we had not used rock salt in the lab. She said those white flakes you mixed with the oil, and she picked up some and handed them to me. They were of course KOH flakes.

She is still alive, and will be reproducing soon.

Brent Ashcraft

[jpc]

JPC, you have to pay very attention if you use KOH, it is very dangerous !

You wet a sea urchin with a little water, and you deposit on the zone that you want to clean glitter of KOH, with a crowbar (pliers).

You wait that the glitter is dissolved completely. The rock is going to split. You rinse well, you rub a little if you want with a brush, and you begin again. But after the complete cleaning, you have to let soak for a long time into some water by changing it every day to eliminate any track of KOH.

I know nothing in chemistry, but I think that it can be necessary to neutralize. I do not know with what (vinegar ? Bicarbonate ?).

JUST BE CAREFUL, my friend !

Coco

Thanks, coco...

What is the French word for a Glitter. I'm afraid that one is lost in translation. Does it make nasty fumes?

[Uncle Siphuncle]

coco's explanation is good and i'll add that glitter=flakes. the reaction of koh with water is both caustic and exothermic. one drop in your eye and you may be kissing that eye goodbye. the rock melting process can be pretty fast so check it every 20 mins or the echie test could fall apart. i neutralize with grocery store vinegar for 30 mins then scrub and soak in water overnight. as with all new techniques, prove it out on your lesser specimens.

[Nandomas]

coco's explanation is good and i'll add that glitter=flakes. the reaction of koh with water is both caustic and exothermic. one drop in your eye and you may be kissing that eye goodbye. the rock melting process can be pretty fast so check it every 20 mins or the echie test could fall apart. i neutralize with grocery store vinegar for 30 mins then scrub and soak in water overnight. as with all new techniques, prove it out on your lesser specimens.

you can neutralize KOH with acetic acid and water and after that soaking the fossil in the water

Edited October 19, 2011 by Nandomas

[Coco]

Hi,

JPC, glitter is "paillettes" in french. I don't know if fumes are nasty, but I suppose as all chimical products, we have to be careful.

Thanks Nando for your precision.

Coco

[Nandomas]

Hi,

JPC, glitter is "paillettes" in french. I don't know if fumes are nasty, but I suppose as all chimical products, we have to be careful.

Thanks Nando for your precision.

Coco

Thanks Coco, but be careful, acetic acid could be too strong for the echies :unsure:

[Scylla]

As Brent alluded to above, KOH can make soap when placed with fats. Unfortunately your cells are partially made of fats so saponification makes hydroxide burns more dangerous than acid burns since you make your own detergent that helps disolve the next layer of you after the first layer of you is gone.

[tracer]

As Brent alluded to above, KOH can make soap when placed with fats. Unfortunately your cells are partially made of fats so saponification makes hydroxide burns more dangerous than acid burns since you make your own detergent that helps disolve the next layer of you after the first layer of you is gone.

now see...THIS is why i like to make sure i read all the chemistry stuff. most folks don't know about this and i'm darn sure gonna make sure i don't get saponificated.

but what i normally do is keep looking til i find a good specimen without too much junk on it and then mechanically get the junk off and move on down the road. every time i've played with chemicals something bad has happened. so all you teenage fossilers out there reading this, remember, the universal solvent is dihydrogen monoxide, so try that and a toothbrush before you do anything else. and ask your mom if it's really ok to use your little brother's toothbrush, because you're likely to find out that it isn't.