Acid etching?

[GreatLakesLady]

Hi, I'm brand new to this. I noticed this "creasing" on a couple rocks after soaking them in vinegar. Is it a reaction from the vinegar?

[DPS Ammonite]

Welcome to the Fossil Forum. I made the same pattern on a piece of limestone when I soaked it in HCl. Somehow rising bubble and acid make those patterns.

Edited July 28, 2016 by DPS Ammonite

[CraigHyatt]

The technical term for it is "puckering". When eat a spoonful of vinegar your face does the same thing.

[GreatLakesLady]

Thank you! Puckering is exactly what it looks like!

[CraigHyatt]

Thank you! Puckering is exactly what it looks like!

Aww man. I should have put a smiley. I was just kidding. On the other hand, maybe we just invented a new technical term and we should publish a paper. ;-)

[FossilDAWG]

This is a common pattern when acid is used to etch limestone. I suspect it occurs because bubbles form as the acid dissolves the carbonate, and when the bubbles get large enough they detach and run up the side of the rock. This dislodges some of the insoluble sediment (which was fine mud on the sea floor) that is released from the limestone and ends up covering the surface, so the bubble track is cleaned and more accessible to further acid etching. The result is that the bubble track gets etched a little more than the adjacent sediment-covered surface, forming a physical groove in the surface. Of course the next bubble is more likely to travel up the groove, reinforcing the process. Many such grooves get started over the surface of the limestone, and as they get deeper and wider they merge so the entire surface is covered with grooves with raised ridge-like areas between them.

Don

[supertramp]

This is a common pattern when acid is used to etch limestone. I suspect it occurs because bubbles form as the acid dissolves the carbonate, and when the bubbles get large enough they detach and run up the side of the rock. This dislodges some of the insoluble sediment (which was fine mud on the sea floor) that is released from the limestone and ends up covering the surface, so the bubble track is cleaned and more accessible to further acid etching. The result is that the bubble track gets etched a little more than the adjacent sediment-covered surface, forming a physical groove in the surface. Of course the next bubble is more likely to travel up the groove, reinforcing the process. Many such grooves get started over the surface of the limestone, and as they get deeper and wider they merge so the entire surface is covered with grooves with raised ridge-like areas between them.

Don

yes, clearly resembling the rillenkarren karst morfology https://www.google.it/search?q=rillenkarren&biw=1280&bih=685&source=lnms&tbm=isch&sa=X&sqi=2&ved=0ahUKEwjLtIen8ZfOAhXFDsAKHQ6sAagQ_AUIBigB, a good example of a positive feedback process

ciao

[tmaier]

You left it soak overnight, right?

If you use vinegar (or any acid) you need to just use short exposures, dilute it if there is too much bubbling, and turn it around as it bathes. If you just leave it in the bath, you get these bubble tracks. It's tempting to just plop a specimen in the bath and wake up the next morning, but what you will find is not pretty.

The only time this is OK is when the fossil is petrified with silica, but most fossils are a carbonate and are soluble in the acid. What you are doing with the acid is trying to use differential expose and solubility to remove the softer and porous limes stone off the harder and less soluble calcite of the fossil. So you dunk it it for a minute, pull it out, brush with soft toothbrush, examine for acid damage, repeat... You are weakening the limestone to mechanically remove it, and stopping before the acid attacks the fossil too much. It's a touchy operation.

After you are done, you can flush it under the flowing water of a sink, and then soak it in a "stop bath" solution. You mix a tablespoon of baking powder in a couple cups of water, and then soak the fossil in it overnight. The solution is a base, and will "stop" the acid. Sometimes if you don't do this you will find that the acid keeps eating for the next couple days. The stop bath attacks and neutralizes the residual acid.

And one last trick to controlling the acid is that sometimes you only want to have the acid work on the surface features of the fossil, so you can soak the fossil in water to saturate it before exposing it to the acid. This prevents the acids from soaking into the fossil and attacking places that you don't want to weaken. This focuses the acid action onto only the surface of the specimen.

[FossilDAWG]

Working with acid can give excellent results but it requires a lot of care and patience. It works best if there is a differential solubility so the fossil is more resistant than the matrix. Sometimes it helps to buffer the acid solution with sodium phosphate, for example when prepping fossils with a lot of phosphate such as bones or conodonts, as this decreases the chance that the fossil will dissolve. Often it is necessary to etch the fossil a bit, rinse thoroughly and dry, then coat the exposed fossil with a protective layer of vinac or B72 or some other consolidant. The fossil can be exposed millimeter by millimeter, but it takes a long time to do properly. Some of the Devonian fish from Gogo in Australia were prepped this way, eventually freeing them completely from the carbonate nodules they were preserved in. The result is a specimen that is as anatomically perfect as a modern fish skeleton, but in some cases it took a couple of years to slowly expose the bone.

Just plunking a fossil into acid, especially concentrated acid, is a good way to wreck it in most cases.

Of course working with acid is dangerous, so you have to be very mindful of personal protection (gloves, lab coat, eye protection) and proper ventilation. You should also neutralize the acid before disposing of it to avoid contaminating waterways.

Don

Edited July 29, 2016 by FossilDAWG

[GreatLakesLady]

Thank you all very much for sharing! As you can tell I still have a great deal of learning to do!

[DPS Ammonite]

Here is what HCl did to a piece of Redwall Limestone from Arizona that had silicified somethings........stromatolites/ stromatoporoids or sponge? Even though the fossils were a dud, I kept the piece because the limestone is a work of art.

Edited August 10, 2016 by DPS Ammonite

[tmaier]

Scrubbing bubbles!

[Khyssa]

Here is what HCl did to a piece of Redwall Limestone from Arizona that had silicified somethings........stromatolites/ stromatoporoids or sponge? Even though the fossils were a dud, I kept the piece because the limestone is a work of art.

I can see why you kept that. It's beautiful!

Kara

[JohnBrewer]

Here is what HCl did to a piece of Redwall Limestone from Arizona that had silicified somethings........stromatolites/ stromatoporoids or sponge? Even though the fossils were a dud, I kept the piece because the limestone is a work of art.

That is so cool!