Prepping Bones In Hard Siltstone

[Doctor Mud]

Hi everyone,

I picked up a piece of silstone from Glen Affric beach in New Zealand (attached picture). This is a Miocene site and some of you may have seen some of the awesome crabs that come from this locality.

The siltstone is hard and may be calcerous (need to test with some acid). It is also full of bones. Many large bones (some can be seen in the picture) and lots of smaller fragments. I'm pretty sure the larger bones are from a whale or dolphin. Not sure which parts yet.

My question is: Should I prep this with a scribe or would acid be useful? I've never used acid prep before but I thought maybe it would be advantageous when working with rock that is full of bone. It might be easy to keep on hitting them with the scribe.

Has anyone done any acid prep before? I'd love to hear any suggestions.

Thanks!

[bone2stone]

I would venture to guess that the hardness of the material would make that determination.

Considering you are out in the middle of the Pacific, that is a pretty hard question.

Texas got some pretty hard "rocks" I been crackin, scratchin, chipping and working on stone

for a while, I know now that no two rocks will work out the same.

Some I have put away till I can figure out a way to deal with them.

I'm working on getting an abrasive air unit.

Just be careful that specimen is pretty big.

A combination of both techniques and make sure your dilution water to acid is balanced well

to avoid over etching. Experiment with different strengths and on different materials.

Avoid doing the acid thing indoors.

If the material is calcitic enough it will neutralize the acid and render it.

Someone else may have some other point of view,

Abrasive air works on some materials, this would be one of the more expensive methods

but you get incredible results.

Good Luck.

Jess B.

[Doctor Mud]

Thanks Jess,

I've only just heard about the air abrasion technique and it does look effective but expensive since you can't re-use the abrasive grit.

I'm new to working on such hard rocks. Where I grew up in New Zealand, my fossil hunting was mostly in softer greensands and sandstones where you could happily prep fossils with a needle and brush. Nice sharks teeth to be found, early penguin and whale fossils of Eocene to Miocene age.

I did a few preliminary tests on the matrix today. As I expected it is very hard and work will be slow.

I took lots of high resolution photographs before beginning and tested a piece of matrix well away from the bones. I took some of the rock chips and immersed them in acetic acid.....sure enough bubbles - a sign of calcite.

One of the problems with this speciment is the bone/matrix interface - the place where rock meets bone. I am used to a nice shiny surface and when you approach the bone surface with a scribe, the rock flakes away nicely to expose bone. Doesn't seem like this will be the case here.....things may change when I get deeper into the rock since the exposed bone may be more weathered and pitted.

The bone that I can see is porous so the matrix won't flake. Perhaps the best way will be to get close with mechanical prep and continue with acid or air abrasion.

I came across a useful guide for acid prep for anyone who is curious (attached file). A great step by step guide on what to do and not do. Sounds like the author might have had some painful learning experiences (leaving the plug in the sink with overflows, loosing bones down the sink.....)

Well I will continue slowly to work out the best way to unvail what this fossil has to offer. It's a bit like Christmas not knowing exactly what waits inside...a 10 million year old present from nature. I'll keep you guys posted now and then if I make any breakthroughs or progress.

j.2151-6952.1972.tb00449.x.pdf

[Auspex]

Better test the acid's effect on the bones...just sayin'.

[Doctor Mud]

Better test the acid's effect on the bones...just sayin'.

Cheers Auspex. Yep it would be pretty disasterous to have the bone dissolving away!

I read the guide that I attached above and apparently you can work with bone that can be dissolved in the acid - you just have to take a few steps to avoid the bone being dissolved. 1. A Protective coating. 2. Only use the acid to remove the last bit of matrix 3. Add a buffer to your acid

1. Apparently you coat the bone in a protective coating - something like Butvar? I'm going to test PVA beads dissolved in acetone as a coating. I got these from the Black Hills institute. ~ $10 per pint which can be dissolved in acetone to make an all purpose prep liquid that can be thinned to suit your needs. It can be used as a glue or as a consolidant. And most importantly it is reversable using acetone to dissolve it. I heard about it from another post on this forum.

You coat all the exposed bone and a few mm of the matrix where it meets the bone.

2. You need to use a scibe to remove the bulk of the matrix and only use the acid to finish off.

3. I also read somewhere else that you can add a phosphate buffer to your acid to help prevent bone being dissolved. I guess that would be based on what your bone is actually made from.

It almost seems like (used this way) acid prep is a chemical version of air abrasion - except I bet you have far more control with the air abrasion unit!

I hadn't heard about the air abrasion before joining this forum and I'm sure its something I will get in the future!

[6ix]

How did you get on? I have some coral I got from same spot sitting in a jar of water seeing if it will eventually soften...

[tooth_claw]

I have acid prepped a couple of smaller specimens but am no expert in the technique. However, the methodology is sound, this is how most acid prep is commonly done. I can only recommend caution and patience as two vital ingredients, avoid the temptation to do too much at once and let the specimen dry completly before adding more consolidant-it will not penetrate into the bone if wet and leaves a thin 'skin' that peels off. You will kick yourself if you 'over acid' the specimen!

Looking forward to seeing what is entombed within the block.

[Doctor Mud]

How did you get on? I have some coral I got from same spot sitting in a jar of water seeing if it will eventually soften...

Hi 6ix, do you have a photo of your coral?

Any luck with the water? Depends on what sort of rock you have

But the hard siltstone from around there might be quite resistant to water.

I tried the siltstone I have with white vinegar and this may be a bit weak. Leaving it sitting for a while may work better but

I am going to try with the scribe again. The main thing is that the rock doesn't seem to want to flake

off. Might have to try varying the angle of attack. I worked on reptile bones once where the bone was so porous and the surface wasn't smooth so you had to virtually grind right down to the surface.

Good luck with the coral !

[Doctor Mud]

I have acid prepped a couple of smaller specimens but am no expert in the technique. However, the methodology is sound, this is how most acid prep is commonly done. I can only recommend caution and patience as two vital ingredients, avoid the temptation to do too much at once and let the specimen dry completly before adding more consolidant-it will not penetrate into the bone if wet and leaves a thin 'skin' that peels off. You will kick yourself if you 'over acid' the specimen!

Looking forward to seeing what is entombed within the block.

Thanks tooth-claw

If might seem that acid prep is a magic fix. But it appears that (if done properly) can actually take longer than physical prep.

I put the block aside while I worked on a crab from the site. Almost done that so time to look at the bones again.

I'm going to give mechanical prep another go. Got to work on getting that rock flaking off the bone.

Edited June 24, 2013 by Doctor Mud

[6ix]

Not sure if its the same substrate as yours - I dug it out of the cliff itself... I notices it was cracking a little while drying so I biffed it in a jar of fresh water.

The first photo is from Sat night... the bottom two are from 10 minutes ago - its gotten really soft now, I doubt the coral will withstand it

I also have some specimens of it in a very hard rock - I'm guessing thats probably what you have instead.

Hi 6ix, do you have a photo of your coral?

[Doctor Mud]

Hi 6ix. Looks like softer siltstone than the stuff I am working with.

I've started work on the chunk of matrix. I've removed a few small chips that were full of tiny bones and soaked them in a small container with white vinegar.

At first I thought nothing was happening but pretty soon the rock started to slowly fizz.

I find that only a small amount of matrix actually dissolves or falls off, but it does make the siltstone soft enough that you can gently remove it with a wet paint brush. I've found that I get a few mm off each time and there are many small bones that are being exposed nicely. I leave the rock in the vinegar until it stops fizzing - which is about a day for the size of chips and amount of vinegar I'm using.

I plan to use mechanical prep to remove the bulk of the matrix and finish it off with soaks in vinegar.

How did your coral go?

[6ix]

Was fun - the soaking softened the clay/silt a heap. Got most of it off with a toothbrush and drawing pin. The Flabellum was harder than expected and pyrite/hematite has protected it..

PHOTOBOMB!!