Coelacanth Posted February 20, 2016 Share Posted February 20, 2016 (edited) Hello, I have recently acquired several unprepped fossils from the Orkney Islands (Coccosteus cuspidatus, Osteolepis macrolepidotus), and I was wondering how one would go about cleaning them up a bit. The bone is replaced by black apatite monohydrate and the matrix is a type of limestone. Based on this, I'm certain that a weak solution of hydrochloric acid could be used to etch the fossil out. Still, I was wondering what the consensus among scientists was about the method of preparing these Orcadian fishes. If anyone here can provide an answer to that question, please do so below. Thanks! Edited February 21, 2016 by Coelacanth Link to comment Share on other sites More sharing options...
Slowpoker Posted February 20, 2016 Share Posted February 20, 2016 A solution of weak acid will do, just make sure not to over-use it. Link to comment Share on other sites More sharing options...
oilshale Posted February 20, 2016 Share Posted February 20, 2016 How to prep fossils from Orkney is not really my area of expertise, but this must be a calculation error - 10.7 mol/dm3 is concentrated hydrochloric acid, almost 34%! You don't want to use that! Thomas Be not ashamed of mistakes and thus make them crimes (Confucius, 551 BC - 479 BC). Link to comment Share on other sites More sharing options...
Slowpoker Posted February 20, 2016 Share Posted February 20, 2016 How to prep fossils from Orkney is not really my area of expertise, but this must be a calculation error - 10.7 mol/dm3 is concentrated hydrochloric acid, almost 34%! You don't want to use that! Thomas 34% sounds okay if used very sparingly, and maybe a far lower percentage for areas closer to the bones, to minimize damage if it should occur. Link to comment Share on other sites More sharing options...
oilshale Posted February 20, 2016 Share Posted February 20, 2016 34% sounds okay if used very sparingly, and maybe a far lower percentage for areas closer to the bones, to minimize damage if it should occur. Really? Pure, undiluted HCL? This is already fuming HCl - 38% is the maximum concentration which can be produced and handled at room temperature. Thomas Be not ashamed of mistakes and thus make them crimes (Confucius, 551 BC - 479 BC). Link to comment Share on other sites More sharing options...
JohnBrewer Posted February 20, 2016 Share Posted February 20, 2016 34% is very dangerous and overkill. You'll probably won't be able to control the prep too. A saturated solution of hydrogen chloride gas is 43% (which is concentrated hydrochloric acid) and a pH of less than one (yes you can get a pH of less than 1!). 5% acetic acid, (roughly the strength of distilled vinegar) is pH 2.5ish. One step on the pH scale is 10x. 1 John Map of UK fossil sites Link to comment Share on other sites More sharing options...
Ash Posted February 20, 2016 Share Posted February 20, 2016 34% is very dangerous and overkill. You'll probably won't be able to control the prep too. A saturated solution of hydrogen chloride gas is 43% (which is concentrated hydrochloric acid) and a pH of less than one (yes you can get a pH of less than 1!). 5% acetic acid, (roughly the strength of distilled vinegar) is pH 2.5ish. One step on the pH scale is 10x. So acetic acid is about the same PH as most soda "Faith is to believe what you do not see; the reward of this faith is to see what you believe" - Saint Augustine"Those who can not see past their own nose deserve our pity more than anything else." Link to comment Share on other sites More sharing options...
Coelacanth Posted February 21, 2016 Author Share Posted February 21, 2016 (edited) Oops, misremembered the molar mass of Hydrochloric acid - 36.46, not 26.46. So, let's see - m = (%/100*D*cm^3/MOL)/dm^3 = (.274*1.017*1000/36.46)/dm^3 = approx. 7.6/dm^3. OK, so I meant 7.6 Mol/dm^3, not 10.7. Sorry. P.S In the case of HCl (very strong acid) you can assume that it is 100% ionized in H2O. Therefore, pH= -log (molarity), so -log(7.6) = -0.88 !!!!! (Guess I will have to dilute it a bit.) IFF anyone has any experience with this type of prep work, I was wondering how far to dilute this solution. Edited February 21, 2016 by Coelacanth Link to comment Share on other sites More sharing options...
JohnBrewer Posted February 21, 2016 Share Posted February 21, 2016 So acetic acid is about the same PH as most soda Yes pretty bad stuff. That's why I sup my whisky on the rocks. Much healthier John Map of UK fossil sites Link to comment Share on other sites More sharing options...
JohnBrewer Posted February 21, 2016 Share Posted February 21, 2016 I did read an academic paper recently on acid prep and did save it. Somewhere. I'll dig it out and post the link. Using a buffer is a good idea too to keep the potentially up. John Map of UK fossil sites Link to comment Share on other sites More sharing options...
Coelacanth Posted February 21, 2016 Author Share Posted February 21, 2016 Thanks for the tip John! Do you by any chance happen to have a link to the .pdf file? Link to comment Share on other sites More sharing options...
jpc Posted February 21, 2016 Share Posted February 21, 2016 Nice to see there are some chemically literate people here. Link to comment Share on other sites More sharing options...
Ash Posted February 21, 2016 Share Posted February 21, 2016 Oops, misremembered the molar mass of Hydrochloric acid - 36.46, not 26.46. So, let's see - m = (%/100*D*cm^3/MOL)/dm^3 = (.274*1.017*1000/36.46)/dm^3 = approx. 7.6/dm^3. OK, so I meant 7.6 Mol/dm^3, not 10.7. Sorry. P.S In the case of HCl (very strong acid) you can assume that it is 100% ionized in H2O. Therefore, pH= -log (molarity), so -log(7.6) = -0.88 !!!!! (Guess I will have to dilute it a bit.) IFF anyone has any experience with this type of prep work, I was wondering how far to dilute this solution. Goodness me, that's what i gotta learn to be able to do if i get into college "Faith is to believe what you do not see; the reward of this faith is to see what you believe" - Saint Augustine"Those who can not see past their own nose deserve our pity more than anything else." Link to comment Share on other sites More sharing options...
JohnBrewer Posted February 21, 2016 Share Posted February 21, 2016 Goodness me, that's what i gotta learn to be able to do if i get into college When I went to college I had to learn moles without using a scientific calculator! Log tables and longhand for me then...Here's the PDF file, pretty old but may be of use. toombs1959.pdf John Map of UK fossil sites Link to comment Share on other sites More sharing options...
Ash Posted February 22, 2016 Share Posted February 22, 2016 Thanks, John! I didn't do well at Chem in high school, for some reason couldn't wrap my head around moles. But going by improvements in algebra I'm thinking this time around I can do it. "Faith is to believe what you do not see; the reward of this faith is to see what you believe" - Saint Augustine"Those who can not see past their own nose deserve our pity more than anything else." Link to comment Share on other sites More sharing options...
oilshale Posted February 22, 2016 Share Posted February 22, 2016 Two papers about transfer preparation of fossils - within there, you will also find some information about acid prep: Challenges encountered during acid resin transfer preparation of fossil fish from Monte Bolca STANDARD PREPARATION TECHNIQUE FOR FOSSIL FISH FROM THE ROMALDO MEMBER STANDARD PREPARATION TECHNIQUE FOR FOSSIL FISH FROM THE ROMALDO MEMBER.pdf Have fun Thomas 1 Be not ashamed of mistakes and thus make them crimes (Confucius, 551 BC - 479 BC). Link to comment Share on other sites More sharing options...
oilshale Posted February 22, 2016 Share Posted February 22, 2016 (edited) Hello, I have recently acquired several unprepped fossils from the Orkney Islands (Coccosteus cuspidatus, Osteolepis macrolepidotus), and I was wondering how one would go about cleaning them up a bit. The bone is replaced by black apatite monohydrate and the matrix is a type of limestone. Based on this, I'm certain that a weak solution of hydrochloric acid could be used to etch the fossil out. Still, I was wondering what the consensus among scientists was about the method of preparing these Orcadian fishes. If anyone here can provide an answer to that question, please do so below. Thanks! If you are sure the fossils are silica and not Ca-phospate, you can use acid prep without problems. However, many fossils are not pure silica but Ca-phosphate, so you might consider acetic acid (5% or so) buffered with phosphate. By the way, I never heard about apatite monohydrate - I guess you are talking about hydroxylapatite [ Ca10(PO4)6(OH)2 ]. Hydroxylapatite (or hydroxyapatite) is soluble in acids - so you need to use a phosphate buffer. Thomas PS: I would prep them with an air scribe or with a sand blaster - not by acid prep. Edited February 22, 2016 by oilshale Be not ashamed of mistakes and thus make them crimes (Confucius, 551 BC - 479 BC). Link to comment Share on other sites More sharing options...
Coelacanth Posted February 22, 2016 Author Share Posted February 22, 2016 I am not talking about hydroxylapatite, I am speaking of dehydrated hydroxylapatite with the chemical formula [Ca10(PO4)6(OH)]. I had a sneaking suspicion that the acid could dehydrate it further, then dissolve the remaining apatite, but I wasn't sure (Ph.D. in physics, not chemistry). Thanks for the advice, though. The bone is very friable, so I was concerned that if I used an air scribe pieces might flake off. However, I did a little research, and the consensus seems to be the usage of ~4% phosphate buffered HCl for the detail, but to remove the overburden with a scribe. Thanks for the .pdfs, though! Link to comment Share on other sites More sharing options...
oilshale Posted February 23, 2016 Share Posted February 23, 2016 I am not talking about hydroxylapatite, I am speaking of dehydrated hydroxylapatite with the chemical formula [Ca10(PO4)6(OH)]. Well..er.. this forum is all about fossils and not about chemistry - so I shouldn't mind. But sorry, this chemical formula [Ca10(PO4)6(OH)] is just plain wrong; a compound with such a stoichiometry cannot exist. You said you are speaking about dehydrated Apatite - this would mean 1 unit of water (H2O) left the molecule. But your formula indicates that 1 unit of OH- left the molecule (OH is negatively charged!). But then, the remaining part (your "dehydrated Apatite") would be positively charged which is of course not possible. This formula is simply nonsense. Better repeat Chemistry 101. If you dehydrate hydroxylapatite you might end up in [Ca10(PO4)6]2O - at temperatures in excess of 1400°C. Thomas Be not ashamed of mistakes and thus make them crimes (Confucius, 551 BC - 479 BC). Link to comment Share on other sites More sharing options...
JohnJ Posted February 23, 2016 Share Posted February 23, 2016 I didn't read the pdfs, yet, but when using acids to prep invertebrates, I always pre-soak in water to saturate the specimen. Putting a dry specimen into an acid treatment just begs for future problems. Something to consider.... 1 The human mind has the ability to believe anything is true. - JJ Link to comment Share on other sites More sharing options...
fossilcrazy Posted February 25, 2016 Share Posted February 25, 2016 Air abrasion would be the prefered method of preparation. This Osteolepis from the Orkney Islands has had no acid treatment. Scales are nice and reflective, even after air abrasion. I haven't tested the matrix, but I would expect very little fizz from acid, since it seems very much siliceous. 1 Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now