Chemical Preparation

[Crusty_Crab]

I am writing a short manual on preparation methods for our club and I am including a short section on chemical preparation. This is most commonly done with acids on carbonate rocks. For many newbies, chemical preparation is attractive since all you need is a weak acid that everyone has in their kitchen (vinegar or lemon juice) and don't have to buy expensive equipment. I try to avoid it if at all possible since it works on a very narrow class of matrices, you have relatively little control over it, you may dissolve the fossil itself if it hasn't been silicified and its irreversible. Consequently, I don't have much experience with it. I have tried brushing Muriatic Acid (concentrated HCl) on limestones and immersing them in weak acids like household vinegar with mixed results.

 

I have a few questions:  does acid preparation only work on carbonates (limestone CaCO3, dolomite CaMg(CO3)2 or siderite FeCO3) or are there other fossiliferous matrices that can be prepared using acid?

 

Are there other legitimate ways to prep using acids other than brushing it on or immersing a fossil in a weak solution?

 

I am aware of using organic solvents to prepare tar pit fossils, but this is something outside of most amateurs. Are there other chemical preparation methods other than organic solvents on tar pits or acids?

 

 

 

 

[Ludwigia]

1 hour ago, Crusty_Crab said:

Are there other chemical preparation methods other than organic solvents on tar pits or acids?

Potassium hydroxide (KOH flakes) can be used in combination with a few drops of water to dissolve marly stones encrusting fossils. There is also a tenside product called Rewoquat which does the same job. They are both basically strong detergents.

[Crusty_Crab]

10 minutes ago, Ludwigia said:

Potassium hydroxide (KOH flakes) can be used in combination with a few drops of water to dissolve marly stones encasing fossils. There is also a tenside product called Rewoquat which does the same job. They are both basically strong detergents.

Very interesting, KOH is a base and appears to work by breaking the bonds linking individual clay particles, which I associate with hydroxides (OH-) or basic. Perhaps this is an example of a stronger base causing a weaker base to act as an acid and thus causing a reaction? Regardless, are there any real world examples of this in action? 

[Ludwigia]

1 hour ago, Crusty_Crab said:

are there any real world examples of this in action? 

This method is used very often on fossils in Middle Europe. I've used it many times myself. Caustic potash, as it is also called, is a base which causes limey marl to swell up and disperse and has nothing to do with acids as far as I know.

[FossilDAWG]

The successful application of KOH depends on the nature of organic material in the matrix, and it varies tremendously depending on the source of the fossil.  I have used it with great success on Silica Shale fossils.  On the other hand it did nothing for fossils from the Verulam Formation.  You just have to try it and see.  Put flakes of KOH on the area to be cleaned, and sprinkle with a bit of water.  Here in Georgia it is usually humid enough that I could skip wetting the flakes as they are very hygroscopic and pull enough moisture out of the air.  I would leave them on for a couple of days, rinse (a lot), scrub with a toothbrush, and repeat.  Patience is an essential part of the process.

 

I have not been able to find a source for Rewoquat in North America.  BTW, what is a "tenside product"?  Anyway years ago there was a detergent called Quaternary-O that did a great job of breaking down shale, but it has been off the market for at least 30 years.  Other quaternary ammonium compounds may work but I have not heard of any that can substitute for Quaternary-O.  I think these products are going extinct because of environmental toxicity but I'm not sure of that.

 

Don

[Ludwigia]

1 hour ago, FossilDAWG said:

BTW, what is a "tenside product"?

Basically a soap.

[Crusty_Crab]

2 hours ago, Ludwigia said:

This method is used very often on fossils in Middle Europe. I've used it many times myself. Caustic potash, as it is also called, is a base which causes limey marl to swell up and disperse and has nothing to do with acids as far as I know.

 

1 hour ago, FossilDAWG said:

The successful application of KOH depends on the nature of organic material in the matrix, and it varies tremendously depending on the source of the fossil.  I have used it with great success on Silica Shale fossils.  On the other hand it did nothing for fossils from the Verulam Formation.  You just have to try it and see.  Put flakes of KOH on the area to be cleaned, and sprinkle with a bit of water.  Here in Georgia it is usually humid enough that I could skip wetting the flakes as they are very hygroscopic and pull enough moisture out of the air.  I would leave them on for a couple of days, rinse (a lot), scrub with a toothbrush, and repeat.  Patience is an essential part of the process.

 

I have not been able to find a source for Rewoquat in North America.  BTW, what is a "tenside product"?  Anyway years ago there was a detergent called Quaternary-O that did a great job of breaking down shale, but it has been off the market for at least 30 years.  Other quaternary ammonium compounds may work but I have not heard of any that can substitute for Quaternary-O.  I think these products are going extinct because of environmental toxicity but I'm not sure of that.

 

Don

To an acid-base chemist, I'm sure the world can be described through acid-base reactions. Since I'm not one, it seems that both are strong detergents that work in the way that surfactants do, with the addition that Quaternary-O has low sudsing potential. Have you tried boiling with another strong surfactant such as tri-sodium phosphate (TSP)? If the low sudsing properties are important, is it possible to add a bit of liquid fabric softener or vinegar?

[Ludwigia]

You appear to know more about the subject than you were first letting on, so I'm sure that you can find out the answer to this question yourself. I myself can't answer it, other than to say that when you mix an acid with a base, they tend to neutralize each other and you end up with a salt.

[Crusty_Crab]

7 hours ago, Ludwigia said:

You appear to know more about the subject than you were first letting on, so I'm sure that you can find out the answer to this question yourself. I myself can't answer it, other than to say that when you mix an acid with a base, they tend to neutralize each other and you end up with a salt.

I honestly didn't know about using KOH or any of the detergents before this conversation and got so excited about learning about a new method that I immediately did some internet searches. I just googled them and saw that they were surfactants and low sudsing. I googled surfactant and TSP came up. Similarly, vinegar and fabric softener came up when I searched low suds. I have no idea if they are effective together, but I did search to make sure mixing vinegar and TSP does not make a deadly reaction like mixing bleach and ammonia.