Polyurethane Removal?

[mudbug]

I have a mammoth tooth that, some years ago, was coated with polyurethane. Can the polyurethane be removed from the fossil? Should it be removed? If so, how should I do it?

[Ridgehiker]

I have a mammoth tooth that, some years ago, was coated with polyurethane. Can the polyurethane be removed from the fossil? Should it be removed? If so, how should I do it?

Not to ignore your question but are you certain it is polyurethane and not some other coating like shellac?

[mudbug]

Yes, the person who gave it to me told me that he used polyurethane with this fossil as well as others in his collection. It was the only thing he used to protect his fossils. I believe it was a fairly common practice with amateur collectors at the time.

[LordTrilobite]

If the mammoth tooth comes from a location like the North Sea it will be full of salt crystals. North Sea fossils often fall into pieces when left untreated. So the coating might be needed to keep the fossil in one piece.

[Ridgehiker]

If the mammoth tooth comes from a location like the North Sea it will be full of salt crystals. North Sea fossils often fall into pieces when left untreated. So the coating might be needed to keep the fossil in one piece.

Yes, this is also my experience. I have a mammoth tooth from the North Sea and it needed to be stabilized. In contrast, I collected one along a river bank in Nova Scotia and it is solid.

[mudbug]

No, it is not from the north sea. It is a land find and looks solid. I'd like to remove the polyurethane to use a more appropriate consolidant. Do I understand correctly that the polyurethane could cause problems with the long term preservation of the tooth?

[Opisthotriton]

Yes, but attempting to remove the polyurethane would probably do more damage than waiting 50 years and dealing with the aging polyurethane then. If it looks ok now, best to leave it be.

Polyurethane is not easily reversible, which is why it should not be used in the first place. It will crack and yellow with age. But if it looks fine now, then it should continue to look fine for many years to come. When it does yellow and crack, there can be chemical and physical damage to the fossil, so some people might want to strip the polyurethane before that happens. Personally I'd leave it alone for now. Unless anyone here has experience with using paint stripper on polyurethaned fossils?

[Ptychodus04]

I have had success with multiple local applications of acetone to remove old poly and lacquer. Brush it on, let it sit for a minute, scrub it with a tooth brush. Lather, rinse, repeat so to speak. Don't get in a hurry. It takes a long time. But, I have had splendid results removing the offending substance.

[caldigger]

I just had a good experience with coating removal. One of the St. Clair fern plates I received on trade had been coated with a matt spray to seal it. This unfortunately changed the coloring and all but took away the normal white color of the leaves ( darkened them to almost black) I figured there wasn't much to lose, damage done right? So I experimented with Acetone gently rubbing on the back and as the coating came off, the colors returned. The plate now looks like it is supposed to with the signature white fern leaves. A successful experiment.

Not saying this will work with all your fossils, but it did with whatever the coating that was used.

Edited September 9, 2015 by caldigger

[amour 25]

Post a pic.

There is one of mine for contrast.

Coated with Butvar 76

[oilshale]

Coating and coating will not always behave the same way. The behavior depends on the chemical composition of the coating. Many polymers used for coating (Butvar, shellac, PVAC, Vinac, Paraloid...) are linear polymer chains, are therefore soluble and can be removed with solvents.

Other polymers such as Epoxy and Polyester are initially also linear chains. After adding an activator/catalyst or a second component, the polymer chains will react with each other and will form a highly crosslinked system (think of a three dimensional chicken wire...). This network can not be easily broken up - the polymer is insoluble.

Most Polyurethane lacquers belong to the second group of polymers. They are two component systems and have to be mixed before application. The two components will react with each other - the result will be a highly crosslinked and very durable coating. The coating can swell in solvents and will get soft, but it will not dissolve. Trying to remove such a coating can end up in a mess and would probably do more damage.

I'd leave it alone for now.

Thomas

[oilshale]

Forgot to mention: Yes, Polyurethane will crack and get yellow with age. This is due to a reaction induced by sunlight. You can not prevent this reaction but you can retard it: Try to avoid bright, direct sun light and keep it if possible under glass (to protect from UV light). A dark(er) spot would be the best place.

[erose]

Forgot to mention: Yes, Polyurethane will crack and get yellow with age. This is due to a reaction induced by sunlight. You can not prevent this reaction but you can retard it: Try to avoid bright, direct sun light and keep it if possible under glass (to protect from UV light). A dark(er) spot would be the best place.

Sunlight is obviously the strongest source of UV but quartz lamp halogens can also be strong. One way or the other keep it out of direct light.

[mudbug]

OK I guess at this point I will leave it alone.

Jeff

That's a really impressive tooth. Mine isn't nearly as nice. I'd post pics of mine but I recently moved and haven't found it yet, It was one of the first projects that I wanted to tackle as soon as I find it. I guess it's not so important now.

[mudbug]

OK, I've been offered another mammoth tooth. An exceptionally nice one but, it has been coated with silicone on the sides and shellac on the grinding surface. I never heard of anyone doing that before. Is this reversible?

I'm not sure maybe I should have started another thread.

[oilshale]

OK, I've been offered another mammoth tooth. An exceptionally nice one but, it has been coated with silicone on the sides and shellac on the grinding surface. I never heard of anyone doing that before. Is this reversible?

I'm not sure maybe I should have started another thread.

Silicone? Oh my gosh! What kind of silicone is it? Is it elastic and flexible? If so, this stuff is crosslinked and not really soluble anymore. You could try it with toluene (or with chloroform, but I wouldn't recommend this - too dangerous to use for ordinary people unless you are a chemist. Chloroform is considered as carcinogenic!). There is a DOW silicone remover - D2025 or so. But not sure if this stuff can do the job.

If it is a sort of grease, you are lucky. This silicone grease can be dissolved in acetone.

Shellac shouldn't be any problem with acetone.

Edited September 13, 2015 by oilshale

[Ptychodus04]

OK, I've been offered another mammoth tooth. An exceptionally nice one but, it has been coated with silicone on the sides and shellac on the grinding surface. I never heard of anyone doing that before. Is this reversible?

I'm not sure maybe I should have started another thread.

If it is exceptionally nice, one wonders why it would have been coated with such a horrible thing as silicone in the first place...

[mudbug]

Some people think they need to put some kind of preservative on every fossil. It is a really nice tooth. I have already purchased it.