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What is the white stuff on seashells?

How to clean your seashells

OK, so you’ve taken our advice, gone shelling and scooped up some sensational keeper shells. Shells that can be centerpieces, shells that can be used for crafts and shells that you’ll most certainly remember for years to come. But now what?

Well, now’s the time to help those shells really come back to life by cleaning them and getting as much shine as possible. Today, we’re going to share a few tips as to how you can clean your shells and make them look as radiant and beautiful as possible. Let’s jump right in!


Using vinegar is easy, it’s simple and it’s cheap. Use a small amount of vinegar into a bowl or cup and use a toothbrush to gently scrub the shell. You might have to do this a few times to get the desired effect. Once you’re done, simply wash the shell with soap and water. That being said – be sure to NOT soak your shell in vinegar as it’ll have a corrosive effect. Simply using the acidic nature of the substance to clean out the dirt and then cleaning it out should do the trick.

Hydrogen peroxide

HP is always a winner. Simply place the shells in a small bowl and use just enough HP to cover the shells. Let it soak for a few hours until you notice a small film at the top. HP has invasive properties that make quick work of bacteria. Then simply rinse your shells and set them aside.

Bleach/Water combo

While this isn’t the most natural way to clean your shells, it’s most definitely effective. That being said, don’t leave them soaking in this solution for long as the bleach can fade the color and also leave that not-so-pleasant odor behind. That being said – if you’re looking for a quick solution that will work – this one might be for you.

One to avoid

There are a lot of blogs out there that will recommend Muriatic acid as a method to clean shells. We recommend that you stay away. Does it work? Absolutely. Does it produce spectacular results? You bet. Is it totally and unnecessarily dangerous? You bet. Acid can burn your skin and if you get any on you, you have to neutralize it with other chemicals and well, you get the picture. Don’t do it. Shelling is fun but that’s a little too hardcore for our tastes. While it’s not a solution like these other mentions, it’s certainly a tip that we hope you abide by. Just stay away.

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There are millions of ways to clean seashells and these are just a few. Some folks boil shells, some bury them, freeze them, put them in the dishwasher – it can get wild! But stick to these three primary methods and we can guarantee gorgeous, radiant shells. Good luck!

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Coquina — The Rock that Saved St Augustine

A close-up photo of coquina stone

Who would think that a fort made of seashells would last 300 years? Who would think that a fort made out of seashells would last three days under cannon fire? But the Castillo de San Marcos, made of local coquina stone, did just that. What exactly is this strange rock? How was it formed, and where did it come from? And how did this rock shape the history of St. Augustine?

How Coquina is Formed

Thousands of years ago, the tiny coquina clam donax variabilis lived in the shallow waters of coastal Florida, as they still do today. These are the small pink, lavender, yellow, or white shells one sees along the beach at the waterline. As the resident clam died, the shells accumulated in layers, year after year, century after century, for thousands of years, forming submerged deposits several feet thick. During the last ice age, sea levels dropped, exposing these shell layers to air and rain. Eventually, the shell became covered with soil, then trees and other vegetation. Rain water percolating through the dead vegetation and soil picked up carbon dioxide and became carbonic acid, the same ingredient that makes soda fizz.

As this weak acid soaked downward, it dissolved some of the calcium in the shells, producing calcium carbonate, which solidified in lower layers, much like how flowstone and stalactites are formed in caves. This material «glued» the shell fragments together into a porous type of limestone we now call coquina, which is Spanish for «tiny shell».

Workers use bars and pickaxes to cut coquina from a quarry.

A Stronger Fort is Needed

Although found in very few places in the world, conditions were just right for coquina formation along the east coast of Florida. The Spanish knew about this rock, and while they might have picked up loose chunks, the people of St. Augustine were primarily soldiers, not stonemasons, and so this rock sat mostly unused and unappreciated for years. Wood was more plentiful, at first, and easier to work with. But then the British, settling to the north, edged into the Carolinas. Spanish Florida was only a short sail away. Something more than a wooden fort was needed to protect St. Augustine and to keep the British from taking over Florida and using it as a base of operations to attack the Spanish treasure fleets and the more wealthy colonies of the Spanish Caribbean.

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As a result, the Spanish began construction on the Castillo de San Marcos in 1672. The coquina stone was quarried in the area of present-day Anastasia State Park on Anastasia Island. Military engineers and stonemasons were brought from Spain. Convicts and additional soldiers were brought from Cuba. Oyster shells were burned into lime and mixed with sand and water to make mortar.

Workers cut stones and use pulley systems to raise the blocks into place as Castillo is constructed.

Slowly, the walls rose. Since no one had ever built a fort or any large building out of coquina, they had no idea how strong it would be. At least it would not burn, and the termites wouldn’t eat it. But how well would seashells last under cannon fire? No one knew, so they built the walls an average of 12 feet (3.7m) thick. The walls on the ocean side are as thick as 19 feet (5.8 m)! The first phase of construction was completed in 1695. At the time, the Castillo looked very different than it did now. The walls were roughly five feet shorter than they are today, and the rooms were half their current size. The larger courtyard allowed for the townspeople to take refuge inside when under attack.


They did not have long to wait before the coquina walls were tested. In 1702, Governor James Moore of Charleston led his English forces against St. Augustine and the Castillo. He captured the town and set his cannon up amongst the houses to bombard the fortress. But a strange thing happened. Instead of shattering, the coquina stone merely compressed and absorbed the shock of the hit. The cannon balls just bounced off or sunk in a few inches. The shell rock worked!

The Rock that Changed History?

Even when General Oglethorpe tried his hand against St. Augustine in 1740 and bombarded the Castillo for 27 days, the walls held firm. The rock made of seashells turned out to be an excellent building material. When the Spanish decided to fortify the southern approaches to St. Augustine by building Fort Matanzas later that year, they again used coquina stone, and, like the Castillo, this smaller fort was never captured.

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What if the Spanish did not have this stone? If not for coquina, perhaps the British would have captured St. Augustine much earlier than 1763, when they finally gained Florida by treaty. If the British gained Florida earlier, it might have changed the course of the American Revolution. Maybe our country would still be a part of Great Britain as a commonwealth like Canada! Our history might have been very different but for this little clam known as donax, and for coquina, the rock that saved St. Augustine.

What is the white stuff on seashells?

Original Forum Question : How do you clean regular sea-shells without bleach? (I like the colors!) DO you just rinse them in water or is there something special you should do?

Bleach is the best all around cleaning solution for shells. It doesn’t harm the colors of the shells. It does remove periostracum, but on a shell with periostracum you can’t see the colors anyway. Still, some serious collectors do like to have some specimenas with periostracum, and such specimens have to be cleaned without bleach. Bleach also removes many kinds of encrusting organisms as well as algae, ordinary mud and dirt, and most importantly, it removes any remaining soft parts of the animal from within the shell, thereby avoiding unpleasant odors. You don’t have to use the bleach full strength. 1 part bleach to 10 parts water is sufficiently strong for most cleaning jobs. But stronger solutions can be used without harm to the shells. There are some exceptions. I don’t use bleach on very thin, translucent shells, or on shells with a nacreous interior like pearl oysters and abalone. If you are nervous about using bleach, try it on a few less desirable specimens until you gain some confidence in the technique. After bleaching, just flush with water inside and out, and dry.

If the whole animal is in the shell, it has to be removed before using bleach. The usual ways of accomplishing this are: cooking — start with room temperature water, put the shells in, bring to near boiling, then cool gradually. Avoid sudden temperature changes, or the shells may crack. Don’t drop them into near boiling water, and don’t remove them from very hot water and immediately rinse them with cold water. After cooking and cooling, the animal can usually be shaken or flushed or picked out of the shell with a narrow sharp instrument.

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freeze/thaw — overnight in the freezer, then thaw at room temperature, after which the animal can be shaken/flushed/picked out of the shell.

microwave — some collectors like this method. I have tried it a few times. It’s fast, and it seems ok for reasonably solid shells. But more fragile shells are likely to crack/break/explode, in my experience. put the shells inside a plastic container to prevent snail parts spattering all over the inside of your microwave.

You may find some more tips in the shell cleaning section of this website (beginners’topic).
(Answer by M. Paul Monfils via the Forum)

Original Forum Question : I recently found some shells on my honeymoon in the Carribean and need some advice on cleaning. The first is a cowrie that I found while snorkling. It is dead and has no meat inside of it, but it is encrusted with hard coral. I have always read that you need to be careful when cleaning cowries, but I have never found any advice on how to clean them. The second is a sea biscuit that was found freshly dead (I had to scrape some of the spines off it). I imagine that it could be put into a bleach solution, but I do not want to bleach it white and lose it’s dark color. Any advice on cleaning/preserving would be greatly appreciated. Thanks in advance.

Yes that cowrie is certainly in need of some cleaning! The problem in removing this kind of calcareous encrustation is that it is composed of the same material as the shell — calcium salts — and therefore any chemical that will dissolve the encrustations will also dissolve the shell. So chemical cleaning methods are out. Only physical methods can be used on something like this. That having been said, it is possible, depending on the type of encrustation, that a pretreatment with bleach will loosen or soften the encrustations somewhat, making subsequent physical methods a bit easier. This is because such encrustations often have a small proteinaceous component, and the bleach will dissolve out the proteins though it won’t dissolve the actual calcium salts. With or without pre-bleaching, physical methods must follow. If you have access to one, I would first try an ultrasonic cleaner, to see how much of the encrustation will be removed by that means. If any of it comes off, continue to use the USC for a longer period of time. After that, if some of the white stuff still remains, patient picking with a sharp tool or scraping with a small sharp blade is about the only way of getting the stuff off, bit by bit. Most likely the gloss of this shell is already damaged, and no matter how carefully you clean it, it won’t look like a live-taken cowrie.

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Now, the sea biscuit — The test (shell) of this animal is actually white. The dark color is due to a layer of microscopic brown spines attached to the test, and associated muscle fibers and other soft tissue. Using bleach removes all the spines and tissues, and leaves just the white test. However, if you want to keep the specimen «natural», it isn’t difficult to do. I usually soak such specimens in formalin for a few days, then alcohol for a few days. But I realize not all people have access to formalin, and also it is smelly and irritating and somewhat toxic. If you do have formalin and know how to use it, I believe that gives the best preservation. However, good results can also be obtained using alcohol alone. In that case, soak the specimen in alcohol for about a week, changing the alcohol at least once, after the first couple of days. Ethyl alcohol is best but isopropyl alcohol is adequate. Avoid methyl alcohol, which is sold as shellac thinner. The alcohol concentration should be at least 70%. «Rubbing alcohol» from the pharmacy works satisfactorily. Once the specimen is preserved in the alcohol, it can be removed, drained, and then dried thoroughly in a warm, dry area. Outdoors is good, weather permitting, but it can be dried indoors too. However, in specimens prepared in this way the spines are not too securely attached to the test, and tend to gradually fall off, especially if the specimen is handled much. So I use an additional procedure to prevent that. I remove the specimen from the alcohol, soak it in water overnight to remove the alcohol, then soak it in 10% Elmer’s Glue-all (10% glue, 90% water) for a couple of hours, then drain and dry. The glue dries invisible, but strenghtens the spine-to-test attachment. Other brands of white glue, like Sobo Glue, are equally good. (Chemically this is polyvinyl acetate). If you use glue, dry the specimen on a non-stick surface like wax paper, so you don’t end up with the specimen glued to whatever surface it is lying on.
(Answer by M. Paul Monfils via the Forum)

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