Written by the TreasureGuide for the exclusive use of the Treasure Beaches Report.
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| BEFORE AFTER Monogram Side of Lima Half Real Before and After Cleaning with Acid. |
In an earlier post I discussed using a tumbler to clean coins. Today I'll discuss using Muriatic acid and white vinegar to clean coins.
When found, the above half-reale was completely covered on one side (the side shown in the photo) with a very hard calcareous crust. The other side was very different. The side not sown above, was covered with a sand crust that was easily removed.
The calcareous crust, although difficult to remove, actually protected the surface details of the coin from corrosion over the hundreds of years on the beach.
The hard crust was removed by soaking the cob in Muriatic acid, which is one common method used on silver reales. I didn't take the cleaning as far as is commonly done. You can see that it is still black.
I've posted this reale before, and I also posted the following excellent instructions as provided by Bill P. before. Today I'll add some additional comments.
Bill's Silver Cob Cleaning Method
Use this method at your own risk. This writer accepts no responsibility for misuse or ignorance or neglect for any person or party and for informational purposes only.
This process uses diluted Muriatic Acid which is commonly used around swimming pools. It can be purchased pretty much anywhere pool chemicals are sold.
A few precautions must be addressed first though. Here is a link to Bob Vila's Info on this acid https://www.bobvila.com/articles/muriatic-acid/ .
As stated in the Bob Vila article, I cannot emphasize enough that water should NEVER be poured into acid to dilute it. The reverse is the only way it should be done...acid into water. This acronym should be in the forefront of your thoughts when using any acid...AAA - Always Add Acid!
How to neutralize acid; baking soda neutralizes acid very quickly so make sure you have enough on hand.
Muriatic acid is readily available at most home improvement stores in ½ and 1 gallon sizes. It is typically a 31% strength from the bottle. I generally cut this in half to roughly 15% strength and it seems to work perfectly but you may want to experiment (again, AAA).
I suggest that you treat one cob at a time. I use a shot glass so not much acid is required. Place the appropriate amount of acid into the shot glass and gently place the cob into the container (no splashing please). Depending on how heavily it's encrusted, it should start fizzing immediately. It may take several treatments to dissolve the encrustation but in my experience once is enough.
Remove the cob from the solution and if you only have one cob, neutralize the acid in the glass and on the cob at once. The cob will have a dull gray color.
Next take a small amount of baking soda (not baking powder), place it in the palm of your hand, add a couple drops of water to make a paste. Then gently rub the cob between your fingers until a bright silver sheen is revealed. It's that simple.
In the case of stubborn encrustation there is another method that works although not as reliable as the acid treatment. However it is much safer because there are no caustic chemicals involved.
You will need baking soda, aluminum foil and hot tap water. Make a paste with the baking soda and water, fold a small piece of foil in half, fill the folded foil with the paste and sandwich the cob in between. The whole thing may become warm as the aluminum and baking soda react. There also may be some fizzing. Once this is complete, you polish the cob as previously stated.
It is my belief that using electrolysis can damage a cob. Years ago I watched a video when the early 1715 salvors were attempting to clean clumps of encrusted cobs. They dumped the clumps into a tub and poured muriatic acid over all of it. I never saw the finished product I'm sure it was effective.
You may want to try this first on a common silver coin instead on a cob...for your own peace of mind.
Silver and copper are insoluble in dilute Muriatic acid at room temperature.
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I don't usually remove all the dark silver sulphate, and I don't usually make my cobs bright and shiny. Rubbing a coin with a baking soda past will shine the high spots while leaving dirt or a darker surface in the low areas. I like just a little of that contrast to bring out the features. Below is an example of a four-reale.that was cleaned that way.
I used a microscope to view the cob.
You can see a few specs of sand remaining as well as the contrasting brightness of the high and low surfaces. There are a lot of interesting features to be seen when you use an amount of magnification.
The aluminum foil method works very well. I've used it too.
Lately I've been using white vinegar (5% acid) instead of Muriatic as my first choice. The vinegar is safer for people and furniture. I once got Muriatic acid on a dining room chair. Not good. You should use Muriatic acid outdoors anyhow. It can give off fumes.
Remove surface dirt, encrustation or corrosion physically first as much as you can safely do that. Removing surface crust first will speed up the chemical process, but it will also enable you to get a better idea of what you are really dealing with. You will learn about the metal as well as the type and amount of cleaning that needs to be done. Extreme care must be taken to avoid damaging the coin when removing crust by force.
I have seen found coins with a crust that was completely impermeable to the acid. The crust had to be physically removed before acid cleaning was possible. The crust retained the negative image of the coin when removed.
Any of these methods will work. Select the one that is most appropriate for the object being cleaned and your goals and circumstances. There is no hurry. You can avoid many mistakes by proceeding carefully and slowly. If you stop too soon and are not happy with the coin, you can always continue, but if you go too far and make a mistake, you might not be able to recover.
You might experiment with coins you don't care too much about and gain confidence in your chosen method before applying it to your cherished finds. For your better examples, if you don't feel that you have enough experience, you can get your coin professionally cleaned.
If you are going to sell your cobs, you will probably want them looking bright and shiny. Augie of Sedwick Coins told me that buyers prefer them that way.
Sometimes you start out hoping for a Marie Osmond type of face lift but end up with more of a Nancy Pelosi type of effect. Some cobs are simply nicer to start with.
In sharp contrast to this post about very old dug coins, when it comes to modern collector coins, cleaning coins in any way is discouraged. Never clean those kinds of coins. It is important to know when and when not to clean coins.
See Treasure Beaches Report: Pt. 2. (2020 and Beyond). : 5/22/23 Report - Cleaning Coins and Three Silver Treasure Coast Reales From Different Centuries. Tropical Storm Time.
And this post provides some additional hints for encrusted coins. The Treasure Beaches Report Direct From Florida's Treasure Coast.: 1/21/16 Report - Preparing and Cleaning Encrusted Coins. Another Front Going To Come Through.
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On the Treasure Coast there is still a small surf. There will also be a decent late afternoon low tide.
There was some tropical development in the Yucatan, but nothing that would affect the Treasure Coast.
Good hunting,
TreasureGuide@comcast.net