Cleaning Coins: Alkali Washing with Sodium Sesquicarbonate
Safe and effective restoration of ground-find coins.
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Restoration of an 1890 Indian Head Cent using the Alkali Cold Wash method. Click and hold the vertical bar, slide it right to reveal restoration results.
In this blog post, we will be using a restoration method with a long and successful history in the preservation of ancient copper coins. In general terms, it is referred to as an Alkali Wash. An Alkali Wash can be defined as a category that includes any cleaning agent that is alkaline in nature. Alkali washing is a commonly used method for removing oil, grease, wax, metallic fines, and dirt from the surface of metals. Alkaline agents are the polar opposite of acidic agents on the pH scale, they are far more kind to metal than acids.
The pH scale ranges from 0 to 14. A pH of 7 is considered neutral, it is neither acidic nor alkaline. A pH of 0 is highly acidic while a pH of 14 is highly alkaline. For example, common dish soap is considered neutral with a pH level typically around 7. Conversely, battery acid is very acidic with a pH level close to 0. Even store-bought Lemon Juice is considered highly acidic with a pH level somewhere around 2. It is well-documented that acidic agents can be damaging to metals and more specifically, while they may clean, they do so by removing a layer of surface metal. In my opinion, acids should have no role in coin restorations. The safest pH range to consider when selecting a cleaning agent is somewhere between 7 and 10, I personally would not use any agent outside of that range.
We will be using Sodium Sesquicarbonate as our alkaline agent. It is based on a naturally occurring mineral which is mined as a raw material (Trona) used to manufacture sodium carbonate (soda ash) and other products. Sodium Sesquicarbonate is considered non-toxic and is used as a compound in many products including foods and cosmetics. A 10% solution of Sodium Sesquicarbonate - when combined with distilled water - results in a pH level of around 9.9. For this article, we will be using two solutions, one at 2.5% and the other at 5%. At these reduced solution rates, the pH levels are estimated to fall between 8 and 9.
There are two paths to consider for Alkali Washing: a cold wash treatment or a hot wash treatment. A cold wash treatment is simply the Alkali solution being used at room temperature. With a hot wash treatment, the solution is heated to 140-150 Fahrenheit. I seldom use the hot wash treatment as it tends accelerate the process quicker than my comfort allows. I only recommend using a cold wash treatment as it affords more precise control over how far you take the coin restoration. Furthermore, I would never consider a hot wash treatment on copper alloys like Indian and Lincoln Cents, Nickels, or any coin where copper is the primary metal. They should always be processed cold. Silver or gold alloys may be processed hot or cold.
For mix strength, I rarely use a solution ratio above 2.5% on copper alloys. Although it might take a bit longer to process when compared to a 5% solution, it is one of the safest methods I have found for preserving a patina that may be hidden underneath the ground-find crud. You should always verify a coin’s composition before processing with a solution above 2.5%.
Towards the end of this article, I provide several restoration examples. I think the results speak for themselves. I will also take two coins - one under the 2.5% cold wash and the other using the 5% cold wash - and document each step in the process. These coins are recent ground-finds that are in need of restoration, and I know beforehand that they might be challenging coins with potentially undesirable outcomes. The silver alloy (1940 Mercury Dime) has indications of fire damage while the copper alloy (1873 Indian Head Cent) is significantly underweight, bent, and has not seen the light of day for around 150 years.
As a final word of caution before we begin, Alkali Washing can easily lead to undesirable results if not stopped at the right time. While you cannot actually damage a coin by removing surface metal, you can take restorative work too far. If the coin is a submission candidate, this can make the difference between a straight-grade and a details grade. Practicing on low-value coins will provide you the experience and confidence to take restoration work to the next level.
General Recommendations for Alkali Washing
Know What You Are Up Against
Prior to starting an Alkali treatment, I like to give the coin a thorough review. The first thing I’ll do is weigh the coin to see how it compares to specs. Unfortunately, there is no secret formula to divulge here; wear and the composition of any caked-on material can skew measurements significantly. However, it might help me understand what I’m up against.
For example, an overweight coin - let’s say 10% over specs with tolerance adjustment added - might be hiding a nearly uncirculated gem under the crud. If details are partially visible thru the crud, I can assume the surface material is relatively thin further increasing the possibility of a healthy coin. These factors may warrant a 1.5% solution mix to slow things down a bit. Conversely, assume a coin is 10% underweight and no details are visible. The coin could be heavily worn, there could be significant metal loss thru corrosion, or it might be some combination of both. I’ll still process a coin like this until I can make out a date, mint mark or confirm the existence of a listed error. If potential value is not there, the coin typically gets tossed in the cull bucket.
But more importantly, a visual inspection can give me solid indicators of success or failure. Is bare metal exposed; pinkish or rusty looking little knicks and notches? If so, corrosion has potentially eaten away at the surface metal already and processing might only expose a pitted, bare metal disaster. If I see any bare metal the coin goes directly under the microscope. Those areas are magnified and analyzed. If I see surface pitting the coin is tossed in the cull bin. If I do not see pitting, there is a slight chance the whole coin could be pit free - albeit void of plating or luster - and I’ll still process.
The following 1900 Liberty Nickel is a perfect example. It was found next to a creek below the high-water mark. It saw a lot of freeze/thaws over the years and took a beating. However, there was no indication of pitting on the exposed metal, so the coin was processed. It is not going to win any awards, but it is mostly pit free (Reverse has some pitting) and not worthy of the cull bin.
Restoration of a 1900 Liberty Nickel using 2.5% cold wash treatment. With no signs of pitting on exposed metal, this coin was processed. Unfortunately, some pitting was uncovered on the Reverse.
Recognize Corrosion
Corrosion and copper go hand-in-hand; it is indeed rare that you would ever dig a copper coin without some form of corrosion on it. Without diving too deep into the technicalities, the process of corrosion starts when a new copper coin is exposed to oxygen and the gases it may contain. This oxidizes the copper and forms a film (oxidation layer) on the metal surface. For corrosion to occur, two things must be true: that oxidation layer exists, and the object is exposed to some form of moisture. Corrosion is less likely to occur on gold and silver however, it does happen.
For copper alloys found in the ground, you will primarily encounter Verdigris. It will be green to blue-green and may cover an entire coin or be isolated to specific areas. Have you ever dug up an older Wheat or Indian Cent and thought, what an amazing green patina? Well, that green patina is actually corrosion, a very mild and uniform layer of Verdigris. I personally rank corrosion into three categories. A copper coin may have any combination of these:
Superficial - A very thin flat layer of corrosion that can be removed with a little effort.
Developing - Corrosion that is coalescing in certain areas with blobs or buttons of growth. This usually takes some effort to remove however, it can often be done without damaging surface metal.
Terminal - This is typically well-established corrosion that has had plenty of time to ‘consume’ metal, fatten up, and build some roots below the surface. Terminal corrosion can still be removed however it will usually leave a pit in its place.
Corrosion Examples
Several examples of corrosion at differing levels. Each location that you dig likely has a unique combination of minerals, chemicals, and moisture that determines how corrosion unfolds.
It is sometimes difficult to visually differentiate between Developing and Terminal corrosion. The best indicators I can relay is that Terminal corrosion tends to be brighter in color, shiny, and more rounded, or uniform in shape. It looks healthy and is sometimes as hard as a rock! On the other hand, Developing corrosion has a slightly grainy texture and a more inconsistent coloration and shape to it. While corrosion may not be visible in the beginning, as you progress through the steps recommended below you should always be on the lookout. A few considerations when encountering corrosion:
When pits are evident alongside visible corrosion, the coin is likely a lost cause. I will only continue processing if the coin is a key date or higher value variety.
If the corrosion is stubborn, consider increasing the Alkali solution ratio while shortening the soak time.
Some forms of corrosion (Bronze Disease) can be spread to other copper alloys. Always isolate coins with corrosion.
Process Each Coin Separately
It is best to process each coin in a separate container - from presoaking to Alkali Washing - to avoid possible cross-contamination with unknown chemicals or minerals that may exist in a coin’s surface material. It also provides visual reference with how things are progressing for a specific coin as each phase unfolds. For example, as the Alkali solution is clear in the beginning, any color changes after a period of time can be indicative of progress. I will dive a bit deeper into those color changes later.
For my Alkali Wash container, I like to use BPA-Free Snack Containers with a snap-lock lid. They will fit any U.S. coin and likely, most foreign coins. You could also use 60mm Petri Dishes, small Soy Sauce cups, or something similar. Just ensure that the containers are clear or translucent, and they come with a lid. A lid is imperative as you’ll want to prevent evaporation. If the solution’s water is allowed to evaporate, it will become stronger and may have unintended consequences on your coins.
Have an End Goal in Mind
While you can rarely predict exactly how a restoration will finish, it is vital to know when to stop the process. A few questions to ask yourself: Is the coin a candidate for grading? Is it destined for a holder in your private collection? Do you plan on selling it raw? Is it a rarity? I have learned from experience that answering these questions prior to starting restoration efforts can help you realize the best outcome. With and end goal in mind, a successful strategy can be developed. This may include extending presoak duration, reducing the Alkali solution ratio and/or shortening the soak times, or letting the coin sit for a week between rounds so you can take a fresh look at it. The following images highlight some of my recent experiences from the initial goals for the restoration to the strategy used for processing. The ground find is on the left while the restoration results are on the right.
Gain Experience on Worthless Coins
Hopefully this is a foregone conclusion but, I adamantly recommend starting your Alkali Washing endeavors on worthless coins having differing types of surface buildup. As the composition of surface material can change from one ground find to the next, it is important to understand how an Alkali Wash interacts with different types of encrustations. Imaging each coin before and after a wash phase can prove invaluable. It can help build a reference library that may set expectations on length of time, results, and more so for future restorations. Once you have a solid understanding of how Alkali Washing works, only you can make the choice to use the process on coins that may hold value.
Start With Lower Solution Ratios
The stated solution ratios of 2.5% for copper alloys and 5% for silver alloys are maximums that should seldom be exceeded. It is perfectly acceptable to start the process at lower ratios and step up from there. In fact, I like to start some of my healthy-looking copper alloys at 1% just to see if I get color change after an hour or two. If I do, I’ll continue at 1% and will only increase the ratio if it stalls.
Know When to Move On
Process mistakes will happen and can be overcome. I’ve made an untold number over the years, and with each one I learned, adjusted, and perfected the process further. Just understand that not all coins will respond to treatment. Success with silver alloys will always outweigh their copper counterparts. There are just far too many variables involved to accurately predict how a coin might restore based on visual cues alone. If you encounter an unresponsive coin, set it aside and move on.
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The Alkali Wash Process
Step 1 - Distilled Water Soak
Each and every coin I process gets an initial soak in distilled water; typically, between 24 and 48 hours. Water is the absolute safest coin solvent available. I have found it to be surprisingly effective at breaking apart some of the coin’s organic surface material. You should only use distilled water since tap water has far too many contaminants that may deposit onto the coin’s surface.
Always start with a fresh pour of distilled water.
Frequently agitate the surface material by swirling around or rocking the container back and forth.
Change the distilled water at least once during the soak.
Let the coins air dry before proceeding to the Acetone soak.
Step 2 - Acetone Soak
*** Please familiarize yourself with the safety data sheet on Acetone. It is a highly volatile solvent ***
After the Distilled Water soak, and the coins have been given ample time to air dry, they should be submerged in 100% Pure Acetone for at least 24 hours. They can soak longer as Acetone will only dissolve organic material. You could literally leave your coins submerged for days, weeks, or months without adverse effect.
Always use a fresh pour of Acetone.
A glass container with a tight-fitting lid is essential. Acetone can dissolve most plastics, and it will evaporate quickly if left exposed in the open air.
Keep your container of Acetone and coins in a cool, dark place since light (direct or indirect) may have an adverse effect on the coins.
Never comingle coins. Specifically, never put a silver alloy, or a nickel-plated coin, in Acetone with a copper alloy, or a copper-plated coin.
This phase requires some clarification and specific recommendations. Initially, when I say 100% pure Acetone, I do not mean Nail Polish Remover. Never, and I really do mean never, use Nail Polish Remover. Furthermore, 100% pure Acetone does not necessarily exist. I’d venture to guess the absolute purest lab grade forms of Acetone might come pretty darn close, but it is virtually impossible to manufacture a product at 100% purity. Furthermore, marketing labels are only words, if it says ‘100% Acetone’ they are implying that the contents include 100% of their version of Acetone. You need to consider the actual purity of the Acetone in question.
That said, I approach the Acetone phase differently depending on the coin. For the vast majority of my restoration work, I use store bought Acetone (KleanStrip) that likely has a purity somewhere around 95%. A gallon of KleanStrip cost around $30 and should be available at most hardware, or paint stores. If I am working on a high-value coin, I will only use a lab grade variety with a stated purity of 99.5%. As you might imagine, the cost is significantly higher ($90 per gallon), so it is used sparingly.
After 24 hours (or more), remove the coin from the Acetone and while holding it by the rim, let it air dry. Acetone evaporates quickly, in 5 to 10 seconds all moisture from the coin’s surface should be gone. Personally, I have never had a negative reaction to Acetone on my skin. I always use my uncovered fingers to pick the coin up by the rim. If you plan on using a glove, or rubber-coated forceps when working with Acetone, you must ensure that the material is Acetone resistant.
Before and After Examples - Ground Find After Distilled Water and Acetone Soaks
1918-S Walking Liberty Half Dollar
1883 Indian Cent
Optional - Spot Applications with VERDI-CARE™
I find it important to know exactly what I will be working on prior to choosing a solution ratio. And every so often, a date or mintmark may still be unreadable after the Distilled Water and Acetone soaks. In these scenarios, I will typically spot apply VERDI-CARE™ until they are readable. If I find the coin is a key date, or has high value varieties, I might adjust my solution ratio lower to slow down the Alkali Wash process and give me precise control over the outcome. Spot applications require the utmost patience and, since you’ll be poking and prodding at a small area, you’ll need a steady hand.
If you are not familiar with VERDI-CARE™, I highly recommend adding it to your coin restoration toolset. I have found it to be extremely helpful; from doing spot applications to removing stubborn buildup that an Alkali Wash might not resolve. It is also an excellent coin conditioner, and coin sealer for longer term storage. While their current formula is geared towards general circulation coins, I do know that they are working on a dirt coin version.
VERDI-CARE™ provides guidance for performing spot applications, you can download it here.
My personal preference is to use Bamboo toothpicks for the majority of my spot application work. They tend to last long while maintaining a pointed tip. You should always presoak your toothpick in VERDI-CARE™ or distilled water for 15-20 minutes before use. Only use an up and down tapping motion when attempting to remove buildup. Horizontal scraping motions - like a push or a pull - create the risk of patina damage or a bare metal abrasion (scratch).
For delicate situations - like removing a small deposit surrounded by healthy patina or ‘skin’ - I’ll use a Silicone Gel Dental Floss Pick in a poking or circular motion. These have a fairly robust ball of Silicone on the end that lessens the risk of abrasion. However, a watchful eye is required as the Silicone ball is simply covering a harder plastic core. A failure of the ball could put the hard plastic in contact with your coin.
Step 3 - Mixing the Solution
Making the solution - whether at 1%, 2.5%, 5%, or any other ratio - is straightforward. It is based solely on the combined weight of the sodium sesquicarbonate and distilled water. Distilled water weighs 1 gram per milliliter so an example for a 5% solution would be 5 grams of the agent and 95 milliliters (grams) of distilled water. A 2% solution would be 2 grams of the agent and 98 milliliters (grams) of distilled water. Using 100-gram increments makes the math incredibly easy.
And by no means do you have to be exact with the distilled water, if you are off a few milliliters, it will make little difference. However, if you have 5 grams of agent and only 80 milliliters (grams) of distilled water then your solution is at 5.88% (5/85) which might be stronger than desirable for some coins. Always error on the side of a weaker solution - 5 grams of agent with 110 milliliters (grams) of distilled water creates a weaker solution (4.35%).
Step 4 - Alkali Washing
Immerse in Solution
The steps behind the Alkali washing phase are not complicated. To get started, put enough solution in a container to ensure the coin is well covered, put on the lid, and monitor. If the bulk solution has been sitting for an extended period of time, it should be stirred before use to ensure no settling has occurred.
Monitor Progress
When using an Alkali Wash, monitoring the coins during the soak phase is the key to success. While silver and gold alloys require minimal intervention, I’ll typically never let a copper coin soak longer than 4 hours without inspection. For those new to Alkali Washing, I’d recommended cutting this down to 1-2 hours until you have gained firsthand experience with different types of surface material, and how quickly, or slowly each reacts to treatment.
Furthermore, never leave any coin submerged in the solution for longer than 12 hours straight without a rinse, and a dry-out cycle. Allowing copper coins to completely dry out in between rounds of Alkali Washing is crucial. This is more specific to coins with a higher percentage of copper - like Indian Head or Lincoln Cents - since they might have a patina and a saturated patina could potentially be damaged or removed during handling. Remember that the patina on copper coins acts as a protective barrier.
During inspection, the first thing to look at is the color of the solution. A change in color is indicative of the process working. The color itself is determined by the composition of the encrusted material. Most of the time it will be a shade of green to brown/green. If the solution is clear to mildly colored, it is suitable to continue processing the coin in the same solution. If it is darker, it is best to swap the solution out for a fresh batch. If the solution has not changed color after a 8-hour period, you can assume that the Alkali Wash has run its course, and the coin should be considered complete and removed.
When working with patinated coins, it is important to consider how healthy any exposed patina looks before continuing with the process. For example, if the patina is only partially exposed and within the exposed areas you see bare metal, you do not have a healthy patina. The coin is likely headed for the cull bucket as further processing could lift more patina. When you encounter an unhealthy patina, immediately terminate the current wash phase and reduce solution strength by 50%.
Patina is healthy.
Patina is not healthy.
Agitate Surface: Swirl, Dab, Brush, and Flip Coins Frequently
While not required, I have found that agitating the surface material during a soak can accelerate its removal. Before you agitate, make sure to let your coins soak for a while first so the solution can penetrate. I use a natural bristle artist paint brush for agitation. Specifically, one made of camelhair. You can also use a horsehair brush. Both have ultra-soft bristles that will not leave scratches on the coin’s surface. That said, they are somewhat more expensive when compared to other, synthetic options. Personally, I would never consider nylon or polyester to be a safe alternative. For those hesitant to use a brush, you can simply swirl the solution and coin around in the container. Something akin to stirring cream into a cup of coffee without a spoon. The video that follows provides an example of surface agitation using a camelhair brush.
Ghost Scratches
One potential issue when working with silver coins is the emergence of micro scratches as you progress between phases. This might rear its ugly head as you start to break through any encrustation and get closer to the oxidation layer. The first few times I encountered these abrasions I retraced my steps over and over. I never swiped the coin with my thumb, did I? A cloth? Did someone in the past actually clean this coin?
While never a certainty, in the few dozen times I’ve experienced this phenomenon the abrasions have disappeared in subsequent wash rounds. I’ve concluded that soil type, freeze thaw cycles, and the continuous movement of the coin while underground is the most likely culprit. As each year passes while an object is buried, a new layer of buildup is deposited on the coin. If the host soil is abrasive enough, friction from movement will cause micro scratches to form on that year’s layer. They typically vanish when the offending layer is removed.
This can present a dilemma when they crop up, or still exist, and you are at a desirable stopping point. It becomes a dammed if you do, dammed if you don’t moment. If the coin is destined for your private collection, then just take it to a point that is visually appealing to you. However, any coin destined for grading becomes a challenge. If the abrasions are visually distracting to you, rest assured that a coin grader will consider them to be far worse. Conversely, if you decide to move forward with another round and subsequent results are not optimal, there is no turning back. Alkali Washing - along with any other coin restoration method - is a one-way street. Choose your speed and stopping points wisely. The example that follows offers a perfect representation of the abrasions I refer to, and the outcome that is possible.
Abrasions appeared on this coin after the 2nd wash cycle. Visually you can associate the scratches with a layer that rests under the newer encrustation layer. From past experiences I was confident the abrasions would fade. In the final restoration results, there is not any evidence that the abrasions ever existed. A welcome outcome given this 1936 Mercury Dime is a DDO.
Wrapping Up a Cycle
So, you finished a wash cycle, what’s next? The steps that follow should be executed in the order shown. I consider them mandatory for my restoration efforts. Deviation may result in a final restoration that just might have been a little better.
Distilled Water Bath
Always use a fresh pour of distilled water. You don’t need a lot and distilled water is cheap. Remember, an Alkali Wash is based on salts, and they need to be removed. If the solution is allowed to dry in place, it may leave a white film of salt residue on your coin. You can use your fingertips or coin tweezers to grab the coin by the rim and submerge. Swirl it around in the distilled water for 20 to 30 seconds. Remove and shake off any excess water, there is no need to dry it.
Acetone Bath
I consider the post-Alkali Wash bath a critical step for two reasons: it will remove any remaining solution salts and moisture, and it can eliminate additional organics exposed during the most recent Alkali Wash phase. Similar to distilled water, you absolutely want a fresh pour of Acetone. Place the coin in your glass container of Acetone and let it soak for at least 30 minutes. After 30 minutes, remove it from Acetone and while holding it by the rim, let it air dry.
Dry Out (Copper Alloys with Patina)
When working with patinated coins it is imperative to have a complete dry out period between wash cycles. The same holds true when the restoration work is complete. I suggest at least 24 hours for wash cycles and up to a week of dry out upon completion. I like to put my completed coppers on a windowsill with a lot of sun. The coin will often darken and sometimes, certain areas may darken quicker than others. Do not be alarmed if you experience uneven darkening, it will typically become uniform by the end of a week. Do not encapsulate or restrict airflow until that week has passed.
This 1910-S Lincoln Wheat Cent highlights the effects of uneven darkening on patinated copper. The image on the right was taken 24-hours after the image on the left.
Reference Images
If at all possible, I highly recommend taking images of your project in between each round and on completion. It provides a visual record of progress between rounds, and the starting and ending conditions. If you don’t see progress between rounds, then you should consider stopping the process. As you restore more ground finds and build a larger reference library, you can start matching new restoration candidates with prior work and leverage, or modify, successful strategies used in the past.
Inspection
Probably the most important step albeit the one I can provide the least amount of guidance on. Refer back to the section on having an end goal. What I can impart is, if you think a candidate is close to complete but want to squeeze just a little more out of it, don’t. Set it aside for a while and forget about it. Revisit once the excitement has waned and you can look at it with a fresh perspective. The biggest gotcha with coin restorations is that you can never reverse the process. It is always better to fall two steps short as opposed to one step over.
Round of VERDI-CARE™
Sometimes I’ll do a round of VERDI-CARE™ in between Alkali Wash phases. Especially when Alkali refuses to address stubborn areas. The 1890 Indian Head Cent from our introduction to Alkali Washing is a perfect example. Additional Alkali Washes absolutely refused to penetrate and lift the remaining Obverse deposits from K-8 to K-10. A 30-minute spot application of VERDI-CARE™ lifted the vast majority of that area successfully. It completed the restoration of the coin.
Make it stand out
Make it stand out
Repeat Alkali Wash or Complete
Make the decision whether to continue processing or complete your restoration. Consider adjustments like stronger or weaker solutions or, shorter or longer soak times. Don’t forget to let your patinated projects dry out between phases. And again, on completion all copper alloys should get an open-air sunbath for at least a week. A windowsill is a great resting place.
If the coin is destined for your collection, an application of VERDI-CARE™ or a coat of Microcrystalline Wax is recommended for added protection. Any coin destined for grading should get another soak in fresh Acetone for at least 24-hours. This can help remove any lingering indication of restoration efforts. Grading companies are notoriously good at identifying ‘cleaned’ coins and leaving residual chemicals on the surface only makes their job easier.
Additional Restoration Examples
My results are not indicative of any results that you may experience with Alkali Washing. I have highlighted successful restoration examples however, for every success there are corresponding failures. You should fully expect failures as you progress into the realm of coin restorations. Embrace the failures, learn from the failures, and realize that each coin is unique, and the final results may be directly impacted by the decisions you make along the way.
Restorations of silver alloys tend to have the best results. That said, you may uncover hidden damage that was not visible prior. This 1900-S Barber Dime conserved wonderfully however, a gouge was exposed on the Reverse. It is barely visible in the starting image.
The Obverse of this 1929-D Walking Liberty Half Dollar was shown before. I purposefully saved the Reverse to highlight just how effective Alkali Washing can be. A layman may look at this Reverse and consider the coin a lost cause that is not worthy of restoration. Not so I say, the results speak for themselves.
Any patina on a copper alloy is a form of corrosion. However, it is superficial and ultimately acts as a protective barrier to prevent further corrosion. I caught this uncirculated 1907 Indian at an interesting time. The red-brown colorations are original surface areas, you can see how they are being overtaken with an appealing green patina.
Alloys are just that, alloys! By no means are the steps discussed specific to coins. Alkali Washing of a Sterling Silver Ring and a Bronze WW1 U.S. Army Collar Disc are shown above.
The Test Cases
1873 Indian Head Cent
A challenging coin from the start, this Indian conserved poorly on the Obverse but surprisingly well on the Reverse. A slighter weaker solution from the onset may have helped but ultimately, I believe the warp (Obverse convex, Reverse concave) contributed. Missing patina on Obverse corresponds with most of the convex high areas. When metal is bent (ignoring annealing), the atoms have to move. In this example, the atoms on the convex side (Obverse) are spread further apart while on the concave side, they are pressed closer together. The metal’s density on the Obverse is weaker than the Reverse thereby making it more susceptible to patina loss. That said, it remains the oldest Indian I have ever found with a metal detector so a sealer coat of VERDI-CARE™ and into a capsule it goes. I’m happy to add it to my collection!
1940 Mercury Dime
More likely than not, you have already encountered fire-damaged coins when metal detecting. If not, I can almost guarantee that you will at some point. In my opinion, fire-damaged silver alloys are often the most difficult restoration candidates since heat adds another dimension to your efforts. Silver has the ability to absorb oxygen at high temperatures and conduct the oxygen to the interior of the metal itself. In this scenario, the oxygen can then bond with the copper atoms present causing a deep, black fire scale to occur. This fire scale can be very difficult to remove.
When unearthed, both the Obverse and Reverse showed indications of fire damage. It was not until a Distilled Water and Acetone phases that the fire damage was confirmed. While the coin ultimately conserved fairly well both Round 1 and 2 were extended soaks. Each was processed around 36 hours which is longer than desirable, but an acceptable timeframe for silver alloys. Take note of the second set of images as they represent the telltale signs of fire damage.
