How to Properly Store and Preserve the 2012-Mo 100 Pesos Southeast Railway Numismatic Heritage Coin: A Conservationist’s Guide to Toning, Oxidation, PVC Damage, and the Great Cleaning Debate

I’ve seen far too many valuable pieces ruined by careless cleaning or neglect. If you own the 2012-Mo 100 Pesos Southeast Railway Numismatic Heritage coin — or any significant commemorative issue — you already know the stakes. Here’s how to keep it safe for the next generation.

As a conservationist who has spent decades examining, grading, and preserving numismatic treasures from around the world, I can tell you that the single greatest threat to a coin’s long-term value and historical integrity is not time itself — it is us. The well-intentioned collector who reaches for a bottle of jewelry polish. The hobbyist who tucks a rare commemorative into a flimsy plastic flip bought at a flea market. The investor who stores a prized piece in a damp basement. These are the real enemies of numismatic heritage. And when it comes to a coin as historically significant and visually striking as the 2012-Mo 100 Pesos Southeast Railway Numismatic Heritage issue from Mexico, the stakes for proper preservation are especially high.

This commemorative piece, struck at the Mexico City mint (mint mark “Mo”), celebrates the rich railway history of southeastern Mexico — a tradition that stretches back to the original 1950 Ferrocarril (Railroad) Peso and even earlier experimental patterns like the 1947 5 Pesos pattern, one of which is known to exist in only a single example graded at Mint State 62. The 2012 issue is a modern tribute to that legacy, and it deserves to be treated with the same reverence and care that a conservationist would afford to any artifact of cultural importance.

In this guide, I’ll walk you through everything I’ve learned about preserving this specific coin and others like it. We’ll cover the science of toning and oxidation, the insidious threat of PVC damage, how to choose the proper holders, and — perhaps most controversially — whether you should ever clean a coin at all. Whether you’re a seasoned collector who just received a fresh grade on your 2012-Mo or a newcomer who has just acquired your first commemorative issue, this article will give you the actionable knowledge you need to protect your investment and honor the history embedded in every ounce of metal.

Understanding Toning: The Beautiful Enemy

One of the first things you’ll notice about the 2012-Mo 100 Pesos Southeast Railway coin — especially if you’ve seen the images posted by collectors in online forums — is the toning. Toning is the gradual change in color that occurs on a coin’s surface as a result of chemical reactions between the metal and its environment. On silver and silver-alloy coins, this can manifest as anything from a light, golden-yellow hue to deep, iridescent blues, purples, and magentas that collectors call “rainbow toning.”

Why Toning Happens

Toning is essentially a thin layer of metal sulfide that forms on the coin’s surface. The specific colors depend on the thickness of this layer and the wavelength of light reflected through it — a phenomenon known as thin-film interference. Factors that accelerate toning include:

• Sulfur compounds in the air — often from cardboard, paper, or certain plastics used in older coin holders
• Humidity and moisture — which speed up chemical reactions on the metal surface
• Proximity to other reactive materials — rubber bands, certain fabrics, and even the oils from human skin
• Temperature fluctuations — which can cause condensation and accelerate oxidation

Natural Toning vs. Artificial Toning

Here’s where the conservationist in me must be blunt: not all toning is created equal. Natural toning that develops slowly over decades or centuries is generally considered desirable and can even enhance a coin’s eye appeal and market value. Artificial toning — achieved by exposing a coin to chemicals, heat, or fumes to simulate age — is considered damage by virtually every major grading service, including PCGS and NGC.

When I examine a toned coin, I look for the following characteristics of natural toning:

• Gradual color transitions — natural toning shifts slowly from one hue to another, rather than appearing in sharp, unnatural bands
• Even distribution — while not perfectly uniform, natural toning tends to follow the contours of the coin’s design
• Patina consistency — the surface should feel smooth, not sticky, powdery, or raised
• Historical plausibility — a 2012-dated coin should not display the deep, multi-layered toning patterns typically seen on 19th-century silver dollars

Actionable takeaway: If your 2012-Mo 100 Pesos is beginning to tone, don’t panic. Light, attractive toning is a natural part of the coin’s aging process and can actually make your piece more unique and desirable. However, if you suspect artificial toning — perhaps the coin was stored near a sulfur source or treated by a previous owner — consult a professional grader before making any decisions.

Oxidation: The Silent Destroyer

While toning is a surface-level phenomenon that can sometimes be aesthetically pleasing, oxidation is a more serious concern — particularly for coins stored in suboptimal conditions. Oxidation occurs when metal atoms lose electrons to oxygen or other reactive elements in the environment, forming metal oxides that can eat into the coin’s surface over time.

How Oxidation Differs from Toning

It’s important to distinguish between the two. Toning is a surface reaction that creates a thin, often colorful film. Oxidation, when it progresses beyond the surface layer, can cause pitting, corrosion, and permanent loss of detail. On the 2012-Mo 100 Pesos, which features intricate design elements celebrating the Southeast Railway — including locomotive imagery, regional maps, and commemorative text — even minor oxidation can obscure fine details that are critical to the coin’s grade and historical narrative.

Environmental Factors That Accelerate Oxidation

In my experience grading and conserving coins from tropical and subtropical regions — including many Mexican issues — I’ve found that the following environmental factors are the most common culprits:

1. High humidity — Relative humidity above 60% dramatically increases the rate of oxidation on silver and copper-nickel alloys
2. Salt air — Collectors living near coastlines face an additional threat from chloride ions in sea air, which are highly corrosive to most coinage metals
3. Pollutants — Industrial pollutants, including sulfur dioxide and nitrogen oxides, can react with coin surfaces to form corrosive acids
4. Improper handling — Fingerprints contain oils, salts, and acids that can initiate localized oxidation spots, sometimes called “fingerprint corrosion”

Preventing Oxidation on Your 2012-Mo 100 Pesos

The good news is that oxidation is largely preventable with proper storage and handling. Here are my top recommendations:

• Control humidity — Store your coins in an environment with relative humidity between 30% and 50%. Use silica gel packets or a dehumidifier in your storage area
• Avoid direct contact — Always handle coins by the edges, and consider wearing nitrile gloves when examining high-grade pieces
• Use inert storage materials — More on this in the section on proper holders below
• Inspect regularly — Check your collection at least twice a year for early signs of oxidation, such as dark spots, white powdery deposits, or a dull, hazy appearance

PVC Damage: The Collector’s Nightmare

If there’s one topic that makes every conservationist’s blood run cold, it’s PVC damage. Polyvinyl chloride (PVC) was once the most common material used in coin flips, albums, and storage pages. It’s cheap, flexible, and transparent — seemingly ideal for displaying and protecting coins. But PVC contains chemical stabilizers that, over time, break down and release chlorine gas. This gas reacts with the metal surface of coins to form a sticky, greenish film known as “PVC residue” or “PVC damage.”

How to Identify PVC Damage

PVC damage is one of the most commonly encountered problems I see in coins that have been stored in older holders. On the 2012-Mo 100 Pesos, PVC damage might appear as:

• A greenish or bluish-green sticky film on the coin’s surface, often most pronounced in the recessed areas of the design
• A hazy, cloudy appearance that cannot be wiped away with a dry cloth
• An acrid, plastic-like odor when the coin is removed from its holder
• Surface pitting in advanced cases, where the chlorine has actually begun to corrode the metal

What makes PVC damage so insidious is that it is cumulative and largely irreversible. Once the chemical reaction has begun, it will continue to progress even after the coin is removed from the PVC holder. The green residue can sometimes be removed with a careful application of acetone (never rub or wipe — more on this in the cleaning section), but the microscopic pitting it leaves behind is permanent and will affect the coin’s grade and numismatic value.

How to Avoid PVC Damage

The solution is simple: never store your coins in PVC-containing holders. Here’s what to look for and what to avoid:

• AVOID: Soft, flexible vinyl flips and pages — these almost always contain PVC
• AVOID: Older albums and folders from the 1960s through the 1990s — many of these were manufactured with PVC-based plastics
• USE INSTEAD: Mylar (polyethylene terephthalate) flips — these are chemically inert and will not damage your coins
• USE INSTEAD: Archival-quality cardboard holders with Mylar windows — the standard for professional numismatic storage
• USE INSTEAD: Hard plastic capsules made from acrylic or polycarbonate — ideal for individual high-value pieces

Actionable takeaway: If you’ve recently acquired a 2012-Mo 100 Pesos that was stored in an older holder, inspect it immediately for signs of PVC damage. If you detect any greenish residue or a plastic odor, remove the coin from the holder right away and place it in an inert Mylar flip or hard capsule. The sooner you act, the better your chances of preventing further damage.

Choosing the Proper Holders: A Conservationist’s Toolkit

Now that we’ve covered what to avoid, let me walk you through the storage options I recommend for the 2012-Mo 100 Pesos and similar commemorative issues. The right holder does more than just protect the coin from physical damage — it creates a stable microclimate that minimizes exposure to humidity, pollutants, and harmful chemicals.

Individual Coin Storage

For individual coins, especially those that have been graded or that you plan to have graded, I recommend the following options in order of preference:

1. PCGS or NGC certified slabs — If your coin has been professionally graded, the slab itself is one of the best storage solutions available. These hard plastic holders are chemically inert, airtight, and provide excellent protection against physical damage. The images posted by collectors in forums — showing their freshly graded 2012-Mo 100 Pesos in official slabs — are a testament to how well these holders preserve a coin’s condition and mint luster
2. Air-tight acrylic capsules — For ungraded coins, air-tight capsules from manufacturers like Leuchtturm, Lighthouse, or Capital Plastics provide excellent protection. Look for capsules with foam inserts that hold the coin securely without touching the surface
3. Mylar flips (2×2 format) — These are the workhorse of numismatic storage. They are inexpensive, chemically inert, and allow you to label the coin with relevant information (date, mint mark, grade, provenance). I recommend the 2×2 size for the 100 Pesos, which is a larger commemorative issue
4. Cardboard 2x2s with Mylar windows — A step up from flips, these provide a rigid frame that protects the coin from bending and creasing. They are ideal for coins stored in bulk boxes

Bulk and Display Storage

If you’re building a collection that includes the 2012-Mo 100 Pesos alongside related issues — such as the original 1950 Ferrocarril Peso or the rare 1947 5 Pesos pattern — you’ll need a system for organizing and displaying your holdings. Here are my recommendations:

• Archival-quality coin albums — Look for albums specifically labeled “PVC-free” or “archival quality.” Brands like Whitman, H.E. Harris, and Dansco offer PVC-free options with Mylar-lined pages
• Coin trays and boxes — For certified slabs, use the standard plastic trays designed to hold PCGS or NGC holders. These stack neatly and allow you to organize your collection by date, mint, or theme
• Display cases with UV-filtering glass — If you want to display your Southeast Railway collection, use a case with UV-filtering glass or acrylic to prevent light-induced toning and fading

Environmental Controls for Your Storage Space

Even the best holders cannot fully compensate for a poor storage environment. Here are the environmental controls I recommend for serious collectors:

• Temperature: Maintain a stable temperature between 65°F and 75°F (18°C to 24°C). Avoid attics, garages, and basements where temperatures fluctuate wildly
• Humidity: As mentioned earlier, keep relative humidity between 30% and 50%. Use a hygrometer to monitor conditions
• Light: Store coins in a dark environment. Prolonged exposure to direct sunlight or fluorescent lighting can accelerate toning and discoloration
• Air quality: If you live in an area with high pollution or near the coast, consider using an air purifier in your storage room

To Clean or Not to Clean: The Great Numismatic Debate

This is the question I’m asked more than any other, and it’s the one that generates the most passionate disagreement among collectors, dealers, and conservationists. The short answer is: do not clean your coins. The longer answer is more nuanced, and it’s worth exploring in detail — especially for a coin like the 2012-Mo 100 Pesos, which may accumulate dust, fingerprints, or minor surface contamination over time.

Why Cleaning Is Almost Always a Bad Idea

When you clean a coin, you are removing metal from its surface. Even the gentlest cleaning methods — distilled water, acetone, or a mild soap solution — can disturb the original surface of the coin, leaving microscopic scratches, hairlines, or an unnatural sheen that experienced graders can detect immediately. The major grading services (PCGS, NGC, ANACS) will downgrade or even refuse to grade coins that show evidence of cleaning, often labeling them as “Cleaned,” “Improperly Cleaned,” or “Altered Surface.”

I have personally examined thousands of coins that were “improved” by well-meaning owners, and I can tell you that the vast majority were made worse by the attempt. A coin that might have graded MS-65 in its original state can drop to AU-58 or lower after cleaning — a difference that can represent hundreds or even thousands of dollars in market value and collectibility.

When Professional Conservation May Be Appropriate

There are rare exceptions where professional conservation — not cleaning, but conservation — may be warranted. These include:

• Removal of PVC residue — As mentioned earlier, PVC damage is progressive and will continue to harm the coin if not addressed. A professional conservator can carefully remove PVC residue using acetone immersion (not wiping or rubbing) without damaging the underlying metal surface
• Stabilization of active corrosion — If a coin is actively corroding (for example, “bronze disease” on copper-based coins), a conservator may be able to halt the process using specialized chemical treatments
• Removal of harmful contaminants — In cases where a coin has been exposed to acidic or caustic substances, professional intervention may be necessary to prevent further damage

Note the distinction: conservation is not cleaning. Conservation aims to stabilize the coin and prevent further deterioration, not to make it look “shiny” or “new.” A good conservator will never attempt to restore a coin to its original mint luster — that luster, once gone, is gone forever.

What You Can Do at Home (Safely)

If your 2012-Mo 100 Pesos has accumulated dust or light surface debris, here are the only at-home methods I consider safe:

1. Gentle rinse with distilled water — Hold the coin by its edges and rinse it under a slow stream of distilled water. Do not rub or wipe. Allow it to air dry on a soft, lint-free cloth
2. Acetone immersion for PVC residue — If you have confirmed PVC damage, you can immerse the coin in pure acetone (not nail polish remover, which contains additives) for a few minutes. Allow it to air dry. Do not rub the surface
3. Proper handling technique — Always hold coins by the edges, over a soft surface (in case you drop them), and avoid touching the obverse or reverse surfaces

What you should NEVER do:

• Do not use commercial coin cleaners, polishes, or dips (such as EZest, Penny Brite, or similar products)
• Do not rub the coin with a cloth, tissue, or your fingers
• Do not use baking soda, vinegar, lemon juice, or any household cleaning product
• Do not use ultrasonic cleaners — these can cause micro-fractures and disturb the surface
• Do not attempt to “improve” the coin’s appearance in any way before submitting it for grading

Building a Complete Southeast Railway Collection: Preservation Across Generations

The 2012-Mo 100 Pesos Southeast Railway Numismatic Heritage coin does not exist in isolation. It is part of a rich numismatic tradition that includes the original 1950 Ferrocarril Peso — the coin that forum collectors have called “a real Ferrocarril” — and the extraordinarily rare 1947 5 Pesos pattern, of which only a single example is known to exist. If you are building a collection around this theme, you will need to apply the same preservation principles across all of your pieces, regardless of their age or metal composition.

Special Considerations for Older Issues

The 1950 Ferrocarril Peso, being a silver coin that is now over 70 years old, may have already developed significant toning, wear, or environmental damage. When acquiring older pieces:

• Inspect for PVC damage — Older coins are more likely to have been stored in PVC holders at some point in their history
• Check for prior cleaning — Look for unnatural brightness, hairlines, or a “washed out” appearance that may indicate past cleaning attempts
• Assess the toning — Determine whether the toning is natural and attractive, or whether it is masking underlying problems
• Verify authenticity — For rare issues like the 1947 pattern, authentication by a reputable grading service is essential to establishing provenance and protecting your investment

Documenting Your Collection

As a conservationist, I cannot overstate the importance of documentation. For each coin in your Southeast Railway collection, I recommend maintaining a record that includes:

• Date of acquisition and source (dealer, auction, private sale)
• Grade and certification number (if professionally graded)
• Photographs — both obverse and reverse, taken in consistent lighting conditions
• Storage history — what type of holder the coin was in when you acquired it, and what you have transferred it to
• Condition notes — any observations about toning, oxidation, or other surface characteristics

This documentation serves two purposes: it helps you track the condition of your collection over time, and it provides valuable provenance information for future owners — whether they are your heirs, a dealer, or a museum.

Conclusion: Preserving History, One Coin at a Time

The 2012-Mo 100 Pesos Southeast Railway Numismatic Heritage coin is more than a collectible — it is a tangible piece of Mexican industrial and cultural history. It commemorates the railways that transformed southeastern Mexico, connecting remote communities and fueling economic growth across the region. When you hold this coin, you are holding a piece of that story, and you have a responsibility to preserve it for the next generation.

The principles I’ve outlined in this guide — understanding toning, preventing oxidation, avoiding PVC damage, choosing proper holders, and resisting the urge to clean — are not complicated, but they require discipline and consistency. The collectors I’ve seen who do the best job of preserving their collections are not necessarily the ones with the biggest budgets; they are the ones who take the time to educate themselves, invest in quality storage materials, and treat every coin in their care as a historical artifact worthy of respect.

Whether your collection consists of a single 2012-Mo 100 Pesos or a comprehensive assemblage that spans from the 1947 pattern to the present day, the preservation advice in this article will help you protect your investment and honor the numismatic heritage that these coins represent. Handle them with care, store them with intention, and pass them on to the next generation in better condition than you found them. That is the conservationist’s creed, and it is the highest compliment any collector can pay to the history embedded in their coins.

Related Resources

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