Why Metal Seed Phrase Backups Matter
Paper is the default medium for recording a seed phrase during wallet setup. It works, until it doesn’t. A fully developed house fire can exceed 600°C (1,100°F), and paper ignites at around 230°C (450°F). A burst pipe or basement flood turns paper to pulp. Even slow degradation from humidity, sunlight, or simple aging can render handwritten words illegible over a span of years. For a backup that must remain readable for decades, paper is an inadequate material.
Metal seed backups solve this by recording your recovery words in stainless steel or titanium. Stainless steel only starts to melt at around 1,400°C (2,550°F) and titanium at 1,668°C, above the roughly 1,000°C (1,800°F) of a room fully on fire, and neither runs nor smudges. Indoors, neither rusts, though salt from sea air, road salt or wet ground can still pit stainless steel. The metal is only half of the job, though: the backup also has to keep your letters in place, and that is where these two products differ most. When your seed phrase is the sole backup for your Bitcoin holdings, the physical medium carrying those words needs to be at least as durable as the cryptography protecting them.
The Cryptosteel Capsule and the Billfodl are two of the best-known metal backups. Both hold loose letter tiles in a stainless steel case, and both cost around $100 or less. They differ in shape, capacity and ease of use and, most of all, in how they held up in independent stress tests.
Cryptosteel Capsule: Design and Specifications
The Cryptosteel Capsule is a cylindrical stainless steel container that stores seed phrase characters on individual metal tiles threaded onto a steel core. It is made by Cryptosteel, a company registered in Manchester, UK, whose original Cassette was one of the first dedicated metal backups on the market. Cryptosteel now sells the Capsule as the Seed24, which it describes as “essentially the same product as the regular Capsule”: the same capacity, dimensions and materials, plus a core that detaches from the shell and holes for tamper-evident seals. The stress-test results below are for the original Capsule.
Material: Stainless steel. Cryptosteel calls it “space-grade” and says it uses AISI 303 for the tube and AISI 304 for the core and letter tiles.
Design: The capsule is cylindrical, roughly the size of a thick marker. Metal tiles, each marked with a single letter or number, slide onto a central steel core. Separator tiles divide words. The core is fixed to a threaded cap: it slides into the capsule body, and the cap screws shut. The closed capsule is compact and can be stored in a safe or hidden in tight spaces.
Capacity: Up to 128 characters. That is enough for the first four letters of each word in a 24-word BIP39 seed phrase (96 letters, plus separators between the words). Since BIP39 words are uniquely identifiable by their first four letters, storing the full word is unnecessary. Any room left can hold a short wallet label. Never put your passphrase there: it belongs in a different place from the words.
Assembly: Setting up a Capsule is a manual process. You pick the right letter tiles from the set, slide them onto the core one at a time, insert separators between words, and close the capsule. Plan for 30 to 60 minutes. The tiles are small, so work on a tray with good lighting. Tile order on the core must match word order in the phrase; an error means sliding tiles off and starting that section again.
Price: The Seed24 Security Kit lists at $80 and was on sale for $50 when we checked cryptosteel.com on 7 November 2026.
Billfodl: Design and Specifications
The Billfodl, made by Privacy Pros LLC, takes a different approach. It is a flat stainless steel tray with rails on both faces. Letter tiles slide into the rails, 12 words on each side, each word in a numbered position and four words to a rail. A gate that you unlock with a flathead screwdriver keeps the tiles in, and a cover plate swings over them on a pivot. That is how the unit Jameson Lopp tested in 2018 looks in his photos; the maker’s page, checked on 7 November 2026, also lists 12 words on each side.
Material: 316-grade stainless steel (also known as marine grade). Grade 316 adds molybdenum to the chromium-nickel alloy, which improves its resistance to chloride corrosion (saltwater, road salt, chemicals). Like other stainless steels, it only starts to melt at around 1,400°C. The tiles are laser-engraved stainless steel.
Design: The flat form fits alongside documents in a safe or safe deposit box. The cover plate protects the tiles when closed. Swing it aside and the words sit in numbered positions, so the phrase reads like it does on paper: left to right, top to bottom.
Capacity: 96 characters: the first 4 letters of up to 24 words. There is no spare capacity. Each slot is fixed to a word position, which removes any doubt about where one word ends and the next begins.
Assembly: You unlock the gate with a flathead screwdriver and slide the tiles into the rails from the side; the numbered positions make it straightforward to check the phrase afterwards. Plan for 20 to 45 minutes. The tiles are easy to handle, though the rails can be tight.
Price: $99 (billfodl.com, checked 7 November 2026).
Side-by-Side Comparison
| Feature | Cryptosteel Capsule (now Seed24) | Billfodl |
|---|---|---|
| Material | 303 and 304 stainless steel | 316 marine-grade stainless steel |
| Form factor | Cylindrical (marker-sized) | Flat tray with a swing-aside cover plate |
| Character capacity | Up to 128 characters | 96 characters (first 4 letters x 24 words) |
| Tile type | Steel tiles threaded on a core | Laser-engraved tiles slid into rails |
| Assembly time | 30-60 minutes | 20-45 minutes |
| Ease of verification | Must unscrew and slide the core out to read | Swing the cover plate aside and read the tiles directly |
| Lopp heat test (torch flame, hotter than a typical house fire) | A: every tile stayed in place | C: tray warped, letters fell out |
| Lopp acid test | A: no data lost | A: no data lost |
| Lopp crush test (20-ton press) | B: tube bent and cap jammed, tiles intact inside | D: nearly all letters fell out |
| Lopp overall grade | A− | C |
| Price (7 Nov 2026) | $80 list as Seed24 ($50 on sale) | $99 |
| Maker | Cryptosteel (UK) | Privacy Pros LLC |
| Spare capacity | Some room beyond a 24-word seed, for a label | No |
| Tamper evidence | Seed24 includes numbered tamper-evident seals | None built in |
Durability Testing: What the Stress Tests Show
Jameson Lopp, a Bitcoin security researcher, has stress-tested about 75 metal backups and publishes a grade for each test. His method today: a propane torch flame of about 1,093°C (2,000°F) for up to 10 minutes, a little hotter than a typical house fire, then a plunge into water; up to 12 hours in 16% muriatic (hydrochloric) acid; and a 20-ton hydraulic press. Each product was tested in its own round. The Billfodl was in his first, published in July 2018: more than 20 minutes under a butane torch that can reach 1,371°C (2,500°F), then half an hour in acid. The Capsule was in his third, in December 2019: about 1,093°C for up to 10 minutes, then three hours in acid. The two products came out very differently:
- Heat. The Capsule “performed admirably”: every tile stayed in place, and Lopp could still unscrew the cap by hand (grade A). The Billfodl’s steel did not melt, but “the heat managed to warp the tray enough to loosen the letters and a bunch of them fell out”. Lopp counted that as “a catastrophic failure” (grade C).
- Crushing. The Capsule held for several tons, then bent under the full 20. Lopp judged the data inside intact but did not open it. The cap no longer unscrewed, so reading it would mean cutting the tube open and sliding the core out without letting tiles fall off (grade B). The Billfodl deformed only slightly, yet letters fell out, and by the time Lopp pried it open “pretty much all of the letters had fallen out” (grade D).
- Acid. Neither lost anything. No acid got into the Capsule, and the Billfodl was unaffected (grade A for both).
Overall, Lopp grades the Capsule A− and the Billfodl C. In neither case did the steel melt. What failed was the part of the design that holds the letters, and that is the thing to look at in any metal backup. It is also why punched plates, where each letter is a dent in the steel, score best (see below). Grades are from Lopp’s reviews, checked on 7 November 2026; the Billfodl was tested in 2018 and the Capsule before it became the Seed24, so later versions of either may differ.
These tests show why the choice of backup medium is a serious seed phrase storage decision: not all metal backups are created equal, and construction details matter as much as the material grade.
Other Metal Backup Alternatives Worth Considering
Several other products did as well as the Capsule or better in Lopp’s tests:
Seedplate (by Coinkite). A 2 mm steel plate, cut and etched in Canada, printed with a grid of letters. You mark the first four letters of each word with a center punch, 12 words per side. There are no tiles to fall out, and it works with any BIP39 wallet, not only Coinkite’s Coldcard. Lopp graded it A: no data lost in the heat, and 20 tons bent it only a few millimetres. Reading it back means noting which letter each dot sits on, which is slower than reading tiles. $59.97 for the 24-word plate and $39.97 for the 12-word Tiny 12, both on sale, plus $14.99 for Coinkite’s automatic center punch (store.coinkite.com, checked 7 November 2026).
Blockplate. The same punched-grid idea on a thicker 304 stainless steel plate. No tiles, no moving parts, nothing to assemble, and Lopp graded it A with no data lost in any test. Its online store was unavailable when we checked on 7 November 2026, and the 24-word plate was already listed as sold out in January 2026, so check before you plan around it.
CryptoTag Zeus. Two 6 mm titanium plates (titanium melts at 1,668°C / 3,034°F). Instead of letters, you punch each word’s number on the BIP39 list, from 0001 for “abandon” to 2048 for “zoo”, and look the words up again when you restore. Lopp graded it A. Punching is loud and permanent, so mistakes are hard to correct. €149 for the starter kit (cryptotag.io, checked 7 November 2026).
All of these products serve the same purpose: recording your seed phrase in a medium that will outlast paper by decades and survive disasters. For guidance on how metal backups fit into a broader self-custody architecture, our overview of cold storage and risk management covers the complete picture.
Which One Should You Choose?
The right choice depends on your specific circumstances:
Choose the Cryptosteel Capsule (Seed24) if you want letter tiles. Of these two, it is the one that kept every tile through Lopp’s heat test. The compact cylinder fits hiding spots a flat case can’t, it holds up to 128 characters, and the Seed24 comes with numbered tamper-evident seals. A hard enough blow can jam the cap, so keep it somewhere it won’t be crushed, such as a safe.
The Billfodl is the easier of the two to fill and to check: swing the cover plate aside and read the tiles. But surviving a fire is the main reason to buy a metal backup, and in Lopp’s tests the Billfodl lost letters in the heat and nearly all of them in the press. We don’t recommend it for a new backup. If you already use one, add a second complete copy of your words on a punched plate or a Capsule, kept in a different place.
Choose CryptoTag Zeus if maximum physical durability is your top priority and you are willing to pay more for thick titanium and punched numbers. Best for long-term, set-and-forget storage.
Choose a punched plate (Seedplate or Blockplate) if you want the best result for the money. Both scored A in every Lopp test, with nothing to assemble and no loose parts. Seedplate is the one you can order today.
Regardless of which product you choose, the metal backup is only one component of a complete security strategy. It must be stored in a physically secure location with controlled access. For high-value holdings, consider using metal backups as part of a multisignature setup where the compromise of any single backup does not compromise your funds. And for those thinking about what happens to these backups across decades, the Bitcoin Inheritance Planning lesson covers the planning that metal alone cannot solve.
Protecting Your Metal Backup from Physical Threats
A metal seed backup resists fire and water, but it does not resist theft. Anyone who finds your Cryptosteel or Billfodl and can read the tiles has full access to your Bitcoin. Physical security of the backup is as important as its material durability.
Store metal backups inside a safe with a fire rating of at least one hour, bolted down if you can, so it can’t simply be carried away. A bank safe deposit box is another option, though it introduces counterparty risk and access limitations during bank closures or legal proceedings.
For additional protection, consider using a BIP39 passphrase in conjunction with your metal backup. Even if the 24 words engraved on your Cryptosteel or Billfodl are discovered, the attacker still needs the passphrase to access the wallet containing your primary funds. This layered approach — something you have (the metal backup) plus something you know (the passphrase) — significantly raises the bar for physical attackers. Keep the passphrase’s own backup in a different place from the words. The mechanics of this approach are explained in our guide on wallet passphrases and recovery best practices.
Frequently Asked Questions
Can a house fire destroy a Cryptosteel Capsule or Billfodl?
The steel survives: stainless steel only starts to melt at around 1,400°C (2,550°F), while a room fully on fire reaches about 1,000°C (1,800°F) and NIST measured 1,200°C (2,200°F) in one full-scale house-fire test. What can fail first is the way the letters are held. In Jameson Lopp’s torch tests, both hotter than a typical house fire, the Cryptosteel Capsule kept every tile in place, while the Billfodl’s tray warped and a number of letters fell out, which he counted as a catastrophic failure. If fire is your main worry, choose the Capsule or a punched plate over the Billfodl. For context on how these backups fit your broader security model, see our custody security guide.
Do I need to store all 24 words, or just the first 4 letters of each?
The first 4 letters of each word are sufficient. The BIP39 word list was designed so that every word is uniquely identifiable by its first four characters — no two words share the same four-letter prefix. Both the Cryptosteel and Billfodl are designed around this principle. Storing full words would require significantly more capacity with no security or recoverability benefit.
Is 304 or 316 stainless steel actually better for seed phrase storage?
For a backup kept indoors, in a safe or a dry room, both grades last a lifetime. Grade 316 adds molybdenum, which resists salt and chemicals better than 304, so it has an edge only if you bury your backup or keep it near the sea. In Jameson Lopp’s acid baths, neither the Capsule nor the Billfodl lost anything. The Billfodl’s weakness in his tests was heat and crushing, not corrosion, so the steel grade matters less than how a product holds its letters.
Should I buy multiple metal backups for redundancy?
Creating a second metal backup and storing it at a geographically separate location protects against localized disasters (fire, flood, theft at one location). However, every copy of your seed phrase is a potential attack vector — if anyone finds any copy, they can access your funds. The decision to create redundant backups should weigh disaster risk against theft risk for your specific situation. For high-value holdings, a multisignature wallet setup may be a better approach to redundancy than multiple copies of the same seed. Our analysis of hardware diversity in multisig security covers how to distribute trust without multiplying single points of failure.
