Putting your seed phrase on metal is a popular choice when combating the elements of time and mother nature. It's strong, durable, and resistant to moisture and heat and psychologically, it's not as likely to be thrown away.
Choosing a Metal
When choosing a metal, you want a metal than can withstand the heat of an unexpected event like a house fire (over 1200°F) and have relatively good corrosion resistance in the event of some sort of environmental exposure.
Corrosion is the deterioration of the material due to a chemical reaction between the metal and its environment. One of the most common forms is iron oxide. The iron present in the metal interacts with the oxygen in water or air to create iron oxide or that brownish-red stuff better known as rust.
Stainless steel (300 series) is a good option and is a great balance between cost and durability. Stainless steels are composed of at least 10.5% chromium. Chromium reacts with the oxygen in the air to create a regenerative protective layer (chromium oxide) thats prevents the underlying metal from corroding (even after getting damaged). Stainless steel has a melting points well above 1200 °F (2750 °F).
You also want to consider the thickness of the metal you're using. "Gauge" is a standard of thickness for metal. The lower the number, the thicker the plate.
Obviously, the thicker the metal (lower the gauge) the harder it is to bend. But, you also want a thick metal to ensure the markings don't show through the other side. For example, something over 0.10" or 2.5 mm thickness should be sufficient for this.
Types of Seed Phrase Storage
Metal seed phrase storage comes in typically two flavors:
- Permanent (One time use)
Each work a bit differently and have their unique nuances.
Note: Only metal seed storage that requires the user inputting their seed phrase data in an offline, physical environment will be discussed. Methods that rely on a third party to "store" or engrave your seed for you results in substantial risk and exposure of your assets.
Permanent Seed Storage
This type of seed storage physically deforms the metal and cannot be undone. If you decide to utilize a new seed phrase, you'll need to secure discard your old metal seed storage and make a new one.
This is usually done utilizing a hammer and letters stamps to stamp your seed phrase onto a sheet or plate of metal.
For the best impression, you want a flat face hammer and stamps that are made of a harder material than the metal your stamping on.
If you're having trouble with getting a good marking, the issue is that most likely are running into is the metal of the plate and metal of the stamps are similar hardness.
Hardness is a measure of the ability of a metal to resist permanent deformation. If you smash two objects together, the softer object will be damaged/deformed by the harder object (with the harder object relatively unscathed).
You want this. You want deformation in the plate your stamping in (since you want to make a permanent mark). You do not want deformation in the tool as it means all the force you’re using is going into damaging the tool, not the plate.
Thus, your tool should be of a harder material and your plate should be of a softer material. If done correctly, stamping leaves a deep marking such that many surface layers must be removed to be illegible.
Etching is also a popular option. This includes utilizing a handheld engraver/etching pen to write your seed phrase on metal. Again you want the tip of the etcher/engraver to be of a harder material than the plate.
But compared to stamping, handheld etching typically cannot get the depths that stamping can provide. Etching creates a shallow mark that may not be as durable.
The most ideal etching/engraving (in terms of impression durability) would be to utilize proper tooling such as laser engraving or CNC machining to get a deep mark but that would risk exposing your seed to a third party or machine with saved data.
Reusable Seed Storage
Reusable seed storage relies on utilizing pre-engraved/stamped single letter tiles and inserting them into a capsule, case, or container to spell out each of the words in your seed phrase.
Since the tiles aren’t permanently affixed, you can use this style seed storage again and again if you decide to utilize another seed phrase.
The reliability of this type of seed storage dependent on
- the metal that the container and tiles are made of
- the impression quality of the markings
- the structural integrity of the container
If the container is compromised and the tiles are able to become loose, you're a bit out of luck in terms of recovering your seed phrase. So, if you decide to choose this type of seed phrase storage, ensure it has been structurally tested.
In terms of seed phrase storage, you want to keep it simple. The more pieces and moving parts something has, the more points for failure. You want something that's reliable and will last (like any good tool).
You also want to avoid seed storage that relies on some sort of "encoding".
There are some devices that do some do some encryption, code, or letter/number conversion so if someone finds it, they wouldn't understand how to "crack their code" or know what it's for. Again, this introduces complexity that is another potential point of failure. Remember, the biggest security risk is not other people, it's yourself. You are more likely to not remember what you did and lose access.
Also keep in mind, the “cooler” your seed storage looks, the more likely it’ll be seen as interesting enough to ask about or valuable enough to take. There’s a difference between clean (legible) vs fancy.
For commercial options, Jameson Lopp reviews many seed storage devices and tests them for crush, heat, and corrosion resistance.