Use of word "patina" by knife enthusiasts

[TenGrainBread]

Is anyone else mildly frustrated by the haphazard and imprecise use of words like "patina" and "rust" in the knife enthusiast world? I'll grant that, all things considered, it doesn't matter too much. And I'll grant that I'm being a grouch. But I'll go on anyway :D

Patina - The appearance of age on the surface of an object. Iron, steel, copper, leather, wood, skin, rocks, etc... All can have "patina". It's not a technical term or synonym for a specific type of surface finish.

Rust - Red iron oxide. Fe2O3. It forms on the surface of ferrous metals (metals containing iron) when oxygen reacts with the iron, converting the iron molecules to iron oxide.

There are other types of iron oxides. The most common in the industrial world is black iron oxide. Unlike Fe2O3, this oxide is Fe2O4, aka magnetite. The extra oxygen atom changes the size of the oxide.

Red oxide can also be corrosive. In many cases the molecules of rust are larger than the neighboring iron molecules, causing the oxides to break off. That's is what's happening when rust causes pitting or flaking corrosion. If the conditions for the conversion to this iron oxide remain, the oxygen will continue to react with iron and convert it to iron oxide until the base metal is gone. Ambient conditions like the presence of moisture and heat can affect the speed and nature of this chemical reaction.

Black oxide can be used to protect ferrous metals from further oxidation. Black oxide molecules will not flake or pit, and can be retained on the surface of the metal to protect the unconverted iron underneath from oxygen and moisture. And red oxide (rust) can be converted to black oxide using various natural or chemical treatments (like Naval Jelly, aka phosphoric acid). There are various types of black oxide treatments, including gun bluing.

Stainless steels resist the formation of iron oxides because there are other metals alloyed in them (the most common of which is chromium) that form their own non-corrosive oxides on the layer of the steel, protecting the iron underneath from forming its own oxides. These protective oxides are often formed at a thickness that makes them invisible to the naked eye. They also form much faster than iron oxides when exposed to oxygen - the chromium oxide layer that protects your stainless steel regenerates much more quickly after you do some sanding or grinding on your stainless steel than any corrosive iron oxide would.

To bring this full circle, the colors you see on your carbon steel or tool steel blades from sweat, moisture, heat, etc... appear because iron oxides are starting to form on the surface of your blade. The layer of oxide refracts the light, causing coloration. The thickness of the oxide layer decides how the light is refracted and therefore what colors you see. This is all the result of oxidation, including but not limited to rust. That's why it irks me when people say stuff like "Oh, that's patina, not rust". When it comes to steel, patina is oxidation, of which rust is a type - patina is a parent category for rust, not a separate type of finish.

[Abyss_Fish]

I see your point, but what annoys me more is people use the word patina in the knife world referring exclusively to rust. Wish it applied further, as my Caribbean is developing quite the patina at work :D

Also I’m assuming this confusion described is because most people’s first encounter of the word patina is in relation to bronze patina in sculpture and such. So people assume that any coating on a metal caused by the elements is a patina. Separate from rust, which is more likely associated with like, car rust, and therefore neglect, in people’s minds. It’s a problem of connotation I suppose.

[skeeg11]

Patina is the popular word to use when you don't want to admit you don't take care of your stuff and let it rust. :p

[Pokey]

I’ve heard a patina on blades is a form of black rust. Here’s a link to a company with descriptions and photos of rust. https://www.armorvci.com/corrosion/types-of-rust/ (They didn’t use subscripts on their website for the make up of the elements. So, Fe304, for instance, is really Fe₃O₄.)

I’ve included a photo of a PM2. Does it have a patina, or is it rusted?

[TenGrainBread]

I’ve heard a patina on blades is a form of black rust. Here’s a link to a company with descriptions and photos of rust. https://www.armorvci.com/corrosion/types-of-rust/ (They didn’t use subscripts on their website for the make up of the elements.)

I’ve included a photo of a PM2. Does it have a patina, or is it rusted?

Interesting article.
Your knife looks like it has a thin layer of Fe2O3 (red iron oxide) beginning to form. It's got a sort of reddish tint to it. That's not what is known as black oxide, but rather rust.

[Pokey]

Patina is the popular word to use when you don't want to admit you don't take care of your stuff and let it rust. :p

Well yeah, I have a '73 LandCruiser with typical 'Cruiser "patina."

[Pokey]

I’ve heard a patina on blades is a form of black rust. Here’s a link to a company with descriptions and photos of rust. https://www.armorvci.com/corrosion/types-of-rust/ (They didn’t use subscripts on their website for the make up of the elements.)

I’ve included a photo of a PM2. Does it have a patina, or is it rusted?

Interesting article.
Your knife looks like it has a thin layer of Fe2O3 (red iron oxide) beginning to form. It's got a sort of reddish tint to it. That's not what is known as black oxide, but rather rust.

The colors change as the light hits it from a different angle. It's mostly gray with hints of pastel blues and greens. (This is beginning to sound like the Home & Garden channel. :D )

This is actually from cutting onions, apples, avocados and tomatoes. It's formed even after using the knife, then washing it with dish soap and drying it immediately. The interesting thing is it changes with time; different patterns show up, then after the next time it's used it changes. I think the acids in the food come into play here, too.

[MotoBro]

I get kinda annoyed when I see the term “forced patina”. Generally they’re just referring to acid washing/etching the blade, which in my opinion is intentionally damaging the blade. I understand it creates an appealing look for some people and that it only removes a very thin layer of steel from the surface so it probably won’t hurt the performance. But to me it just seems like a bad idea unless it came from the factory that way.

I’m all for modding your knives the way you see fit, after all it’s your money and you can do what you want with it. But as someone who buys a lot of knives secondhand from people on Reddit & eBay, I always avoid anything that’s been acid washed/etched or any type of “forced patina”.

[ChrisinHove]

I’ve heard a patina on blades is a form of black rust. Here’s a link to a company with descriptions and photos of rust. https://www.armorvci.com/corrosion/types-of-rust/ (They didn’t use subscripts on their website for the make up of the elements. So, Fe304, for instance, is really Fe₃O₄.)

I’ve included a photo of a PM2. Does it have a patina, or is it rusted?

Great link! That makes sense.

[araneae]

A forced patina in reference to cutlery steels is forced oxidation. The mingling of colloquial terms and scientific ones is the issue. Rust and patina are both descriptions of oxidation. "Rust" is colloquially used to describe ferric oxide, the reddish oxidation we see on steel.

The statement that "When it comes to steel, patina is oxidation, of which rust is a type - patina is a parent category for rust, not a separate type of finish" would be more accurate to say "Patina and rust are both descriptors of oxidation, resulting from a redox reaction of the blade steel. Patina is usually a reference to a lightly oxidized finish, while rust generally refers to the reddish corrosion associated with pitting and advanced damage."

A forced patina can help prevent pitting from excess oxidation which most people would call rust. I have Opinels with a forced mustard patina that have been around over a decade with little to no oiling of the blades and no rust and zero pitting have appeared. Same on my super blue Caly 3. Leave some corrosive agent (fruit juice for example) on that blade and the oxidation will advance to a rusty appearance and pitting usually results. I have a number of Old Timer knives as old as myself with both patina and rust pits, the previous owners weren't careful with the blades.

In some cases, I would prefer light oxidation in the form of a patina to the possibility of pitting. Plus I like how a patina looks. You can attempt to force a patina, or let one develop over time, but letting it happen on its own can also result in pitting if you aren't careful.

[Blnd]

Patina is the popular word to use when you don't want to admit you don't take care of your stuff and let it rust. :p

The difference between tool polishers and tool users.

[The Deacon]

Nope, it doesn't bother me at all. Most of the knife related posts I've seen where the word "patina" is used, it is being used to mean a coating of oxidation that is not "rust" on the blade of a knife. Said coating may occur naturally, as the result of cutting certain things, or be induced by the knife's owner. Only places I can recall seeing patina used to describe rust, the objects being referenced were either guns or motor vehicles.

But who am I to judge, I still get annoyed when manufacturers call cars with convertible tops, windows that roll up, and outside door handles "roadsters".

[Bolster]

But I'll go on anyway :D

Glad you did...good info...I learned something today!

[The Meat man]

Doesn't bother me either. When people talk of rust or patina, they are speaking of two different things. I understand what they mean.

A forced patina in reference to cutlery steels is forced oxidation. The mingling of colloquial terms and scientific ones is the issue. Rust and patina are both descriptions of oxidation. "Rust" is colloquially used to describe ferric oxide, the reddish oxidation we see on steel.

The statement that "When it comes to steel, patina is oxidation, of which rust is a type - patina is a parent category for rust, not a separate type of finish" would be more accurate to say "Patina and rust are both descriptors of oxidation, resulting from a redox reaction of the blade steel. Patina is usually a reference to a lightly oxidized finish, while rust generally refers to the reddish corrosion associated with pitting and advanced damage."

A forced patina can help prevent pitting from excess oxidation which most people would call rust. I have Opinels with a forced mustard patina that have been around over a decade with little to no oiling of the blades and no rust and zero pitting have appeared. Same on my super blue Caly 3. Leave some corrosive agent (fruit juice for example) on that blade and the oxidation will advance to a rusty appearance and pitting usually results. I have a number of Old Timer knives as old as myself with both patina and rust pits, the previous owners weren't careful with the blades.

In some cases, I would prefer light oxidation in the form of a patina to the possibility of pitting. Plus I like how a patina looks. You can attempt to force a patina, or let one develop over time, but letting it happen on its own can also result in pitting if you aren't careful.

Quoted for truth.

[jpm2]

I've always known patina as the change of surface and appearance with time and exposure, and sometime wear. I think the term has mostly lost its meaning with knife people.
I don't consider a blade purposely dipped in vinegar or any other substance to change its appearance, as patina.

I think patina can include rust, but the reverse may or may not be correct.

[soulspy]

This all reminds me of a Modest Mouse lyric, "so rust is a fire and our blood oxidizes..."