The unremovable stains on your knife — are they good, bad or is it rust kicking in? This article will break down the patina on your knife for you.
What is Patina on a metal blade and how does it form?
Naturally, Iron is a subject to rusting. This is the inevitable demise of every iron alloy, detail, part or tool — given the time will eventually rust away, and this process can even become quicker if iron bacteria is present. There are different approaches to solve rusting, such as high Chromium content, but patina and subsequent patinization is one of those ways to keep the rust away.
Patina forms naturally, over time. To put it simply — patina is the additional layer of protection on your knife. The original term refers to the finish, which appears on the surface of wooden, bronze, brass, copper, tin and iron items. This layer, however, has nothing to do with rusting. Patina will form over time on high-carbon blades during use, indicating their wear and age.
Photo from kitchenknifeforums.com
Have you ever wondered how you protect high-carbon steel blades from rust? Well, Patina is one of the answers to this question. After formation of this thin protection layer, the high-carbon blade becomes protected from the oxygen molecules in the air, which helps to ward off rusting. With age and use, eventually, most of the high-carbon blades form a patina layer.
If you’ve been wondering how patina can be used aside from protecting the blade — here’s a picture of Minneapolis City Hall copper roof covered in green patina.
Photo from wikiwand.com
Is patina bad for a knife? No, it does not damage the blade, but patina can be mistaken for rust. Common wisdom here is simple — if the stain on your blade has orange dots anywhere, that’s a hidden rusting and you should polish it off or use rust-removing compounds.
What makes steel to form a patina? High-carbon content in steels are known to be responsible for the patina formation at the surface of the blade. Over time, contact with oxygen, water, organic fluids and other substances make carbon form a patina at the surface level. Patina layer is thin, micrometers away from the steel surface and protects it from corrosion.
Stainless steels are protected from patina. Since patina is the form of corrosion in play, knives made from stainless steel can’t form this coating. Instead, they are protected by the layer of Chromium-Oxide at the surface level, along with Nickel additives which prevent rusting altogether. This is not a silver bullet, because even the most resilient stainless steels can rust, given enough time, moisture and bacteria.
Can you force a patina on stainless steel? There is a way — you can try submerging the blade in Ferric Chloride (or PCB Etchant), but it won’t look the same as patina on high-carbon steel. Moreover, this can make steel more susceptible to corrosion and rusting. The way to do this is to put the blade into solution for 2 minutes, but you should cover critical areas beforehand with nail polish or similar solution.
How long to wait until the high-carbon knife forms a patina? Since carbon-rich steels are often delicate in chemical nature, they form patina relatively quickly — under a week or two. Patinization can be started even from touching the blade, as your skin oils can react with carbon atoms in the metal.
There are two opinion camps on patina. Some find it aesthetically pleasing for a blade to have a patina, referring to it as the aging mark of the knife. Others argue that it looks “ugly” or “messy”, while preferring stain-resistant steels more. The bottom line is that patina is used by antiquity collectors to determine age and value of the blade; this, of course, means they are able to determine whether patina was forced or started to form naturally.
What purpose does patina serve? This thin film at the surface of the blade protects it from corroding, while subsequently contributing to the visual appeal of the «ye’ olde» blade. Patina layer can differ in color from the main item, but mostly stays «in check» with the original shade of metal your blade has.
Can you force a patina to appear on a knife? To form a patina, one of the following substances should be present: acids, salts or moisture. Since there’s nothing stopping you from covering your BPS Knives with patina yourself, you can always try to enforce it at home. This isn’t necessary, however, as with time and use your high-carbon knives are going to form this natural layer on their own. Yet, you can always try several methods to force patina on your knife:
- The “quick” protection — put a knife into a Coca-Cola or similar carbon-rich drink; Take it out after the patina is formed;
- Cut through a really juicy meat; This will leave more vibrant and pleasing patina pattern for a blade;
- Put your knife into a vinegar and wait for the patina layer to form; Alternatively, soak a paper towel in vinegar, wrap it around the blade for 10 minutes and clean off the blade afterwards;
- Slice a bunch of apples and leave the juice on the blade; Likewise, you can also try onions, lemon, grapes, orange or any other fruit with high acidity; The onion method gives of «ringular» shaped patterns, and you have to soak knife in it for 24 hours;
- Soak the blade in mustard; It is said that mustard patina is among the best of them all. In certain cases, it can give off the “tactical” like yellow or light-brown pattern along the blade surface.
Photo from instructables.com
Before treating knife to a patina:
- Clean the blade; You need to polish the blade clean with fibrous or paper towel;
- Prepare container for patination solution; Every regular plastic container will do, but if you’re looking for something really acidic it’s better to use glass;
- Make sure you cut out some paper print blanks to leave unique stains;
The older the blade — the more patina it would have. Old, almost ancient, blades will always have some sort of patina on them. Even some of your long-term storage tools might form this layer over time.
Can you remove the patina from a knife? Yes, you can grind it away by using any conventional sandstone, whetstone or sandpaper. Just be sure to keep in check with grind grit, you want #1000 to #2000 grits on a stone or paper for best polishing results. Keep in mind that after you grind off the protective layer of the high-carbon blade, keep it away from moisture for a while.
Stay Sharp.