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Many knives are produced with a blackened blade, while others blacken or darken over time. The main reasons to darken a knife blade are for protection and aesthetics. How is this blackening process achieved on the steel? We will explain some proven methods how to blacken your knife blade yourself.
Blackening a knife blade can have advantages for carbon steel to protect it from harmful oxidation such as rust. Black blades also have an aesthetic appeal for many people. Most of these home blackening processes do not work well on stainless steel.
- Ferric chloride
- Citric acid
There are many methods you can try to achieve a blackened coating on your knife blade. Some methods work well, while others produce mediocre results. We have detailed some of the more effective, proven methods to blacken a knife blade.
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What Is The Black Coating On Knife Blades?
The first step to blackening a knife blade is understanding what the black coating is on the knife blade and why it is desirable.
Oxidation is normally a process associated with corrosive rust on steel and something that we should take steps to protect our knives from.
The interesting aspect is that not all oxidation on steel is corrosive or detrimental to the metal. In some cases, oxidation can be a protective, non-corrosive coating that prevents damage to the steel from other forms of oxidation.
This makes a black coating desirable for protection on certain steel types, but this is not the only reason blades are blackened.
Another reason knife blades are blackened is for aesthetic purposes. Blackened knife blades can be visually appealing and can add to the stealth aspect of the knife.
Blackened blades are often used in military roles to add to the low visibility requirement for a soldier’s equipment and protect the blade in typically harsh environments.
Why Did My Knife Turn Black?
If a knife is not intentionally blackened, it is possible for the knife to blacken or darken over time. This is generally referred to as a patina, and it occurs over a period of time as the knife is used.
A patina is a form of oxidation caused by cutting acidic foods, which results in a reaction with the steel overtime to give the knife blade a blackened or stained appearance.
Many people try to wash or scrub this patina from the knife blade, unaware that it acts as a protective layer on the steel, helping to prevent rust from forming.
Patina will occur on carbon steel knives rather than stainless steel knives. Stainless steel is resistant to the oxidation process responsible for producing the natural patina on a carbon steel blade.
A stainless steel blade will not develop a natural patina due to its alloy composition, but it does not need the same level of protection from rust since stainless steel is resistant to this type of oxidation as well.
TIP: Is patina a good thing or a bad thing on a knife, and should you force a patina or take steps to prevent it? Find out in-depth explanation in the article below:
Patina On A Knife: Prevent Or Force It? In-Depth Explanation
How To Blacken A Knife Blade?
Several methods force the oxidation process to blacken the knife blade, some of which involve heating the blade. We will not cover the methods involving heat since these processes can adversely affect the temper of the steel and effectively compromise the integrity of the steel.
The following methods are the top proven methods for blackening a steel blade and giving the metal a protective and visually appealing oxidation coating.
1. Using Ferric Chloride To Blacken A Knife
Ferric chloride is a chemical that many knife makers use to blacken knife blades. It is common for knife makers to soak the steel in the ferric chloride for a prescribed time to darken the blade.
The length of time it takes to blacken the blade with ferric chloride will depend on the strength of the solution and the duration the steel spends in the substance.
Ferric chloride can be purchased at most knife makers’ supply stores but must be handled with care. It is an acidic chemical that can burn your skin and eyes if the proper precautions are not adhered to.
The acid in the ferric chloride reacts with the surface of the steel, causing the steel to oxidize and turn black during the process.
Different types of steel react differently to the acid requiring varying times to produce results for each steel type. It is best to perform multiple soakings to establish the required depth of oxidation rather than a single long soak.
The knife blade must be cleaned with a solvent such as acetone to remove any fingerprints and oil spots to ensure an even etch in the acid. The steel will not etch or darken where oil residue remains on the steel.
You can leave the steel in the ferric chloride in the solution too long, in which case, the acid will begin to damage the steel. The acid must be neutralized on the knife blade after each soaking with an alkaline substance such as baking soda and a thorough rinse with clean water.
Stainless steel can be darkened using ferric chloride, but the steel must be left in the solution much longer than carbon steel to produce the darkening effect. The acid may damage the knife blade depending on the stainless steel composition.
We recommended that you do not try this option at home on stainless steel knives since the result may be unpredictable. Rather purchase a stainless steel knife with a pre-blackened blade from the supplier.
TIP: Part of knife maintenance is protecting your knives from corrosion, which will extend the knife’s lifespan. Check out the best oils for knives in the article below:
These Are The 2 Best Oils For Kitchen Knives (+ How to Use)
2. Using Citric Acid To Blacken A Knife
Citric acid is a common household substance that is often used in baking. It is sold in powder form for use in the kitchen.
Mix 1 tablespoon of citric acid powder in a cup with hot water. Stir the solution until all the citric acid has dissolved.
Ensure the cup or glass is tall enough to stand the knife blade in the solution to completely cover the area of the blade you want to be darkened. Be aware that some handle materials, such as wood, can be damaged when submerged in the acidic solution.
Before the blade is placed in the solution, thoroughly clean the blade with acetone to remove any oil spots from the steel.
Stand the blade in the solution and wait. You will see bubbles start to develop on the surface of the steel, and the blade will begin to darken. Take the knife out periodically to rinse it off and check the level of blackness. Submerge the blade for longer to get a darker result.
3. Can You Blacken Knife Steel with Vinegar?
Vinegar is a safe household item that can also be used to blacken a knife blade. There are a variety of methods to use vinegar, and we will detail one of the simplest methods.
Use acetone to thoroughly clean the knife blade and remove any oil spots and fingerprints. The best vinegar to use for this process is white spirit vinegar.
Pour enough vinegar into a glass and stand the knife blade down in the glass. Ensure the vinegar only comes into contact with the steel and not the knife’s handle material, which the vinegar could damage.
You will see bubbles develop on the surface of the steel as the oxidation process begins. Leave the knife in the solution long enough till you can see it begin to darken.
Remove the knife, rinse it off with water, and check the oxidation level. If you want a darker finish, repeat the process until the desired level of darkness is achieved. Rub the blade with some baking soda to neutralize the vinegar, rinse under clean water, and dry the knife off.
Stainless steel can be darkened with vinegar, but it will not go as dark as carbon steel. It also takes much longer for stainless steel to darken in the vinegar.
BTW: If you want to know more about Japanese and other types of knives and their sharpening check out the books listed above. These books are recommended by professional sharpeners and knife makers (Amazon links):
- Japanese Kitchen Knives: Essential Techniques and Recipes
- The Knifenerd Guide to Japanese Knives
- Knife: The Culture, Craft, and Cult of the Cook’s Knife
- Sharp: The Definitive Introduction to Knives, Sharpening, and Cutting Techniques, with Recipes from Great Chefs
The 3 methods we have chosen to detail here are the 3 easiest methods you can use at home with a fairly low safety risk. Knifemakers use other methods that require heat and other chemicals that are not safe processes to use at home.
These blackening processes work best on carbon steel with varying results on stainless steel. If you want a blackened stainless steel blade, it is best to purchase a knife with a blade that the manufacturer has darkened.
TIP: Knives are one example of an item that can suffer damage when cleaned in a dishwasher. Find out why and how dishwashers damage knives in the article below:
Guide: Why Dishwashers Ruin Knives & What Knives Are Safe