Maintaining kitchen knives: two burnishing techniques
Unless you have high-quality stainless steel knives - which are very rich in chromium, making them completely resistant to any form of corrosive attacks - your carbon steel knife blades will probably get a patina over time, especially when used on a daily basis for cutting a wide array of foods.
Burnishing is an efficient method for maintaining kitchen knives in order to remove the marks that give your steel blades a dark tint.
There are two techniques used to burnish steel, including the following:
- Mechanical: smoothing asperities and polishing by plastic flow
- Chemical: using chemicals to maintain and polish steel
Mechanically burnishing steel
Mechanical burnishing is similar to conventional sharpening methods, which use stones or sharpening rods. A small tool called a burnisher is used to exert pressure on the blade’s surface.
The mechanical burnishing method polishes the steel through the plastic flow of the surface of the metal, smoothing asperities, the irregular nicks and imperfections, and bringing shine to the blade.
Mechanical burnishing achieves the following results:
- Restores shine
- Reduces rough appearance
- Increases surface hardness
- Sharpens and improves the blade’s cutting edge
In order to execute this kitchen knife maintenance technique properly, you should run the burnisher along the blade’s cutting edge while pressing firmly. The angle used must be the same as the one used when sharpening the blade.
Chemical technique for maintaining kitchen knives
Cold burnishing steel
Cold burnishing steel deposits a protective anti-corrosive layer on the metal surface of the kitchen knife’s blade by immersing it in a burnishing solution called an immersion burnishing bath. This protective layer varies in thickness from 0.2 to 3μm. It gives your blade durable qualities similar to stainless steel while increasing its aesthetic quality.
If you would like to use this technique to maintain your kitchen knives or any other cutlery item, you can easily find this burnishing solution in specialized stores. The burnishing solution consists of four main chemicals - acid, copper, selenium and water - which attack and wear down the surface of the steel.
Finally, a chemical reaction takes place that deposits a protective, porous crystalline coating on the blade’s surface. The coating’s porous properties allow it to absorb protective oils and waxes that are applied to the blade.
Cold burnishing is a very interesting technique because it improves the blade’s resistance to abrasion and its anti-corrosion characteristics without altering its strength or removing material (the blade’s dimensions remain the same).
- Degrease the blade with rubbing alcohol (isopropyl alcohol 90% v/v).
- Prepare the burnishing solution in a 5:1 ratio of water to burnishing solution. Pour the diluted solution into a suitable container.
- Soak the blade in the burnishing bath for a maximum of 5 minutes. Duration varies according to the type of steel. The longer your immersion time, the darker the solution becomes.
- Rinse the blade with lukewarm water. The surface will be covered in a black, uniform layer. To remove this fine powder, rub the blade with a sponge and then dry it immediately.
- Rub the blade lightly with fine-grade steel wool (grain code 0#).
Voilà, there you have it, a kitchen knife that is good as new!
Heat techniques for maintaining kitchen knife blades, using sodium hydroxide
The technique of burnishing kitchen knife blades with a sodium hydroxide based bath is a very effective method but must be performed very carefully, preferably by a professional.
It is very effective when used on kitchen knives made of carbon-based steel, ferrite steel, forged steel or tempered steel}, including all different types of knives (chef’s knives, paring knives, etc.) and different sized blades.
The sodium hydroxide immersion bath must be prepared by mixing the following items:
- 2 liters of deionized water
- 2 kg of powdered caustic sodium hydroxide (also known as lye)
- 200 grams of sodium nitrate
- 200 grams of sodium nitrite
Heat the solution to 120°C/248°F for approximately 10 minutes and then warm it to 140°C/284°F.
Fully immerse the knife’s blade in the solution and pull it out. Repeat these two steps continuously for 1 minute.
Then place the knife in a basin filled with room temperature water then immerse it in the sodium hydroxide bath for an additional 4 minutes. Repeat these steps until the desired color is obtained.
Finish by washing the blade with water and Marseille soap, then dip it in thick mechanic’s oil.