DIY for hot burnishing of metals
The black or matte effect on the metal in addition to the increase in surface resistance to corrosion is also an aesthetic value that is often used on knife blades.
It is a personal taste but in recent years many makers have tried to darken their blades both at an artisanal and industrial level.
Burnishing(sometimes also called bronzing or metallochromy) is the surface coloring of a metal practiced with various methods and having the purpose of providing protection against oxidation as well as improving its appearance.
Usually this process can take place in two ways, burnishing to:
- Cold or Chemical (with products sold commercially such as the Ballsitol Burnisher Kit),
- or Hot.
In this post we are going to see how to do hot burnishing home made or DIY.
ATTENTION!! THAT THIS PROCESS INVOLVES RISKS FOR THE PERSON AND THAT IT MUST BE DONE SAFELY.
Caustic Soda: Sodium hydroxide, chemical formula NaOH, you buy at the supermarket as caustic soda.
It should be handled with gloves and glasses because it is caustic, dissolved in water develops heat.
Potassium nitrate: or potassium nitrate chemical formula KNO3, you buy it at the agricultural consortium, it is a fertilizer is certainly sold in bags of 50 kg, or in the wholesale of food products in kilo bags.
There are no particular risks in handling it.
The official recipe consists of:
- 1000 grams of water
- 500 grams of soda
- 300 grams of nitrate
DIY procedure for hot burnishing metals
Take a stainless steel container and pour in in the following order:
Put the container on an electric or gas stove and bring to a boil by shaking/stirring occasionally to facilitate the dissolution of the products.
The solution so ready you have to boil between 135 and 145 degrees , if you have a thermometer that allows you to measure the temperature is better.
Ps. Consider that the temperature goes beyond 100° C because it is not just water.
Some notes and tips for hot burnishing of metals
- Using the thermometer is important because if you do not reach those temperatures (around 140 ° C) the risk is that you do not get a burnished, dark color, but a color that tends to yellow.
- Example the operation of preparation of the piece is important and the more you brush it (shiny) and the more fat it is the better for the final result;
- Apparently strange coloring always has causes,
- As for the colors:
- the iridescent colors tending to blue are due to temperatures below 140/145 °
- while the reddish colors or sometimes tending to yellowish are due to the concentration of the solution that “lacks” water and therefore the piece “oxidizes” quickly or in jargon they say that it burns (rusts).
- Moving on to the final phase of the burnishing operation, once the piece has been removed from the bathroom, you must wash it for a few seconds in clean water to purify it from the solution, then wet it with a sponge or immersing it with emulsifiable oil that precisely emulsifying with the aqueous traces present on the piece guarantees lubrication avoiding oxidation.
Ps. The emulsive oil can be purchased in stores and specialized hardware for milling and turning machines (it is used for cooling tools) and takes on a milky color. It costs little: about 5/6 euros per liter.
Handle the pieces with rubber gloves so that your hands do not leave traces of grease on the workpiece.
Degrease the pieces to be browned well with professional degreasers such as fulcrum or alternatively dishwashing detergents but better the fulcron.
If you really want to do things in a workmanlike manner, also give it a pass in hydrochloric acid (muriatic acid).
Now the pieces or the piece should be degreased.
Immersion of burnishable pieces
Immerse the pieces in the boiling solution keeping them tied with an iron wire, better if the wire is stainless steel because the stainless steel can not be burnished.
Let the pieces boil inside for 20-30 minutes checking them from time to time to see if the color achieved is the desired one.
During the process the water evaporates, so it is necessary but being VERY CAREFUL to restore with a ladle the water that evaporates.
Be very careful, because diluting the solution lowers the boiling point and therefore the mixture tends to splash and that is why you have to use the ladle with a long handle and that allows you to keep a certain distance.
The process of hot burnishing of metals has been completed:
- extract the pieces,
- wash them in running water. Consider that to neutralize the effect of the acid you have to immerse it (just for a little) in a solution of water and bicarbonate being basic cancels out or even in a solution of caustic soda but here you have to be more careful so I recommend you with water and bicarbonate.
- clean the pieces by scratching them with an abrasive sponge (the classic yellow and green kitchen),
- dry the pieces and
- Oil the pieces well,
Let the pieces rest a couple of days in a lightly oiled rag.
Surface protection of burnishing
Burnishing is a process that serves to protect all steels from rust (excluding stainless steels that cannot be browned).
It is a process that is routinely used on weapons, rifle barrels, barrel, shutter, etc.
Such protection is certainly not comparable to a galvanizing or chrome plating …
The protective layer of a few microns is formed by iron oxides, (hematite) that protect, passivating the lower layers.
Aesthetically the black or matt black burnishing has an aesthetic result on the blade that is really captivating.
Burnished pieces should always be kept slightly oiled.
The burnishing done with this system is certainly better from the point of view of protection than the many cold burnishers that are on the market.
The solution could also be reusable but must be placed in containers suitable for chemicals.
- Always pay close attention
- Always wear glasses and gloves.
- The solution is dangerous due to the operating temperature of 140°C and the presence inside of caustic soda which is highly burning.
- In case of accidental contact with the solution wash the part thoroughly in water and if necessary go to an emergency room.
- To dispose of the burnishing solution once cooled you must dilute it with plenty of water before throwing it away.
Like everything if you know some person or company that performs burnishing professionally it is a correct choice because without good equipment it is not easy to have a professional quality level.
The galvanic method to carry out the burnishing of metals is one of the various professional methods widely used today, even at an industrial level. This method consists of immersing the piece to be treated in an aqueous solution of sodium sulphantimoniate (or “Schlippe salt”), of anhydrous sodium carbonate and then electrolysis for a few minutes at room temperature with a current of 0.35 amperes (A) at voltages between 2.4 and 4 volts (V). This method is also used in the treatment of copper and brass.
So my advice regarding in the specific case of the post of hot burnishing of metals is to do tests on simple details such as screws or bolts, or some waste of hardened steel to be able to see the result without wasting time and material.
After that it is also important to check that the burnishing is stable and with the right “depth” to be sure that it is uniform and stable on the material and does not go away with a slight abrasion.
Personally doing things at home is a personal pleasure but remember that the comparison you have to do with the same object or similar made by a professional.
If you do not reach a certain level of quality, it means that you still need to improve your production process.
You can also try the cold burnishing process of ballistol or Birchwood Casey.
It is chemical burnishing but it has excellent quality levels to be done with this method.
Widely used in the weapons industry but not only but I think the best thing is to do tests on an old knife or on a “wrong” knife before doing it directly on your favorite blade.
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