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- Sep 9, 2003
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I actually got an idea to write an informative post about salt baths about a month ago, when I saw some very puzzling post/ false assumptions regarding them, but the notes I started got set aside. However with the amount of recent posts regarding them I though it may be worth while to finish the writing and post the information that I have.
Many of us would love to believe we are some sort of innovator or pioneer because we use salt baths, or that we were using them before others. The truth is that salt baths are VERY old news in industry. Records go back to their industrial use in America and places as distant as China to the turn of the 20Th century. By the 1930’s they had been around long enough to be so commonplace as to not even be notable.
The first account I have of a custom knifemaker in America describing a use of salt baths is in 1984 at the first ABS hammer-ins in Wyoming, where German smith Heinrich Frank explained how he used a little table top salt bath to heat treat small blades after he had engraved them. By all accounts the mention was pretty much ignored by most knifemakers who found it too esoteric. The first full scale demonstration of salt baths, high and low temp, in use, that I am aware of, was many years later at the New England Bladesmith Guild Ashokan seminar, where smiths such as Dan Maragni, Phil Baldwin and Tim Zowada were quite familiar with the wide scale industrial uses and recognized the potential for knifemakers. Tim Zowada had designed a stackable columnar kiln to be built by the Evenheat Kiln Company in MI specifically for salt bath use. Folks like Al Pendray may have been working with the ideas in their shop at this time, but I believe Ashokan was the first full public demonstration by custom smiths.
It wasn’t too long after this that I got set up with a high temp unit of my own due to my increasing work with swords. If none of my blades would have ever exceeded 10” I doubt I would have developed the motivation to take on the added expense and maintenance of the necessary equipment. For those who not familiar with my work, I have been using these tools for many years, I have helped many others, from individual knifemakers to large production companies, get set up with and properly use them. I didn’t invent anything about them, I am no innovator in their use, but I have plenty of experience and a good understanding of their use in knife/sword shops.
What are salt baths anyhow?
They are not brine solutions, brine is a very fast quenchant achieved by mixing salt (around 9%) with water. Salt baths are a different beast from the normal bladesmiths heat treating entirely. When steel is heated for the quench several problems arise at the necessary temperatures. Common issues in forges or ovens are times required for thorough heating, scaling and oxidation, decarburization, uneven heating and over heating of thinner parts. Most of these issues are connected to the atmospheres. Air is an insulator so heating is slow. Air contains oxygen so oxidation and scaling will always be present, even if you eliminate the oxygen in the heating chamber there will be momentary exposure when the part is removed. Most of the same applies to decarburization, and forges are very prone to overheating tips and edges.
Salt baths were industry's (and eventually some knifemakers) answer to many of these issues. A salt bath is a volume of superheated molten salts that replace the atmosphere and heating chamber of the traditional oven, kiln or forge. The high temperature salts that most knifemakers would use are primarily NaCl based and begin to melt at around 1275F and have a working range up to around 1600F, but can go higher with other elements added to the chemistry. Most knifemakers find a columnar, tube type design to be the most convenient so I will focus on that. Picture a tube filled with crystalline NaCl salts that is sticking out the top of an enclosed heating chamber (either electric kiln, or gas fired). Heat is applied until the salts melt and then the liquid can be adjusted to any temperature you like within their working range.
The blade is then placed not into an evacuated or gaseous open space, but immersed into this superheated liquid. Because it is a liquid (a conductor) the first thing you will note is the speed with which the steel will come up to temperature, indeed the only thing I have seen heat quicker is induction. The next thing you will notice is how evenly the heating will be. Unlike radiant heat traveling through an insulative space where thin sections can overheat and thick areas still not reach temperature, the totally even and conductive nature of the salts will not allow this. The cycling overshoot and undershoot nature of electrical elements is overcome by the conductive and convective effects of the mass of salts.
Then there is the total absence of oxygen or other atmospheres under the surface of the salts. There is no scaling nor oxidation whatsoever because there is no oxygen involved in the heating. That is not to say that poorly maintained salts cannot tear up a blade, they are superheated NaCl salts after all. This is why proper salts prepared for, and intended for, the purpose are important along with regular monitoring and maintenance being essential. Contaminated or tired salts that get out of neutrality will begin to pit and decarburize steel on levels that will make you long for a forge or kiln again. However if you know what you are doing and pay attention to proper maintenance, salts pretty much eliminate oxidation, scaling or decarb.
I take all of my blades to the final hand rubbed polish BEFORE heat treating, and could even sharpen the blades if I wanted to without any concerns that the blades would be just fine afterwards. All I do is a quick rub down with 800X on damascus blades before proceeding to etching, with no concerns about scaling, decarb, or overheated edges or tips.
But why don’t you get decarb and scale when you pull the hot blade from the salts, you may ask? Because the blade is wet with a thin protective coating of salts, now if you left it exposed long enough you may have problems, but that length of time would also negate your entire quenching operation as well.
The inherent benefits should already be apparent, but no salt bath should be complete without a digital controller to give the hand free, unequalled precision. Wired to a relay or valve to control the electricity or gas supply to the heating device is a PID electronic controller to which you attach a thermocouple probe which goes into the salts and sends temperature readings back to the controller. The controller uses this input to control the heat source and regulate the temperature of the salts to with 1F. or 2F. of the set temperature, to give one control over the austenitizing process that you may have never dreamed of.
I have seen it fallaciously implied that things like salt baths can have negative effects on things such as grain size; this is totally false to a level that causes me to seriously consider the motives of anybody who would suggest something so erroneous. Serious steel industries turned to this technology to achieve controls that are still unparalelled in many applications. In the ranges for the steels we work, temperature is the key factor in final grain size, and with salt baths even someone new to heat treating with a few keystrokes can dial in any size they would like to maintain with temperature accuracy not possible with a forge or a torch. It is just a fact that I can punch in any number in the liquid range of the salts and hold it within 1-2 degrees for as long as I please.
Many of us would love to believe we are some sort of innovator or pioneer because we use salt baths, or that we were using them before others. The truth is that salt baths are VERY old news in industry. Records go back to their industrial use in America and places as distant as China to the turn of the 20Th century. By the 1930’s they had been around long enough to be so commonplace as to not even be notable.
The first account I have of a custom knifemaker in America describing a use of salt baths is in 1984 at the first ABS hammer-ins in Wyoming, where German smith Heinrich Frank explained how he used a little table top salt bath to heat treat small blades after he had engraved them. By all accounts the mention was pretty much ignored by most knifemakers who found it too esoteric. The first full scale demonstration of salt baths, high and low temp, in use, that I am aware of, was many years later at the New England Bladesmith Guild Ashokan seminar, where smiths such as Dan Maragni, Phil Baldwin and Tim Zowada were quite familiar with the wide scale industrial uses and recognized the potential for knifemakers. Tim Zowada had designed a stackable columnar kiln to be built by the Evenheat Kiln Company in MI specifically for salt bath use. Folks like Al Pendray may have been working with the ideas in their shop at this time, but I believe Ashokan was the first full public demonstration by custom smiths.
It wasn’t too long after this that I got set up with a high temp unit of my own due to my increasing work with swords. If none of my blades would have ever exceeded 10” I doubt I would have developed the motivation to take on the added expense and maintenance of the necessary equipment. For those who not familiar with my work, I have been using these tools for many years, I have helped many others, from individual knifemakers to large production companies, get set up with and properly use them. I didn’t invent anything about them, I am no innovator in their use, but I have plenty of experience and a good understanding of their use in knife/sword shops.
What are salt baths anyhow?
They are not brine solutions, brine is a very fast quenchant achieved by mixing salt (around 9%) with water. Salt baths are a different beast from the normal bladesmiths heat treating entirely. When steel is heated for the quench several problems arise at the necessary temperatures. Common issues in forges or ovens are times required for thorough heating, scaling and oxidation, decarburization, uneven heating and over heating of thinner parts. Most of these issues are connected to the atmospheres. Air is an insulator so heating is slow. Air contains oxygen so oxidation and scaling will always be present, even if you eliminate the oxygen in the heating chamber there will be momentary exposure when the part is removed. Most of the same applies to decarburization, and forges are very prone to overheating tips and edges.
Salt baths were industry's (and eventually some knifemakers) answer to many of these issues. A salt bath is a volume of superheated molten salts that replace the atmosphere and heating chamber of the traditional oven, kiln or forge. The high temperature salts that most knifemakers would use are primarily NaCl based and begin to melt at around 1275F and have a working range up to around 1600F, but can go higher with other elements added to the chemistry. Most knifemakers find a columnar, tube type design to be the most convenient so I will focus on that. Picture a tube filled with crystalline NaCl salts that is sticking out the top of an enclosed heating chamber (either electric kiln, or gas fired). Heat is applied until the salts melt and then the liquid can be adjusted to any temperature you like within their working range.
The blade is then placed not into an evacuated or gaseous open space, but immersed into this superheated liquid. Because it is a liquid (a conductor) the first thing you will note is the speed with which the steel will come up to temperature, indeed the only thing I have seen heat quicker is induction. The next thing you will notice is how evenly the heating will be. Unlike radiant heat traveling through an insulative space where thin sections can overheat and thick areas still not reach temperature, the totally even and conductive nature of the salts will not allow this. The cycling overshoot and undershoot nature of electrical elements is overcome by the conductive and convective effects of the mass of salts.
Then there is the total absence of oxygen or other atmospheres under the surface of the salts. There is no scaling nor oxidation whatsoever because there is no oxygen involved in the heating. That is not to say that poorly maintained salts cannot tear up a blade, they are superheated NaCl salts after all. This is why proper salts prepared for, and intended for, the purpose are important along with regular monitoring and maintenance being essential. Contaminated or tired salts that get out of neutrality will begin to pit and decarburize steel on levels that will make you long for a forge or kiln again. However if you know what you are doing and pay attention to proper maintenance, salts pretty much eliminate oxidation, scaling or decarb.
I take all of my blades to the final hand rubbed polish BEFORE heat treating, and could even sharpen the blades if I wanted to without any concerns that the blades would be just fine afterwards. All I do is a quick rub down with 800X on damascus blades before proceeding to etching, with no concerns about scaling, decarb, or overheated edges or tips.
But why don’t you get decarb and scale when you pull the hot blade from the salts, you may ask? Because the blade is wet with a thin protective coating of salts, now if you left it exposed long enough you may have problems, but that length of time would also negate your entire quenching operation as well.
The inherent benefits should already be apparent, but no salt bath should be complete without a digital controller to give the hand free, unequalled precision. Wired to a relay or valve to control the electricity or gas supply to the heating device is a PID electronic controller to which you attach a thermocouple probe which goes into the salts and sends temperature readings back to the controller. The controller uses this input to control the heat source and regulate the temperature of the salts to with 1F. or 2F. of the set temperature, to give one control over the austenitizing process that you may have never dreamed of.
I have seen it fallaciously implied that things like salt baths can have negative effects on things such as grain size; this is totally false to a level that causes me to seriously consider the motives of anybody who would suggest something so erroneous. Serious steel industries turned to this technology to achieve controls that are still unparalelled in many applications. In the ranges for the steels we work, temperature is the key factor in final grain size, and with salt baths even someone new to heat treating with a few keystrokes can dial in any size they would like to maintain with temperature accuracy not possible with a forge or a torch. It is just a fact that I can punch in any number in the liquid range of the salts and hold it within 1-2 degrees for as long as I please.
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or a smaller version of it 
