Whether you have a long history in operating black oxide equipment or just now venturing into the field, we believe that you can gain some knowledge from the information we have provided below. It is not uncommon for those who operate equipment such as a black oxide system everyday get buried in the job and the ritualistic process and will forget or simply overlook some basic observations and techniques, and we hope this will provide some folks a little helpful insight.
The following information provides you some comparison between using your Midnight Black™ Liquid black oxide solution or any black oxide solution in an electric heating system compared to a gas heated system. Our Midnight Black™ solution will work just as well in either system, but the information below you may find helpful regardless of the type of heating source you are using.
As a prerequisite to properly understanding the difference between a gas heated black oxide system compared to the electrically controlled BlackWorks® system, you need to have some basic understanding of how it all works.
First, black oxide and bluing is the same thing, they are used interchangeably depending upon who is saying it. People in the firearms industry referrer to it as Bluing, mainly because most salts are chemically mixed to provide a slight “Blue” color to the steel rather than a blacker looking color. When it comes to industrial applications, it is usually referred to as “Black Oxide” or blackening. Same thing, same process, just called different names.
Secondly, the process of black oxide finishing is achieved by submerging an iron steel part into a heated bath of what is commonly referred to as a “Salt” bath. It is referred to as “Bluing Salts” because for one it leaves a “Blue” finish or black as we mentioned earlier and it contains Sodium Nitrate, commonly referred to as “Salt”, as in salt peter, which was used in preserving the ham you had for Christmas dinner or making gunpowder, we prefer the later.
The black oxide solution comes in either a powder form or a liquid form. Either form will work for a gas heated or electrically heated system. We however highly recommend our Midnight Black™ in the liquid form due to its longevity and its ease of use in an electric heating system and its extreme black finish properties.
The black oxide solution, regardless of powder or liquid form contains three main ingredients, Sodium Hydroxide (NaOH) (Lye, basically your household drain cleaner as in Drano®), Sodium Nitrate, (NaNO3) and Sodium Nitrite (NaNO2). Along with this chemical mixture, there is also trace amounts of proprietary additives which give you the different colors ranging from light blue to super black.
The combination of chemicals mentioned above are then mixed with water (H2O), which is what is then referred to as a “Salt Bath”. The salt bath is typically heated between 285-310 degrees Fahrenheit. At this temperature and chemical mixture, the surface of the iron metal submerged in the bath is converted into magnetite (Fe3O4). It leaves dark or black like cosmetic appearance and a corrosion resistance finish.
And thirdly, you also need to understand the basic of boiling points of liquids. As most already know, water (H2O) boils at 212°F at sea level. The salt bath we have been discussing boils between 285°F – 310°F. The reason the salt bath is boiling at say, 290°F and not at 212°F, as compared to our water is because of the higher boiling points of the 3 main chemical additives.
To clarify, if you are boiling a pot of water on the stove at home, the temperature will be 212°F, assuming you are at sea level. Whether you put 5,000 Btu’s of heat under the water pot or 1,000,000 Btu’s of heat, the water will not boil greater than 212°F, however your pot might vaporize.
In regards to your salt bath, the same situation also applies; depending upon the concentration of “Salts” in your salt bath, it will increase or decrease its boiling point. To make this simple, we will assume we want the salt bath mixture to boil at 290°F. That means you have to have the exact combination of salts to water ratio. Keep in mind though, once the mixture begins to boil, no matter how much heat you apply to the mixture, it can’t boil it hotter than 290°F. But, this mixture is not quite like boiling your pot of water. In this situation, as the salt bath begins to boil, the water in the solution begins to evaporate. As the water evaporates, your water to salt solution begins to change immediately.
As your salt concentration begins to increase, your boiling point also begins to increase as well. The longer the salt bath boils, the higher the temperature increases due to the evaporation of the water. Not because of more heat, but because the water continues to evaporate, causing the boiling point to increase due to lack of water and higher salt concentration to water ratio. If you were to continue to heat the salt bath solution and not add water it would eventually boil down to nothing except the 3 main chemicals in your salt bath and the temperature would be well over 2,000°F or more.
The way we properly control such a chemical solution to maintain a specific boiling point is we continually add water so that the boiling point remains at a specific temperature that we choose. The tricky part on all of this mentioned is the balancing act between the proper heating of the salt solution and also the adding of water to maintain the desired temperature.
As you can surely appreciate now, it quickly becomes quite a balancing act to keep the right heat level on the tank and keep adding water so that your salt bath temperature stays at the proper setting.
As a normal process of operating a gas heated black oxide tank, the following is the standard operational procedures. We are providing this information to you so that you can have a point of reference in how a standard gas heated tank system works, assuming you may have not operated one before. If you already have had experience in operating a gas heated tank system, please disregard and take a break or feel free to chuckle knowing you have replaced yours with a Blackworks® system.
- Turn your gas burner on High under your salt bath tank following the manufactures instructions for your specific make and model.
- Keep a close eye on the temperature; it usually takes about 45 minutes for a 5 gal tank to reach pre-operating temperature of 275°F. Make sure you do not leave the tank unattended. Leaving unattended can cause a severe hazard and mess if the tank is allowed to overheat and boil over onto the floor. If allowed to run unattended any longer, you will overheat the salt bath and destroy the salt solution, which usually happens when the salts are operated over 312 degrees Fahrenheit.
- Turn down the gas burner to about 70-75% gas flow, follow your personal experiences as to the exact heat adjustment which will determine your exact setting, which is largely determined as to the amount of cold steel mass you are usually running in your tank. The smaller mass of parts, the less amount of heat you need. More parts, more heat, which is required to more expediently return the salt bath back to its proper temperature once you place your parts into the tank. Keep in mind, don’t run the gas burner hotter than necessary for the parts you are running, doing so will only cause the water in your salt solution to evaporate faster, forcing you to add water sooner than necessary. However, you must have enough heat to quickly return the salt bath to operating temperature after adding your steel parts and to maintain a rolling boil.
- You want your salt bath to be at a rolling boil at your preset temperature. For the majority of steels that you run, you will want to have it set at 290°F.
- As the temperature rises above 290°F., carefully and slowly add COLD water with an extended handle ladle or cup. Remember, always add COLD water to the salt solution and do it very slowly. Using HOT water or adding the water too quickly can cause violent eruptions of the salt solution, potentially causing severe burns as well as having the salt solution boil over onto the floor. Always wear protective clothing and eye protection when operating a black oxide system, ESPECIALLY WHEN ADDING WATER.
- Remember, if your tank is at 290°F when you added your parts into the tank, the temperature will drop rapidly due to the cold parts cooling the salt bath. It will take time for the salt bath to come back up to the 290°F. However, while you are waiting for the salt bath to return to the 290°F., which can be anywhere from 5 minutes to 15 minutes, the water has been continually evaporating from the solution. So, as the salt bath returns to the 290°F., you will notice that it’s not vigorously boiling like it was before you put the parts in. The reason is that a percentage of the water in the salt bath has evaporated during the short period of time, causing the boiling point to rise. At this time, you want to begin adding water so that your salt bath begins to have a rolling boil but also to maintain your 290°F. Practice over time will allow you to get a “Feel” of when and how much water to add.
At this time, you want to begin adding water so that your salt bath begins to have a rolling boil but also to maintain your 290°F. Practice over time will allow you to get a “Feel” of when and how much water to add.
- Keep in mind; this is a continuous balancing act when operating your black oxide tank with gas heating. You have to continuously monitor your bath temperature and the boiling point as well. You want your salt bath to always be boiling and for it to be within your specified temperature range, which is usually +/- 5 degrees of set temperature when operating a manual tank.
- Do not place parts into your tank when the salts are not a rolling boil. This can cause greenish and/or reddish residue on your parts. After a tank has been used multiple times, there is a natural build-up of chemicals residue at the bottom of your tank, which is normal from the parts you have placed in the solution. If you place parts in the tank while the solution is boiling, the build-up doesn’t have an opportunity to adhere to the parts being blackened. However, if the solution is stagnant and not vigorously boiling, the build-up residue will commonly adhere to your parts. If you are lucky, it can be wiped off with no ill effects, other times you may have to strip the parts and completely refinish the black oxide finishing, especially if your tank temperature is running on the high side.
We hope this has provided some basic insight into the process of black oxide finishing.
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We have worked very hard to maintain all of our manufacturing processes here in the United States. We will continue to do so as we grow. We will not deviate from our founding principles that we all should strive to produce our products here in the United States. We feel we have proven that one can be competitive in the marketplace without having to import our goods.