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Metal finishing Q&As since 1989

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"Low Temp" Hot Black Oxide?

July 31, 2009

Sir,

My plant is considering in-sourcing hot black oxide and I've been directed to investigate. In doing research, I've found a company that is touting that it has developed a hot black oxide process/chemicals that provides very close to the traditional black oxide results but operates at 190 degrees F instead of the traditional 290 degrees.

Your thoughts or experience on the matter would be greatly appreciated.

Sincerely,

John Bonner
engineer investigating for a mfg facility - Washington, North Carolina

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Hi, John. The traditional cold black oxides have been, in my personal opinion, a poor substitute for real black oxide. They tend to be smutty, gritty, and not as nice looking. We have an FAQ on Black Oxide and Cold Blackening. But hot black oxide is very dangerous due to the water additions potentially flashing to steam instantly when added to a 290 degree tank, and erupting boiling hot caustic on you. If there is a true black oxide that operates at 212 degrees or less, it sounds terrific.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 3, 2009

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August 6, 2009

Over the years I have run into literally dozens of low temp or acid type blues. Ultimately they all sort of do what they claim. They do turn certain steels black. Unfortunately the finish they produce is often less than satisfactory being mottled and often dull. Also the finished produced is about 1/10th as durable as a hot caustic blue. Acid blues are very susceptible to moisture and will rust or gather surface smut if not protected with oil, wax or some form of clear paint. Also parts that are acid blued should not be handled by human hands. The acid found in our sweat breaks down the acid type bluing very quickly. Even if protected with oil or wax human hands tend to strip away the protective layer and leaves acids on the parts in their place.

The only low temp bluing that I have come across that actually works is the old Herters Belgium blue ⇨

This finish is actually more durable than hot caustic blue but it does have its draw backs. Its application is extremely tedious. The part to be blued must first be boiled for five or ten minutes. It is then removed from the boil tank and allowed to air dry. As soon as the part is dry the solution is applied until the part is so cool that the solution no longer rapidly evaporates from the surface. The part is then returned to the boil tank for 15 minutes to allow it to rust further. The part is then removed and carded with #0000 steel wool [on eBay or Amazon (adv.)], placed back into the boil tank and allowed to come back up to temperature. The part is once again removed and the solution reapplied. This process must be repeated 20 to 30 times to get the desired finish and it can take a full day to blue just one part to acceptable standards.

Belgium Bluing has yet another drawback. The overall finish although very durable is satin in appearance. As though the surface was finished with a soft wire wheel. While it looks nice on firearms it is less than desirable for machined parts.

Your best bet is still caustic bluing. It is not as expensive as some might claim. You can buy the salts from many distributors or if you wish you can make your own salts. The recipe is by no means a secret and you don't have to be a chemist to make them. The chemicals for the most part are not controlled in any way and if you happen to be really cheap you can make them from common fertilizer and lye. Caustic bluing is also not as dangerous as a lot of folks think. I have been bluing for 25 years and of course over the years I have had a few small burns but by no means as bad as most of the cooks and chefs get in their daily chores. I have never had a salt burn that bothered me for more than a few hours or required medical attention outside of a thorough wash and a rub down with vinegar [in bulk on eBay or Amazon (adv.)] and I have never had a salt burn that left a scar. Many of the suppliers of bluing salts have packages which include salts, tanks, heaters, instructions and all of the other necessities needed to get started. More importantly, these companies also have tech staff on hand that will help you smooth out any bugs you may encounter.

As for the cold or low temp blues, by all means try them. I have tried them all. There is a simple way to test the durability of bluing. Cut a coin from a one inch 4140 steel rod and polish it to the finish you want. Blue it as per instructions and then carry the coin around alone in one of your pockets until the bluing is gone or stripped to the point where it is no longer acceptable. I have found that hot, caustic blues will last for a few months. Belgium Blues will be double that. Cold or acid blues will wear off in one day to a week or two. Simple and stupid sounding test. Google (Occams razor). There are a lot of ways to arrive at a conclusion or destination but simple and stupid is more often than not the best way. Hope this helps.

Rod Henrickson
gunsmith - Edmonton, Alberta, Canada

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August 7, 2009

Thanks Ted I should have worded that a bit differently. I should know better. But one does tend to get complacent and forget all the ass kickings for doing stupid things we got when we were learning.

I know of a guy that decided to start bluing some of the parts in his machine shop himself. He did everything right and wore a mask, rubber apron [on eBay or Amazon (adv.)] and rubber gloves [on eBay or Amazon (adv.)]. But he got careless one day and added to much water to the solution to fast. It burped and an ounce or two of solution splattered on the top of his head. He has a white spot on his head to this day.

The most sorrowful case I ever heard of was a gunsmith that had been bluing every second day for 30 years and had never had an accident. He got a new puppy that followed him everywhere. After only a few days the pup went lame and it took him months to recover. The pads on his feet went raw. I guess you already know what happened. The guy was a slob and never cleaned up under his bluing tanks. He had been trodding caustic soda [affil links] all over his shop for twenty years.

Caustic bluing can be extremely hazardous if handled carelessly by uneducated people and freak accidents can happen to even the most conscientious folks. Hell, we lost a bunch of good folks in a space shuttle that were guided by the most educated and gifted people in the world.

I guess what I was originally trying to say is that a lot of things have gotten a lot of bad press over the years. But, we should have enough common sense not to dismiss things out of fear. With proper education, good shop practices and a lot of good old common sense caustic bluing is quite safe and still is probably the cheapest and best way to turn chrome molly steel black.

Mr Bonner: Asking folks like me for opinions is a fine way to get your feet wet and get a rough overview of things. But when push comes to shove and if you do decide to use caustic bluing I would advise you to get in touch with one of the companies that manufactures and sells commercial bluing salts. As I have stated they have tech staff that will come to your door and get you started properly. It might cost your company an extra couple thousand bucks but I would defiantly consider this.

I spent two years in an accredited school learning among other things caustic bluing. I have never been seriously hurt on the job and I firmly believe that having some one stand over me and bonk me on the head a few times has probably kept me from losing an eye, finger or worse. I consider the money well spent in the end. :o)

Rod Henrickson
gunsmith - Edmonton, Alberta, Canada