View Full Version : Chemical question regarding barium carbonate

Jaakko Fagerlund
06-09-2013, 11:50 PM
It has been too long since my last chemistry classes, so here it goes.

I've been playing with pack case hardening compound that includes charcoal, barium carbonate, sodium carbonate and calcium carbonate. The idea of the carbonates is to give up carbon dioxide, that together with the carbon promote or "energise" the formation of carbon monoxide that is the transport medium for the carbon to the steel surface.

The temperatures usually given are around 815 to 950 Celsius. Sodium and calsium carbonates both decompose at about 850, but the barium only after the 1000 degrees of Celsius mark.

So what puzzles me is that the barium carbonate seems like it would do nothing in the mix except add some toxicity to the mix. Am I thinking way off with this one or is there something else happening that promotes the barium carbonate to give up carbon dioxide?

Because if my reasoning is correct, I would just leave it out of the mix. But the thing is, it usually covers 80-90 % of the whole carbonate content.

06-10-2013, 01:42 AM
It is a good Q. The barium? Not sure why its in the mix other than being maybe a type of flux. Aids in binding and prohibits massive oxidation? Maybe? JR

06-10-2013, 01:50 AM
Energiser is really a simplistic term for a catalyst, the catalyst enables a reaction to take place by providing a shortcut through the energy hill that a reaction needs to get over to occur, look up catalysis for energy hills, barium or calcium carbonate provide this shortcut, as does sodium carbonate at a much lower temperature but will attack everything.
The catalyst does not get changed by the process, leave the hardening compound on a tray in air tonreactivate after use

06-10-2013, 01:55 AM
Going from carbon dioxide to carbon monoxide is the wrong direction, unless the barium is reacting with the carbon dioxide to form barium oxide, leaving carbon monoxide.

Jaakko Fagerlund
06-10-2013, 04:03 AM
All I know is what I've read about in diferent books on the subject, but the idea of the carbonates is to act as a catalyst/energiser in the mix. The carbon will burn to carbon dioxide and carbon monoxide, of which the latter is the preferred product and most likely to evolve as there is excess of carbon and limited amount of oxygen inside the pack. What the book told is that the carbonates will decompose at those temperatures, thus giving off carbon dioxide that gets transformed to carbon monoxide. How it exactly happens, I don't know for sure (maybe like Evan pointed out), but I'm not interested in that.

What I'm interested in knowing is why the barium carbonate reacts at all the way it is explained to do, if the temperature is below decomposing temperature? Is it because some other atom or molecule robs the CO or CO2 from it?

The sodium and calcium carbonates are easy: they decompose totally at those temperatures.

06-10-2013, 09:32 AM
You are on the right track regarding the carbonates. You have entered into the realm of phase rule chemistry. Changing the proportions of sodium to calcium to barium changes the temperature at which the reaction occurs. The temperature that the reaction needs to occur with the metal is critical. If you have a fancy laboratory you have the capability of measuring all the parameters to get it right. For the rest of us. it's cut and try.

06-10-2013, 10:03 AM
According to the Merck Chemical Index, Barium Carbonate is VERY poisonous and lists a bunch of symptoms which it can cause. Its main use in industry is a rat poison. I would be very careful working with it. At 1300 deg. C it disassociates into barium oxide and carbon dioxide.

06-10-2013, 12:12 PM
From what I'm seeing, the mechanism is dissociation of carbon dioxide. Above 1000 deg C, carbon dioxide dissociates into carbon monoxide and O2. The O2 in a carbon rich environment forms more CO2 and CO. The additional CO2 also breaks down into CO and O2.

The trigger points for the various carbonates determine how much CO2 is generated ata a specific temperature. Above 1300 deg C, the barium carbonate breaks down.

The rate that CO is absorbed by the iron increases with temperature, so a mix including Barium carbonate, the barium carbonate doesn't contribute until the temperature hits 1300 deg C, but at the elevated temperature, the carbon monoxide that results is absorbed faster.

I'm used to seeing carburized parts using methane gas at elevated temperature and pack carburizing using carbon and calcium carbonate. Historic methods included willow charcoal, ground horn, ground hoof and wrapping the part in leather and cooking at high temperature.

The carbon monoxide doesn't normally dissociate into carbon gas and O2 until the temperature exceeds 3750 deg C, except when a promoter atom is present. Lithium, calcium and a few other elements will strip the oxygen off CO at carburizing temperature and then decompose back to the element plus O2 which goes to pick up more carbon as CO.

In any case, it works.

Jaakko Fagerlund
06-10-2013, 12:44 PM
BigBoy1, that is exactly the reason I'm trying to avoid using anything containing barium. There just ain't no healthy materials that include barium in them.

So, I'll just stick with the sodium and calcium carbonates, at least they are safe to handle and quite probably work just as good. I might try to do one barium carbonate test batch and one other and compare the results if I just get my friend to measure case depth on each part.

Found something nice to read, mentions the use of barium free packs: http://books.google.fi/books?id=eIfcHJtv_kcC&pg=PA115&lpg=PA115&dq=activated+carbon+pack+case+hardening&source=bl&ots=Brg3mAoplG&sig=qBuPpMFzysw623fd0EETToN9rxk&hl=fi&sa=X&ei=NNK0UeXNJ-il4ATw3oDoCQ&ved=0CEoQ6AEwAQ#v=onepage&q=activated%20carbon%20pack%20case%20hardening&f=false

Mike Hunter
06-10-2013, 05:09 PM
I do a lot of pack case hardening; you can get very good results with just wood and bone charcoal.

Dont mess with al the other junk

Mike Hunter

06-11-2013, 08:43 AM
The barium question is fairly complex, things like Gibbs free energy and Ellingham diagrams are involved and to be honest deltaG and deltaH makes the fur on the back of my neck stick up, the lectures were torture!
You can go have a look,there's a good site at Cambridge uni interactive about lol the weird stuff, I remember using a lot of it to predict what was happening in the different zones in a blast furnace, compounds can come into existence at elevated temperatures that are non exist ant at room temperature, barium being included

Jaakko Fagerlund
06-11-2013, 12:22 PM
I do a lot of pack case hardening; you can get very good results with just wood and bone charcoal.

Dont mess with al the other junk

Mike Hunter
I assume you are the Hunter from Hunter Restoration? If so, then you are probably more after colors than the hardness and depth that I want. I'm shooting for 1 mm or thicker case and no colors needed, as this is usually on tools that are to be ground anyways.

Bone charcoal is not needed. It contains some carbonates, but mostly phosphates that can have bad results on steel. Easier to just mix carbonates with charcoal (or activated carbon) and have at it. It is the carbonates that make things go faster and I don't like to spend a whole day to get the case I want.

I'm more inclined to using science than 'old timers folklore', as it comes with predictable results that are easy to redo. And the less there is variables, the better.

I'm probably going to do a series of tests once I get hydrochloric acid from somewhere. The tests would be using only carbon, carbon with barium carbonate, carbon with sodium carbonate, carbon with calcium carbonate and then a mix of carbonates with the carbon. After carburization and hardening, I would cut the sample in half, sand it smooth and give it an acid bath to see how deep the case has gone.

The reason for the tests would be to prove if the barium carbonate somehow magically is actually better than the others or not. If the others are on par with the barium carbonate, then I don't feel like using the barium carbonate due to its inherent problems with toxicity at various stages.

06-11-2013, 01:24 PM
I don't know if it will serve your purpose, but if the surface finish is not important you might try coating the surface with borax, just like we do to forge weld, and then coating the surface so wet-ed with cast iron turnings. I used brake drum swarf.

After coating the surface with the flux and turnings, heat the whole mess in a furnace with a reducing atmosphere (I used a gas forge... it's what I have) and lightly forge the surface to achieve more intimate contact. Then back in to heat some more... In my case the test bar was heated for an additional 3-5 minutes or so... no science here the temp was a bight yellow, just below forge welding temperature.

I used this to raise the carbon content in a 1/8" X 1/2" mild steel sample to a point sufficient to fracture it after quenching.

So as to whether or not the surface finish would be too negatively affected only you can speak to. This process will raise the carbon content.

The upshot of the preceding and what follows is that it does not take that long for the carbon to move. I can only surmise that the terrible long times of the various charcoal methods are more due to lack of carbon transport to the surface and difficulty taking elemental carbon up in the steel. Having the carbon in the cast iron particles makes it more readily available to the steel, I believe. I think the lower melting point of cast iron vs steel further helps the process.

I only include this reference to provide a little hard science. :>)

The following article relates to the diffusion of carbon in pattern welded Damascus steel, and while the lowest layer count on one of the charts is 50, which might be a thickness of about .07", the time to homogenize the Carbon content is about 10 seconds.

See :


See also:
to see John's cred's.


Mike Hunter
06-11-2013, 02:03 PM
Case hardening using the pack method has been around for a couple of hundred years. It was very, very, well understood.
Keep in mind that quite a bit of pack hardening was done on everything from ultra precision tools to automotive parts, train parts, aviation components, up to huge parts weighing several tons used on ships. Pack hardening was very well understood 1849s-1960s, times, temps depth of case etc were well documented, and understood by the manufacturing community, and was truly a “Science” with very controlled and predictable results.

Unfortunately a lot of that information has been lost, and now pack hardening is misunderstood and considered black magic or “folklore”.

06-11-2013, 03:18 PM
This book has a large section on pack hardening and is available at used sites. The info was current in the late 60's. It is oriented towards the professional and contains recipes and techniques for different kinds of surface hardening as well as much more: http://www.amazon.com/dp/B0013HJP32

Mike Hunter
06-11-2013, 03:40 PM
Yes, the ASM heat treating books are excellent for those that want to understand how to heat treat metals properly. Takes quite a bit of the “superstition” out of it.

Jaakko Fagerlund
06-11-2013, 06:22 PM
Too bad no-one wants to ship that book outside the US on Amazon. Would have been a nice addition to my bookshelf :)

Mike, pack case hardening truly is a kind of lost art, as more efficient methods have been adopted for commercial use. But trying to find information on just carburizing and hardening is difficult, as the most of the search results are for color case hardening, which is an art and really black magic to get what one wants and doesn't care about the case depth that much.

I really hope to gain some useful information about my tests so that others wouldn't have to think this same thing through.

What comes to using cast iron shavings, it is a thing I won't do. Too much unkown variables, the problem of acquiring cast iron shavings and the fact that carbon and the carbonates are a lot cheaper and easy to get. The carbon moves around in the form of carbon monoxide, so the cast iron doesn't add anything to the process. The only way of improving the process speed is to up the temperature and promote the formation of carbon monoxide. And this is exactly what the carbonates do.

06-11-2013, 06:39 PM
Jaakko, the idea was to convey the carbon migration information. Regardless of how you introduce the carbon, it still penetrates at a rate that depends on concentration and temperature. The source of carbon is irrelevant. Just the concentration and the temperature that you treat at.

Unlike the Barium compunds, the cast iron is harmless unless propelled at velocities exceeding 600 fps or so.

The cast iron, a source of known concentration of carbon per unit volume is free at just about any automotive repair shop.

06-11-2013, 10:06 PM
Just use Barium Carbonate and try not to eat it... There is dread and there is risk and what you dread may not automatically mean high risk... I have 3kg of Barium Carbonate stored here.. I do not dread using it, just use normal handling precautions..

I use it in my mix and get good results..

Jaakko Fagerlund
06-11-2013, 11:46 PM
Just use Barium Carbonate and try not to eat it... There is dread and there is risk and what you dread may not automatically mean high risk... I have 3kg of Barium Carbonate stored here.. I do not dread using it, just use normal handling precautions..

I use it in my mix and get good results..
Yeah, Ld50 is around what, 30 grams ingested, so one really would have to shovel it to get dead, but compared to the other carbonates it is a quite a bit of hazard that I try to avoid if at all possible.

ironmonger, those brake parts are still intact, so it would require quite much work to get it to chips. So no thank you, plain carbon is a better option.

06-12-2013, 09:54 PM
ironmonger, those brake parts are still intact, so it would require quite much work to get it to chips. So no thank you, plain carbon is a better option.

Maybe they do thing different over there... but when one reconditions brake drums or less frequently disk brakes rotors, they are turned on a dedicated lathe and the surface is cut with a carbide cutting tool. The swarf that results is on the order of .040" cubic-ish pieces. Akin to pieces of sand

Just sayin'...


06-12-2013, 10:48 PM
Barium carbonate is a common case hardening accelerator. It's described in the ASM Heat Treat manual.

It's used in the Machinist Bedside Reader "Bullseye Mixture" (case hardening recipe).

I posted this thread back in 2008, asking about the toxicity of barium carbonate at high temperatures:

Barium carbonate toxicity?

Here's the first page of the ASM article:

http://i164.photobucket.com/albums/u15/rtgeorge_album/bariumcarbonate_zps6b7f3e9b.jpg (http://s164.photobucket.com/user/rtgeorge_album/media/bariumcarbonate_zps6b7f3e9b.jpg.html)