sea shells as case hardener

I was reading a post by Charly the Bastard and he mentioned sea shells used to smelt iron. Would they be appropriate (ground up) for use as a case hardening material? A blacksmith friend of mine saves his fingernail clippings for case hardening special items.


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I'm an amateur but I've read a fair bit on metallurgy. I cannot see how sea shells could impart carbon which is what you are doing when you case harden. Rather, I would think that the sea shells acted as some kind of basic flux since they're made of lime, which is basic.

On the other hand, according to my old blacksmithing books, items were case hardened by placing them into an iron box in which they were surrounded by bone fragments or even powdered charcoal. The box, containing the parts, was then placed on the forge fire. Once the fire ignited the contents of the box, and after it burned out and cooled down, the case hardened parts were removed.

I fail to see how sea shells could be used for this purpose.

But the day isn't over. So there's yet time for me to learn something new today.


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That makes sense if they're made of lime, I was hoping they were carbon.

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re: the seashells

Calcium carbonate, that's the bulk of seashell. Seashells make limestone, given enough time and pressure. The primary purpose of the shells was as a flux to bind the mass together, but the carbon may have enhanced the ferrite into a crude steel by 30 or so points. Usually, bloom furnaces didn't get hot enough to get to force the large uptake of carbon that we see in modern pour furnaces. Charcoal just doesn't have the kick that coke does.

Pack carburization; burying apart in coal dust and baking it at the back of the fire, takes advantage of the 'carcon into solution' phase. As the temp comes up in the box, the part is bathed in a carbon monoxide-rich reducing environment, and carbon soaks in from the surface over time. Take iron strips, pack them in carbon dust and bake them for a few hours at medium red. Then weld them into a billet, make a sword. What you have is a high carbon skin over a low carbon core on each strip. Higher carbon lowers the fusion point temp, so the strips weld easily with less fuel. It was fuel that was the point of diminishing returns in antiquity, trees only grow so fast.

Why swords had names:

Think about just how many people were in line for their cut of the swordsmith's price. The smith, his apprentice, the two or three shop drudges, the charcoal burner and his crew, the smelter and his crew, the forresters, the teamsters (yeah, even waaaaay back then), the miners... the line's getting long. This many people sweat over somthing and it 'aquires' a personality, seemingly by osmosis. YMMV.


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Charly the Bastard

Lime is a useful material for many purposes and has a long history of production by burning calcium carbonate rocks in a lime kiln. Where oyster beds were found on the shore (the "sea shells" most likely to be found on these quantities) then they represented a high quality source of calcium carbonate and were often used instead.

There's a nice description of this in the T'ien-kung k'ai-wu (A 17th century Chinese compendium of technical knowledge - the Dover reprint is well worth reading) It also states [on oysters] "After a long period of time the inside melts to form a fleshy lump, called oyster meat, which is delicious to the palate".

Lime may also be used for the smelting of iron. It's little used these days, as a blast furnace is big and hot enough to "be its own lime kiln" and so the "lime" is added as the raw limestone instead. This has both a fluxing effect and also takes part in the reduction chemistry that reduces iron ore to iron.

Lime isn't used as a flux when smithing though. Even forge welding just isn't a hot or isolated enough process to use it as such, so something more active like borax is used instead.

Useful though it is, you can't case harden with lime. You need a carbon source and a reducing agent. Depending on how you do the process, you might need just the carbon source.

There are two ways to do case-hardening. One is to get a sealed iron box, pack it with carbon-donor and then heat it to a dull red in some sort of furnace. then leave it there for a few hours. This is the best process, as used in old industry.

Historically the favoured carbon source for this was finely chopped hooves or rawhide trimmings from a tannery. Bone has very little available carbon in it, so isn't useful here. Tanned leather might work, but a leather tanned with metal salts (most of them thee days) might cause its own problems. Charcoal can also be added (but not entirely).

The simpler process (the only one that's really workshop-practical) is to take a tin of cold carbon-donor, heat the end of your new chisel/whatever and stick it into the magic powder. leave until coold down, then repeat a few times. The problem is that because it's a cold and rapid process, it's very difficult to get the carbon on-board. You need some additional chemistry, typical nitrogen compounds including cyanides. Using plain old charcoal just isn't going to do a thing. Really, you need to find a tin of commercial Kasenit to make this work. Doing it from hoof and horn first principles is strictly for the dedicated re-enactor.

Industrially, case-hardening is now done with liquid tanks of ferricyanides. Better engineering through chemistry.

He should find a horse or cow instead. _Much_ more to use!

If he's a smith then he's probably a closet Norse pagan and is hiding his fingernails away so that Naglfahr "may be long in the building and low in the draught" 8-)

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Andy Dingley

Might be that the shells bring Oxygen as they bake and fluxing it inside the metal. Hard to say. Maybe the flux by itself keeps the carbon from burning off. The reverse of my first thought. Martin Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH, NRA Life NRA Second Amendment Task Force Charter Founder

snipped-for-privacy@tuckl> I'm an amateur but I've read a fair bit on metallurgy. I cannot see

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Martin H. Eastburn

When you heat calcium carbonate (limestone, shells) hot enough, CO2 is liberated leaving CaO, or quicklime. Hit that with H2O, and you get Ca(OH)2, or slaked lime. Leave that around long enough, and it'll absorb CO2 out of the air and turn into calcium carbonate + H2O. I don't think CO2 will carburize anything.

Pete Keillor

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Pete Keillor

I understand that, but wasn't bone charcoal used to case harden?

Why not? Powdered charcoal is ...Carbon!

Modern science and Kasenit are great, but do tell howcome soaking a part at red heat packed in charcoal dust anaerobically just won't work?

~Dave AKA "trims toenails often"

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Oooo... good questions. :)

I'll see what I can find out in this corner of the world.

Speculation: First bones are protien with "added minerals for stiffness;)". Don't know the weight percentage but the first statement is true IMO even if they are only 1% protein. You can take calcium out of a bone but you can't take the protein out and still have a bone.

Calcium phosphate is the main -mineral- in bones (so said my 8th grade health teacher;)

If: pure carbon doesn't soak into steel then it's obvious that the carbon needs a "push" or maybe an "activator". Cyanide seems to supply one or both of those to the carbon. What else can/will do that? I have no idea. :/

Ok, that's what I fiNger.

Just happen to have checked out from the library Metals Handbook Volume 4 "Heat Treating"...

The book's articles are all about "gases".

One of the interesting sections tho is on "carbonitriding".

Nitrogen and phosphorus act like carbon and boron when it comes to hardening steel two of the three together is pretty powerful stuff.

Metallurgy math: 1+1=4. :)

The old-timey;) methods packing the iron in animal products at high heat sounds like carbonitriding at least, to me, don't it you too? :)

The depth of carbon-case, in 16 hours at 1675F can be .040" and get .60% carbon.

I've always had the impression that's what the "real damascus steel" was... thin layers of iron with carbon (and nitrogen?) soaked into the surfaces of those thin layers (part way?) and since that was the outside, is later, the weld, and ends up on the inside so the welds are thin layers of high carbon steel. ??

I read where the iron they were getting from India had a little bit of vanadium naturally alloyed into too?


I see a hundred fires going all at once, burning day and night with various levels of partially finished billets soaking in fires with every spare animal product avaiable in those fires including shit.

Soak the outside surface in a carbon+nitrogen rich fire for a day and when retrieved, fold it, weld it and hammer it out (fold it the other way hammer it out again?) then it goes back into the stinky-ass fire to get re-charged with the "strength of fire" for a day or so.

The billets are brought to the "blacksmith" and his crew of two or three hammerers, the blacksmith would also be acting as a foreman positioning the billet on some sort of anvil and making decisions about how to go about it and when it needs re-heating etc.

Certain hammerers become blacksmiths.

Fire-tenders, billet-trackers at the fire, billet-couriers, blacksmiths (acting as foreman with a hammer-crew), finishers and handle-makers etc.

What do you think?

See something else to add? :) Subtract?

Alvin in AZ

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Charcoal from all sources was used, but needs to be mixed with an "energiser", usually barium carbonate. Steel can't be hardened by carbon alone, as this is effectively immobile. The active carbon is transferred in the form of gaseous carbon monoxide, generated by the breakdown of the carburising compound and the oxygen packed into the sealed box. This takes place with pure carbon, but unworkably slowly. Adding an easily decomposed carbonate such as barium carbonate breaks down to BaO + CO2 and this encourages the reaction C (from the donor) + CO2 2 CO increasing the overall abundance of CO and the activity of the carburising compound.

If it's anaerobic, it's going absolutely _nowhere_. Although the box is sealed, there is plenty of oxygen inside it. This is re-circulated through the CO cycle, but it's vital. The sealing is necessary to stop the CO either leaking out, or being oxidised to CO2 by excess outside air.

Bone contains some carbonates, but is mainly calcium phosphate (as hydroxy-apatite). This doesn't have the beneficial effect on encouraging CO production and it can also supply phosphorus as an impurity into the steel alloy. Bone can certainly be used, but not entirely.

The most effective nitrogenous compounds have more complicated chemistry which I can't claim to understand. AIUI, they can also produce some free cyanide CN as well as CO, which is a more efficient carbon transport. This is a different reaction from the liquid cyanide salt baths, which operate with sodium cyanate (NaCNO). This is noted for a fast carburising action, but a much thinner skin.

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Andy Dingley

Thanks for the expladissertation there, Andy and Alvin!


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