Hi there people!
I'm preparing an Early Iron Age panoplia (which I'll show you soon, it's in its final stages), and defensive parts of it are made out of leather.
These are a couple of greaves, a body cuirass and a helmet.
While greaves will be left unhardened, and cuirass has already been hardened with bee wax, I'm now going to harden the helmet too.
The fact is, that this panoplia is a good occasion to test different methods for hardening leather.

These are the methods I know: using bee wax, using hot water (in different ways) and using heat.

While bee wax has proven to be the better way (the cuirass is now resistant to both moderate blows and rain), heat has proven to be the worst, for it had not changed the helmet's leather in any way.

J. Coles ("Experimental archaeology") suggest that you can harden your leather either by immersing it in hot water (80° C - 176° F) for about 30 seconds, or by scattering boiling water on it. While the second way does harden the leather, but does not make it resistant to the rain, the first method seem to give some results (not as the bee wax does), but it also sound of complex realization (I doubt that in the Early Iron Age people did know when the water reached exactly 80° C).

So we have:
- bee wax (successfully tested),
- heat (unsuccessfully tested),
- immersion in 80° C water (could work, not tested yet),
- using boiling water (doesn't seem a valid method, not tested yet).

What do you know about hardening leather?
Do you know any other method?
Give your contribution!
Greetings,
Mauro.

Hallo nemo, thanks for the quick answer!
I didn't know about sand, it sounds like a good method! It surely intrigues me, I'll test it someday!
I just put the helmet in a kiln, at about 240°C (540° F I think) for 2 minutes, as suggested by J. Coles in his "Experimental archaeology", but it didn't work, nor that seem to be a method available about 2600 years ago 

About your question: first of all, cuirass has been made using different leather than the one used for helmet and greaves. While these are made out of belly's leather, cuirass was made using the back of a bull. It's naturally harder than the leather taken from the belly, but it's a precise choice: back leather is difficult to cut due to its strength, so it's better to use large pieces of it, where cuts are limited, than to work hard to get small sections of it (helmet is done with 6 big sections reinforced by 6 smaller ones, and a circular couple that hold them togheter at the top and at the base - while the cuirass is just 2 big pieces of leather with 5 smaller pieces to protect the groin).

Said that, you're right, I used boiling wax, but didn't need that much.
You put some wax in a pot, wait it for boiling, and sprinkle the liquid wax on the cuirass using a brush, or a wooden spatula.
It saves a lot of wax, that you can use for sprinkling the cuirass again and again - even if it's not the amount of wax that hardens it, but how you sprinkle it (the more uniformly, the better).

More information about this panoplia will come on another topic, don't worry  
Greetings, and thanks again!
Mauro.

My only experience is with raw goat hide which i wanted to soak in warm water so after that i mount it on a drum's frame.The hide was soaking in water in a vessel put on the fire.I was having breakfast while chatting with others,so the water come close to boiling when i reacted that it got too warm The hide shrunk into an incredibly small,creased piece of hide.It was exhaustingly difficult to put it on the frame,it got very elastic,strechy but as it  shrunk so much it was hard to unstrech it.
I think the heat makes it contract so that's what it's making it harder,as it gets thicker and denser.

I used to brush a piece of leather with melted bee wax and next to warm it under a gas cooker in order to get the leather soaked deeply and evenly.

UPDATE:

I just examined the helmet and it has become incredibly hard 
I knocked on it with my fist and it sounds like wood.
It's still greasy by the way, perhaps a sand heating treatment (as nemo suggested) would have prevented this grease dispersion, since sand would have absorbed it. What do you think?

By the way, I'd like to make a drum, too, but the main impediment is to make the frame; jlasud, how did you obtain it?
Greetings,
Mauro.

Actually, I am following this.  I have heard of wetting the leather and forming it to a shape, letting it dry, then applying beeswax with heat to soak in the wax.  I have also read about the Plains Indians making their shields and breastplates from buffalo hide by steaming the green hide over a hole with red hot rocks and vegatation but the story was too vague to tell exactly how they did it.

Thanks!
That's exactly what I had in mind, but I was thinking of using some pine cortex for my frame - I recently found a log that was cut more than a year ago; its cortex came off easily, in 3 pieces, mantaining its curvature. I may try out as soon as I'll find some more leather!

About the helmet: I took it today, after having left it outside for a couple of days and night. The leather is now very hard, but the helmet has become too small for my head! It looks like a Jew kippah now, while it covered my ears before the hardening!
I'll keep it anyway, because I've no more leather to produce another one; I'll just add a pair of strings to tie it under my chin and hope that the helmet will deform a bit by using it 

So this's what I did to the helmet:

2 baths on almost boiling water for about 30 seconds each;
2 days and nights of exposure to strong winds;
2 fast "cooking" under a very powerful lamp until the grease burst and the helmet smoke;
I put some salt on it for the whole last night.

I don't know which, but one or more of the above actions granted the hardening; the only thing that went wrong was the constriction of the entire helmet.
I'm going to post some pictures soon!
Greetings,
Mauro.

In relation to how useful the helmet would be, it is 4-5 hits better than nothing

Leather helmets always used to be the cheaper option and so will never do quite aswell as metal ones, but I agree 4 hits seems a little off. I have a few friends who use leather helmets in our Dark Age re-enactment and they have survived plenty of hits. The leather is solid, like wood as you said, but it has dents in it from where they have been hit hard, so unlike yours it didnt shatter but bent.
I will try to ask around to see how those ones where made, as they really are great quality leather helms, but I dont know how well they would stand up to sharp weapons, but Im pretty positive they could take any 1 handed swing fine and within reason most likely two handed swings (still wouldnt do your neck much good though lol)

Nemo