Friday, January 09, 2009

Applied Knowledge - Quenching

Abstract - The Ancient wisdom says you carry out the task 'just this way'. But how much of that is Science, and how much is smoke and mirrors? A discussion related to quenching of blades.

When I was just starting out in blacksmithing, I had managed to wrangle a job at an 1850's Canadian Settlement Era living history museum. You know the type, a collection of moved historic buildings (mainly out of river valleys after the infamous Hurricane Hazel in the late 1950's here). I had progressed past a trial period chopping wood, hauling water and giving lunch breaks. I found myself posted two days in the blacksmith's shop, then two days in the gunsmith's shop for my working 'week'. These were the days off for the primary craftsmen (who where actually skilled in their areas). Near the end of one day, getting on towards fall, an older fellow, a real codger, came into the blacksmith's shop. I had been forging out a small blade, a 'patch knife' out of a piece of old file. To that point I had made maybe a dozen knives, and felt I was slowing getting the hang of it. (Too little learning a dangerous thing?)
Anyway, I had got the rap down on what I was doing pretty well, as you are endlessly being asked what you are doing when working at a living history museum. He had come in just as I was taking the 'blood red' blade blank and quenching it for hardening (in motor oil).

'What you doin' there, boy?'
I replied some variation on the theme: old file forged to patch knife blade, just getting ready to harden, blah, blah...
'Whatcha usin'?'
Again some variation on 'high carbon, hard but not too hard, using oil, blah, blah... Followed up with 'It normally works for me...'
'Na! Ya should be usin' cow's urine. From a preg-ig-ant cow!'
The typical old fart scowl. I must have muttered something, and the old boy shuffled off.

At the time I thought he was just pulling my leg *. Later I get to thinking. Working on that integration of tool purpose, carbon content, and quenching, as applied to the suggested method .
Ok, urine is typically 5 % salts and nitrogen compounds. (Or some variation, but likely not too far off consistent - if you are collecting the stuff from your cows somehow and ending up with an average sample.) Do pregnant cows produce more consistent composition urine? Or different content? Of course the component chemicals in urine vary a lot depending on diet, but cows eat more or less the same thing every day. (Again not exactly true, but close enough for this tale.)
In theory (at least) there might be some ideal amount of nitrates or hormones present in pregnant cows than are not in not pregnant ones. Again in theory, there could be some chemical action possible between these traces and the metal itself during the quench. But face it, these could only be mere molecules thick, the metal is only at a reaction temperatures for mere fractions of a second as it cools. A single pass of polishing is going to wipe off any modified alloy that might result. (So forget any 'nitrate hardening' effects!)

But it did occur to me that this is just the kind of hoodoo that filled the world before science. How old was that piece of blacksmith 'wisdom'? Once a trade secret between father and son, master and apprentice?

There are a lot of these:
Urine from a redheaded girl (usually virgin).
Urine (stale) from pre pubescent boys.
Quenching in a still living human body (!!)
Aligning the point of the blade to north.

In a world where quality is based on repetition, rather than an understanding of base science, there is no way to separate the wheat from the chaff. You repeat an entire elaborate sequence 'because it worked last time'. There are sure to be some elements that are pure ritual, not technical at all.**
I do differentiate between Science and Accumulated Knowledge. Accumulated Knowledge comes from an endless series of trials and errors, repeating what is found to practically work. The Norse metalworkers were so obviously rich in Accumulated Knowledge. What we today recognize as the Science of Metallurgy is something that grows out of the Industrial Age and the Victorians. No Viking Age blacksmith would be able to talk about crystal structures and elemental content of alloys.

I have always been interested in the root source of legend. There is always some fact behind all these 'traditional methods' which seem strange to our eyes in the modern day.

So as a (fabricated) tale:

When the master swordsmith made a sword, he always did the following when preparing for the hardening quench:
Wait for a full moon night.
Make love to a red haired virgin girl.
He and his assistant drank 5 pints of dark strong ale.
Then they both pissed in the slack tub that would be used for quenching.

The Science is the amount of salt added to the fresh water from the urine. This remains pretty constant each time, due to the processing effect of the 5 ale. The salt content of the 'brine' effects the cooling rate of the metal, thus the resulting hardness. It results in the best balance for the metal used and the blade being created.
But the actual reason the beers were consumed the very first time was that the smith was nervous, and hell, the slack tub was right there. And hell, it worked last time.
It was a full moon the first time he made a sword, so what the hell, it worked last time.
The red haired virgin girl is completely incidental, but the patron pays, and the smith is ugly and generally doesn't get any otherwise.

I had to just look the exact numbers up here:
human blood has about 1% salt
human urine ranges up from there, but not more than just over 2%
natural salt water is roughly 3.5 %
(The things you can find on the internet!) Anyway, the water / urine / blood / sea water progression shows a roughly 1 % increase in salt content each step. Sure to have a noticeable effect on the quench rate. Of the four possibles, it is most likely that fresh water and blood will be the most consistent fluids.

Looking for that information, I also stumbled across a web site (Hong Kong Polytechnic University) that has a nice blend of the technical presented in a clear fashion:
'Heat Treatment of a Mold' by Chan Ka Man Carmen

The most useful part of the two graphs are the lines for 3 % Brine (red) and Tap Water (yellow). The other two suggested fluids, 1 % for Urine and 2 % for blood, will fall some place between those two. You can see clearly that the addition of even small amounts of salt effect both cooling time and very significantly, cooling rates. The second is important, because the rate will have an impact on thicker objects (for example axes).

But me, I generally use 'new' 10 W 30 motor oil (depending). Seems to work ...

* With a special note to a conversation I've been having with Sandy Sempel. He worked in a functioning blacksmith shop as a kid in the 1950's. He related that the apprentices would often make up the most outrageous tales in answer to the frequent 'dumb questions' that came out out of the mouths of people. He suggested that the 'preg-gin-ant cow' might have been one of those, and a test in its own way.
Sandy is behind the quite excellent Frojel Gotlandica Viking Re-Enactment Society web site. The site contains a large visual database of VA artifacts.

** A couple of examples:
I have been told by many Scandinavian smiths that they ALWAYS leave their hammers resting on the anvil after work. Why? The iron mass keeps the Elves from effecting the hammer. From English, tapping three times before starting, again when finishing. Why? To warn away the Devil / 'Wee Folk'. I have had both these tales told to me by many smiths from both areas, often after observing them doing the action and asking why.

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February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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