Saturday, April 09, 2011

'Steel' strikers - some considerations (?)

This came in from my friend (and mentor) Kevin Smith of the Haffenreffer Museum of Anthropology (Brown University, Rode Island). Kevin has done considerable research into a number of pieces of jasper uncovered at L'Anse aux Meadows in the Viking Age occupation layer. These show signs of being used as the stone element for 'flint & steel' fire starter sets. Most importantly, these stones are not native to the region, and are in fact imports from Iceland. This discovery tied another of the Saga story elements into fact - that some of the ships that travelled to Vinland had Icelandic origins.

" I've been asked by Parks Canada to look at another jasper fragment they uncovered during recent excavations at L'Anse aux Meadows that were focused on the prehistoric native components. ...

In thinking about what to do with it next, I've been putting together a mental list of possible projects to do with students ... One thing that I know should be done is some more experimental work on how much debris is produced by striking jasper fire-starters with strike-a-lights, what kinds of debris are produced, and how to distinguish them from other kinds of flakes. "

This may be a much larger kettle of fish that you may realize. Your primary focus is on the stone - but the variation within the actual steel striker is a HUGE 'random' factor.

All our modern metals (as you know) are much different structurally than historic bloomery irons. Varying carbon contents then also gets back into the mix.

Image is taken from the Faganarms online catalogue:
" C866-1067. Found in York England. Identical examples have been recorded in numerous Viking settlements. 2 3/4” overall with scroll tip recurved arms and high peaked striker bar, demonstrating the skill of the forger. "

This is what (I think) I know about strikers and how they work:

The hard flint tears off a small piece of the metal.
The force of the strike actually massively heats this fragment.
The resulting temperature is enough to actually 'burn' this piece. (energetic reaction with the oxygen in the air)
The carbon content alloyed with the iron is what causes the visible spark. The more carbon, the larger and brighter the visible spark.

Now comes the tricky bit.
The exact alloy chosen needs to have just the right resistance to the shearing action of the flint that this tearing process results in 'sparks' (actually the pieces) that are hot enough to effect the tinder. If you use too soft a metal (modern, a 0.2 % mild steel), the hard flint will certainly tear the surface, but the resulting fragments are not heated enough (show no spark).
If you use a high carbon alloy, but harden (quench) the material too much, the flint can't actually tear any pieces off. Few fragments (sparks) produced, those that do occur are too small to retain effective heat as they pass through the air.
The balancing act is to use a carbon content *plus* correct cooling/hardening cycle to produce a surface that falls into the correct 'hard enough to tear with hot fragments' balance.

I suspect there is also an aspect of the physical structure within the stone itself. 'That side doesn't work' is an often heard comment. There are a set of variables that have to do with the actual shape of the stone edge. I don't have enough experience to know what exactly is required here, save to comment that the stone edge needs to be 'sharp' enough to slice off that sliver of metal, but at the same time not so thin as to have the stone shatter on contact.

To make this all even worse.
Individual pieces of stone are hardly consistent. Even within a walnut sized piece of stone, the effective 'hardness' can vary noticeably. Even the same 'type' of stone (read flint) does vary in hardness considerably between physical locations. (This is why 'English' flints are prized for use as gun flints over generally softer 'American' ones.) This effect is going to be even wider with differing types of stone (flint, jasper, ...).

I know from my own (limited) use of flint and steel that there is an effective pairing of a specific steel striker with a specific chunk of flint. Because the flint is consumed more significantly than the steel, you often see the situation where a once effective steel is suddenly considered 'used up' - because the flint itself has been replaced. Unfortunately, I have had the *steel* blamed more often than not for being 'poor quality' (which always makes the metalworker responsible!). "But I always got great sparks with my old steel" is often heard.

Back to the metal.

That theoretical description of how the metal is effected by the striking flint now has to be considered in terms of the actual iron metals used in the Viking Age. These are all bloomery irons, not our modern day Bessemer steels. As such, the historic metals have an entirely different creation process, resulting in a quite different physical structure.
Bloomery iron will always have microscopic layers of glassy slag as threads through the metal. These layers may create what are in effect shear planes through the block. As glass, those layers are hard, but extremely brittle. The number and size of such layers will vary depending on the quality of the individual starting bloom from the furnace. The smelt master would desire an extremely dense and compact bloom as a finished product, but a huge number of individual factors conspire together to determine just what qualities of the product of each individual smelt event might be. So any given bloom might contain quite differing amounts of glassy slag - and deposited physically quite different through the parent metal.

The way a given bloom is consolidated into bar might also have an effect. When starting with a dense bloom (with less contained slag in the first place) it may be possible to compress to working bar with very few compress / fold / weld steps. (I had one bloom piece that only took a single folding over and weld step to produce a quite nice working bar.)
Is there a factor related to just how this 'grain' is worked up during bloom to bar?
The simplest way, if the quality of the starting bloom allows, is to make long draws and folds to re-weld (long rectangle, single fold back on itself). This is going to create slag lines all running in the same orientation, down the long axis of the bar.
Most often, the fragmentary nature of the starting bloom does not allow for this. With flaws running in all directions, you end up dealing with diagonal cracks. The way to deal with this is to use a series of compressions and folds, each set at 90 degrees to each other. The easiest first step is to just flatten the bloom into a irregular plate of fairly uniform thickness. The first fold is then across this width (called a 'book' fold). This is welded, then the piece is drawn out to a long rectangle, at 90 degrees to the initial fold and weld. This rectangle is then folded in half and welded. Last the resulting 'brick' shape is compressed downwards to make another rectangle, again force at 90 degrees to the previous step. Fold and weld to a block. The end result is more or less a 'brick' shape. You can see how this would create a series of shorter slag lines, running in differing directions, but individually quite short.

Just to throw yet another element dealing with potential grain. Normally, the pronounced grain in a piece of bloomery iron (or historic wrought iron) is in long lines running down the length of a bar. (Think of the grain in a piece of clear pine lumber.) The normal process of making an object from bar is to form the shapes by stretching as required long the length. Strikers are more or less a C shape, the centre of the C creating the striking surface. This puts the grain running in more or less the same direction as action of the individual strikes. The net effect should (at least potentially) be long thin shavings.
Or is this what you really want (??). If you forged your source material quite deliberately, you could work the grain at 90 degrees to the direction of strikes. Admittedly, this would be a pain to do, and might also effect the quality of shapes on the two terminal ends of the C. (I don't actually think this would be likely, but thought I'd mention the theoretical possibility.)

Before we leave historic bloomery metal entirely, remember our variation in carbon content.
Its easy with our modern metals to specify alloy content, and get completely even distribution of carbon through the mass. Not so in either case with bloomery irons (despite what some modern knifemakers claim). Each smelt even will produce a bloom that will vary in potential carbon content. Each bloom will also vary in carbon within itself. The easiest way to roughly select for relative carbon (thus potential hardness) is via the 'quench hard, smash, then eye ball select' method. (This best documented in Japanese traditional methods.)
Given what was just discussed concerning slag content, it *may* be that the most effective carbon content would hardly be a standard, but might have to be considered in light overall quality of a given bloom.

My own modern experience with making steel strikers is hardly consistent. At first I followed the 'traditional' advise - which is to use an old file. Honestly, some of those worked well, others not very effective. Thinking back now, I can see the huge range in possible alloy types existing by using such unknown materials might easily have been a bit part of the problem. Better quality modern files are made of more complex alloys (Chrome, Vanadium, who knows?) beyond a simple approximate 1% carbon content.
I did spend an entire afternoon forging strikers, using differing metal alloys and variations in quench method with outdoors instructor Peter Ferri. After a number of not so great pieces, we settled on using commercial O1 (drill rod) stock, then oil hardening it from critical (measured with a magnet). This alloy and method produced the most consistent and effective results. At least for the batch of flints he brought (which yes, were all from the same point source.)

David Cox of DARC starting a fire using a flint & steel - L'Anse aux Meadows NHSC, August 2011

Placed back into our Viking Age context, I suspect what might REALLY happen would be this:
A given smith most often was working from a fixed geographical location. Although he did not (usually) smelt his own iron, I suspect access to iron bars would also be relatively local and regular. He would thus gain a direct experience with how best to work up his available source iron materials to a product most effective to the locally / regular source of stones used in combination. I can imagine him tinkering with his process against a given customer's personal stone.

Although there is a lot of speculation here, it may cast a new light on just what the Icelandic Jaspers found at L'Anse aux Meadows mean in the larger context of Norse material culture?

PS - Any readers who have better experience (or more effective!) producing steel strikers, PLEASE comment. Those with experience working with gun flints on actual antique firearms may actually have some important insights.

No comments:


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

COPYRIGHT NOTICE - All posted text and images @ Darrell Markewitz.
No duplication, in whole or in part, is permitted without the author's expressed written permission.
For a detailed copyright statement : go HERE