Sunday, April 26, 2015

Ancient Metals = Ancient Fluxes ?

On 25/04/15 4:02 PM, Joel wrote:
   ...  Since in our modern world, we use mild steel to mimic wrought iron, there is a need for a wire brush.  What can be used instead as a Viking Age tool?  I've tried sand with limited success.  

What you are appearing to refer too is the forge welding process.
It is quite true that iron oxide scale itself will not fuse. The metal surfaces need to be made, then remain, as clean as possible.
What the addition of a flux does is two things:
1) Coats the surface of the iron to prevent contact with oxygen. No oxygen = no scale formation
2) Under the pressure of the hammer stroke, the (then) liquid flux squirts out - washing with it any dirt or scale that might have gotten between the surfaces to be welded.
Careful control of the fire oxidation / reduction atmosphere also obviously important to reduce potential scale formation.

Recent (say post 1855) practice with our modern steel alloys is to add a flux to assist with both functions above. (The best way to ensure the success of a forge weld is to start with clean bare metal surfaces!)
Here in North America, which has natural supplies of borax, it is borax that is the usual flux applied. This can either be chemically purified (water removed) borax, which requires only small amounts, but also is quite expensive. A lot of smiths (including myself) just use much cheaper 'washing soda' borax from the grocery store (like '20 Mule Team' brand). This is certainly messier - but only a tenth the cost.
The 'traditional' practice from England / Europe is to use a fine white silica sand as the flux. I fully admit that I personally have not tried this. You certainly would need much higher temperatures to get this material to stick, much less melt on to, the metal surface. Problem there is that oxide scale is starting to form * before * a modern steel has gotten hot enough to allow that same sand to fuse to, thus protect, the metal surface.  (One warning bell here - this practice likely pre-dates the introduction of our modern steels.)

Forge welding a piece of bloomery iron. (Image by Neil Peterson ?)
Ancient type bloomery iron especially, but also 'antique' (pre 1855) wrought iron have a quite different physical structure than modern steels. Both these materials always contain small amounts of glassy slag remaining from their initial smelting process. So in effect, both these materials are * self fluxing *, in so much as the glass contained tends to 'float' to the metal surface at forging temperatures. How much so would certainly depend on the initial purity (read quality) of the starting bar. A lower quality bar would contain more slag, which also makes it de-laminate more easily (but in combination should be easier to weld back together). This 'weld while forging' process is often over exaggerated by those less experienced with historic type materials. (You do * not * have to start * every * forging step at welding heat!)

We of course can never know exactly just how individual blacksmiths from ancient times might have undertaken specific forging steps. The pronounced grain in corroded bloomery iron objects can inform us about how the metal might have been folded and welded. On a microscopic level, remaining inclusions of certain slag compositions might suggest addition, and composition of a fluxing agent. It may prove possible to make some guesses based on close examination of 'hearth bottom' slags. Even collection and examination of tiny pieces of slag spattered out from the welding hammering itself.

On your observation on use of modern steel wire brushes to clean metal surfaces. Obviously there is no duplicate of this type of tool in the Viking Age tool box. I would suggest that the possible method would be to use a large 'whetstone', over the surface to be cleaned / welded, likely like modern practice of brushing, when the metal is hot. Remember that many of these whetstones are huge to modern eyes - imagine a block the size of a two by four as long as your arm! If made of a rougher sandstone, such a block would certainly allow for very swift cleaning of scale off a hot metal surface.

Viking Age whetstones - Ribe Museum - Ribe, Denmark.
It is important to remember always that our modern steels do * not * replicate the handling properties of historic metals. What may be easily accomplished with modern alloys and also modern coal / gas forges (and big anvils!) might be quite different than the working processes used with ancient metals and tool sets.

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

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