Friday, March 2, 2012

Why Bloom Iron ?- Four

The most obvious characteristic of bloomery iron is its distinctive

2011 - Bloom Iron Bowl - forged bloomery iron - L 20 cm

As was mentioned in the Feburary 29 post, one of the major differences between bloomery iron and modern industrial steels is that the formation process used in a bloomery furnace always leaves glassy slag trapped within the metallic matrix.
Just how *much* slag is mixed in with the metal from an individual smelt will depend on a large number of factors : type and quality of the ore, furnace design and working sequence. Loosely it is the experience and the skill of the smelt master which defines all the variables. Reguardless, the desired result of any smelt attempt is to produce the highest yield with the least amount of fuel expended - and the densest bloom possible.

Bloom Iron Bowl was forged from a one quarter section of a bloom created in 2005. In this case I wanted to retain as much of the surface and edge texture as possible. For that reason, the cut section from the parent bloom was not subjected to the normal compressing, folding and welding steps. Since the ideal working bar would retain as little slag as possible, there might be several courses of these steps in the 'bloom to bar' process.

Cutting the 7.5 KG bloom
Shot with available light to show the metal heat.

The starting piece was a rough pie shape, with two edges the cut surfaces of the inner portion of the bloom. The other edges were the more ragged external surfaces of the original bloom.
By taking this segment , then basically hammering it flat down into a plate, all the slag inclusions and the bloom's starting ragged edges were retained.
The next step in the process was to take the two more or less flat surfaces of the plate and polish them roughly smooth with an angle grinder.
Once this was done, the plate was again heated to forging temperature, and worked with ball surface hammers over dishing forms to create the overall bowl contour.
The last step was to use a wire brush to clean the fire scale off the inner surface of the completed form. This polished the metal to 'bright', thus raising the contrast between the metallic and darker slag pockets.

Bloom Iron Bowl is an excellent example of the special physical composition characteristic distinctive to bloomery iron.

I should mention that this specific aspect of bloom iron - the possibilities of the unique texture - was first illustrated to me by the work of Lee Sauder

Detail of Tension Deficit by Lee Sauder

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

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