Wednesday, February 29, 2012

Bloom to Bar?

(Day 14)

Bloom iron has a physical texture and integral chemistry quite different from modern industrially produced steels.

Both of these are a result of the creation process, the reduction of raw iron oxide ore inside the direct process bloomery furnace. (see Day 11)
As the individual particles of metallic iron sinter together at the bottom of the furnace, they are accumulating inside a pool of liquid glassy slag. If you stopped this process early in the sequence, then cut open the mass, what you would find is something that looks like this:
'Proto-Bloom' - Early Iron 1, 2004
Preparation, Description & Images by Elizabeth Henricks

The iron starts to collect together with the matrix of the slag, almost like soap bubbles.
As more and more iron accumulates, the developing bloom will become denser and denser. At first the bloom is very lacy, almost as much slag as iron. This is especially the situation with furnaces utilizing low volume, low pressure air blasts.
The iron of course is considerably heavier than the slag. So as more and more is reduced and it clumps together, it presses out more and more of the slag. The ideal is to create a dense 'puck' of metal. The classic shape is 'plano-convex' - a slightly oval half sphere with a flat top :

Vinland 2- October 2009 (5.6 KG)
Side view, showing rough placement in the furnace.

Now, the ideal bloom will be very dense, but some of the slag always remains trapped inside the metallic mass:

Vinland 1 - May 2009 (4.9 KG)
Sliced in half, surface polished (upside down in photo)

The raw bloom thus will have both voids and inclusions of slag. The better the skill of the smelt master, the denser / more solid the bloom will be. Remember that historically, the objective of whole smelting process is to produce working bars of solid iron metal (!).

Replica Viking Age 'Currency Bar'
Forged down from bloom (November 2005 smelt)

This process, 'Bloom to Bar' represents an entirely different stage of the iron production sequence.

Individual raw blooms will vary considerably how lacy or solid they may be. The actual carbon alloy content is likely to vary, even across the same bloom (more on that later).

The process of forging a bloom down to a working bar involves compacting, cutting, folding and welding. It is to gain more direct experience with this aspect of the overall process that this OAC Grant is supporting.

Readers may be noticing a bit of disorganization between the individual postings (hopefully not too much *within* individual postings!). This is because my attempt to provide a daily commentary does mean I am jumping around in terms of actual subjects (rather than a concise continuing narrative). The topic of the daily posts most often does not reflect the physical work being undertaken on that same day.

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

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