Sunday, October 18, 2009

On Larger Charcoal Fires?

This came in as a response to one of my video clips on YouTube - most specifically to 'Forging a Viking Age Broadaxe'

...How come your forge is so effective at getting your metal to welding heat? I blacksmithed for 5 years as a teen, now starting again (forced to use charcoal this time) and I've went through two forges, the latest, a side blast design, that gets *all* the charcoal forging hot, but I STILL after all these years cannot get metal up to white hot. I was doing better at getting to welding heat years ago with coal than now, but that's not saying much. I'm thinking possibly my blower might not be strong enough, though it looks it... (90 cfm) i was so sure this time my forge design would be good enough, but it's not - any tips?

First - Coal gets hotter than charcoal will.
Second - the actual heat zone (ball of heat) with coal is normally larger with coal than with charcoal
Thats somewhat dependant on the set up of your equipment!

Side blast is the usual for any charcoal fuel, as the ash quickly blocks the air flow if you attempt to use a bottom blast as you would for coal.

Remember that there will be a penetration problem with air into a charcoal fire. The heat is created through the interaction of oxygen and carbon. Charcoal is not a dense, so the volume of charcoal needs to be larger to include as much carbon as you would need with coal. Working against that (effecting air penetration, are the sizes of the individual pieces of charcoal). So although it may seem that more air is the answer, in actual fact there is a break point where you just can't get the air across the pile of charcoal. The more or less cold air you dump on the tuyere side ends up effectively cooling the potential heat zone as you increase the blast.
Multiple tuyere points is the traditional solution to increasing the size of the heat zone in charcoal (or really big coal fires for that matter).

My own experience with single tuyere charcoal fires (admittedly with lower volume Norse double bag bellows) is that these create an effective heat zone about the size of a grapefruit (about 4 inches diameter) Obviously equipment design will effect this. For comparison, my working coal forge has a deep rectangular fire box, which gives me an effective maximum heat zone almost the size of a volley ball (about 8 inches diameter).

One of the nice side applications of this knowledge is that you can often spot objects (especially in twisted sections) that where originally forged in charcoal fires by just this shorter heat zone. In twisted sections, there will often be a distinctive tightening and loosening of the 'threads' over those same four inch sections - the mark of work done in the smaller charcoal fire.

Roughly 1830, Fireplace Crane at Stirbridge Village, MASS.

Use of heavy square bar a bit unusual. In this case you can clearly see the effect I describe above. (This image scanned from a slide, sorry about quality.)

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

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