Wednesday, October 13, 2010

Realistic Yields for Bloomery Furnaces

50% yield is more in the ball park.
I have often been underwhelmed with the ore in vs bloom out...and then you forge it to bars and get less still.
The less "solid" blooms have charcoal and waste and unrefined ore so the final yield can be quite low.
(name withheld, as this comment is kind of out of context!)

There are no absolutes in the realm of iron smelting furnaces. So much depends on the nature of the ore employed. Air volumes are a huge modifier, even with identical furnaces. And of course the current Early Iron experimenters are using quite a number of different furnace designs, inspired from different national traditions. Most of us with depth of experience tend to stick to pretty much one pattern of materials, furnace and methods and through trial and error we get most (?) of the bugs worked out. (Not too surprisingly, this is exactly the same as what ancient iron masters did.)

That having been said, are are some observations:

As with any large equipment employing high temperatures, there is a very clear time + volume = efficiency element in any bloomery furnace. Regardless of size, it is going to take a certain amount of raw materials (thats charcoal and ore) to 'kick in' the interior environment before any significant bloom formation is possible. Increasing ore additions above this base amount just serves to add more mass to the developing bloom. Simply put, a larger ore volume smelt almost always results in a significantly higher per cent yield.
In my own work with Norse styled 'short shaft' furnaces, it normally takes about 8 kg of ore (see below) to effectively create the required internal conditions for bloom formation.

The iron content of the ore is extremely important. The thrust in recent North American iron smelting has been to *production*, so often high iron content ores (hematite grit, industrial taconite) are being employed. Natural ore bodies (primary bog ore, rock limonite ) can vary considerably, and rarely reach the same iron concentrations. You can only get out what you put in ! (all things being equal).

One last important consideration, as taught by Sauder and Williams, is air volume. Without large air volumes, effective yield will be directly impacted. This effect has been demonstrated consistently. A secondary impact of the air (both physical design of the system and volumes available) is on the density of the resulting iron mass. A lacy bloom may still have good yield numbers, but it will most certainly suffer significant losses in the bloom to bar phase of the overall process.

I have only observed traditional Japanese method (or modern adaptations of the basic method) a few times. All my own work has been with variations on Northern European systems. Over a good number of personal smelts and observation of other's work, I'd suggest the following:

Small volume smelts (20 - 30 kg) with 'good' ores (60% + Fe) : Yields in the range of 25 - 35 % are typical
Larger volume smelts (45 - 50 kg) with good ores : Yields as high as 45 - 50% are possible.

I'd suggest that for anything over 20% return (bloom from ore), of course 'depending', you should not be embarrassed by the result. Get 35 % and you have done a pretty good job. At 45 % you might be right to brag a bit.


(Image is the 'Redemption' bloom - November 2006. 19 kg ore (mixed 'Lexington' rock with gangue from earlier smelt) to 6.8 kg very dense iron = 35% yield)
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