Seth wrote to Don Fogg's Bladesmith Forum:
I have seen tuyeres adding to or helping to reduce wall erosion. What causes this and what am I looking for in a tuyere to help prevent it?
Right to start, we all need to remember that the exact working of any individual iron smelting furnace is a complex interplay between a very large number of variables! Your individual application of possible raw materials (ore, furnace, even charcoal) may certainly modify this basic level description. (!!)
Mike McCarthy proposed a working model for what is happening inside an iron smelting furnace during a late night discussion at Early Iron 1, back in 2004. I think his concept still stands up in the light of continued observations and experience:
|McCarthy's original diagram - 2005|
|My original notes on the discussion - 2005|
|Air Flow in a Working Furnace|
|Effect of VOLUME|
With low volume air, the core of the furnace may well be hot enough. But the individual ore particles may not have enough time at temperature and reaction chemistry to reduce. They may well fall to the bottom of the furnace without ever hitting an effective reduction zone.
|Effect of PRESSURE|
This is why using a rotary blacksmith's blower is often not found to be very effective for an iron smelting furnace. You may have large volumes - but with almost no pressure, the air simply does not penetrate the furnace.
Most hand powered bellows systems will certainly produce adequate pressures. Many European experimenters will use stones placed on the top plates of their bellows to control with fair consistency their air delivery pressures.
We have found that the ANGLE of the tuyere is critical to an effective iron smelting furnace.
|Effect of ANGLE|
If you place your tuyere too steep, you move the air blast so it will be pointed directly at the developing bloom. Since you have oxygen at high temperature, it effectively acts like a cutting torch, and slicing your bloom apart as fast as it accumulates.
|Effect of Angle on Bloom Formation|
The next variable on the tuyere is how far the tip is inserted proud of the interior furnace wall by 5 cm / 2 inches. This helps to move the effect of the upper lobe of the torus of air away from the furnace wall.
Just to mess that element up, the material that your tuyere is made of will most certainly effect its durability over the life of the smelt:
Although using a piece of simple mild steel pipe will work - you are certain to find it will burn away quickly, moving the hot air to flow back against the furnace wall and start to erode it.
Ceramic tubes are considerably more durable to furnace temperatures. Although these will burn back as well, our experience is that these endure long enough to prevent excessive wall erosion.
Experience has proved the most durable tuyere material is heavy forged copper. Lee Sauder has used the same copper tuyere for dozens of smelts will virtually no damage.
Putting the whole package together, you get something that looks like this:
|Overall Diagram of Working Furnace|
Making the furnace body a flask shape, instead of a simple cylinder is one possibility. The ideal appears to be increasing the diameter at tuyere level to by the same amount that the tip of the stands clear of the furnace wall. (In effect a total of plus 10 cm beyond the diameter at the top.)
A second possibility (developed by Michael Nissen) is to insert a relatively thin plate into the heavier furnace wall - around the tuyere point. In practice, the highest temperature on the wall, thus the most erosion effect, occurs over a rough oval, 10 cm below and to the sides of the tuyere, and extending about 15 cm above it. The 'bellows plate' system relies on the atmospheric cooling off a thin cross section, in the range of 1 cm / 1/2 inch from a plate installed in that area.
Of course I do have to warn everyone that the working systems described above are most certainly not the only way to produce iron in a bloomery furnace!
A lot of combined experience has shown these elements do combine to make a very effective small scale bloomery however.