Air inside a Smelting Furnace

(This altered from a posting to Don Fogg's Bladesmith Forum)

Hannes N., on 12 June 2012 - 10:49 AM, said:

We where mostly successful... some experimental runs with ores of low quality or with an effort to evaluate the influence of one or the other paramenter on the process failed.
Gallery

Here in Ontario, my team (DARC) has been working backwards towards Viking Age styled air delivery systems. As well as using vacuum cleaner blowers (at least three different ones) we have used replicas of the side by side VA blacksmith's bellows, plus two different oversized versions built just for smelting. A couple of times we have attempted using rotary blacksmith's blowers (usually as an emergency when electric blowers failed). A couple of years back I invested in one of the same high volume industrial blowers that is Lee & Skip's standard. I did build a monsterous prototype mechanical box bellows at one point. (For the English here, I've even got a 'Bouncy Castle' blower, but frankly have never used that one!) The only system I have not worked with directly has been the classic 'Great Bellows' style. I have worked around the team from Colonial Williamsburg (Shelton & Steve) when they have been using one of those to good result.

Because my group here is interested in Viking Age iron smelting, possible air systems based on that historic period are of great interest. We have worked a couple of smelts using the 'bellows plate and blow hole' system that Michael Nissen of Ribe introduced me to. Our normal is an insert tuyere. On that we have used ceramic, steel pipe, and most recently the forged copper type that Lee introduced.

We have kept spotty records through all this of air delivery volumes and pressures. I rigged up an in line fitting that goes between the air control gate and the tuyere. This is not the most accurate, but at least in comparison gives us some numbers to work with. The volume is measured via a simple aneonometer (normally used by wind surfers and the like) and an older pressure gauge. Lee had shown me using that kind of wind speed measure (which he had used for his earlier experiments). All our values are being calculated / translated back into standard values (Litre per Minute & mm of mercury)

http://www.warehamfo.../flowrates.html

One of the things I personally have found a bit frustrating in reading reports by other experimenters is that people often do not give any kind of estimates (or even descriptions) of the air systems they use. As you are experiencing, not only how you set up the tuyere, but exactly what size and kind of blower / bellows system you use can make a huge difference in the actual results in a smelt. We all dwell on the variations on ore, but it is safe to say that *air* is perhaps more critical. (Even with a good ore, the wrong air set up or volume / pressure can drastically effect your results!)

Neil Peterson and I have been collecting data, and hope to pull enough together for a formal paper.(Theoretically for the ICMS at Kalamazoo next Spring ???)

Mike McCarthy had postulated (way back at Early Iron 2 or one of the first Smeltfests) that the air into the furnace makes a torus shape. The lobe at the top bigger than the one at the bottom due to heat effects. With a tuyere set horizontal, this hot air washes back onto the upper wall especially to errode it. Tipping the tuyere angles the torus so it sweeps out over more of the interior of the furnace volume. The size of this torus is determined by the volume of air. The penetration of the torus into the interior of the furnace is determined by the pressure.

You can see how this is effecting smelts with low volume and pressure. The air more or less 'bounces off' the charcoal contained in the bottom of the smelter. This limits the size of the 'hot spot' inside the furnace to just against the front wall. Much of the ore falling down the inside ends up at the rear wall, too cold to really add mass to your developing bloom. The effective slag bowl is smaller, and again mainly laying against the front side of the furnace. The developing bloom is itself smaller, and with less reduced and sinter temperature ore available, tends to be lacy in structure.

With lower air you would be consuming less charcoal. Your overall yields of ore into bloom are much lower. Anything you gain in terms of saving charcoal is easily lost from both lower yields - but more importantly from how much more work is needed to effectively compact that lacy bloom into a working bar.

Don't mis-understand. I'm not saying you can not get iron produced with a lower volume smelt. So many people (primarily in Europe) have proven that over and over. The few surviving artifact blooms however look much more like the dense 'puck' blooms that are the result of our high volume air smelts. Something is going on there!

We think there are some big potential cultural implications around the whole question of effective air systems for Viking Age smelting. With so little (or vague) archaeological evidence, this might be more an interesting speculation than anything else.