Preparing an Iron Smelt

I am throwing this up on the BLOG for a couple of purposes. Main reason is that I'm hard into preparing for iron smelting, have a tone of things to get done, and don't have time (or brain power) to get anything else posted up. My readers may also be interested in the amount of work that goes on to prepare for a smelt.

On Saturday (May 26) a small number of folks from DARC came up to help with the preparations for the June 9 smelting experiment.

Our objectives for this experiment are the following:

1) Re-use of existing smelter
The current clay cobb furnace was fired three times in 2006. It looks like with some minor patching the furnace will easily be able to withstand another sequence. We hope to match this furnace to the sequence of patching and re-use seen on excavated furnaces. Objective two relates to this process as well.

2) 'Hot Swap' - a double bloom firing
In the past we have seen that a large part of the charcoal consumed is used to bring the furnace structure up to operation temperature and to create the underlaying slag bowl. If manpower permits, we intend to extract the first bloom, then immediately re-fill the furnace and start straight into a second smelt sequence. A marked improvement in overall raw materials consumed is expected, but it is unknown how this second smelt sequence may develop.

3) Recording debris field
We intend to lay a clean layer of sand over our working area, and make a solid attempt to record both the work patterns and waste deposits that develop. Secondary to this will be collecting all the slags created to better compare total products against ore amounts.

Among may tasks to be undertaken before the smelt itself, preparation of the required ore is by far the most time consuming. On our last trip to Lexington Virginia, Vandy and I picked and packed out 130 kg of rock ore. This material is from one of two abandoned Colonial workings discovered by Lee Sauder and Skip Williams. This is the rock ore DARC has used for a number of earlier experiments, and also the material used for the EARLY IRON series of symposiums.

The first step in preparing the ore is to roast it. Neil and I set up a purpose built fire pit for this, to the left side of the existing smelter. The shallow rectangular pit was walled with brick and had a floor made from heavy steel plate. Richard assisted by chopping all the fire wood required for what turned out to be a several hours long burn. Layers of split hardwood, each roughly wrist thick, were alternated with a layer of rock ore. The fire was assisted with an air blast from our trusty blower.

This shows Neil pulling out the larger pieces of heated ore using the bloom tongs. The average pieces were roughly fist size, the largest maybe three times that. Ideally the pieces were heated long enough to bring them up to a dull red. In the fire they needed to at least show an obvious shift from red to a dull black. First the burning wood was removed and set to the sides. The hot pieces were then dumped into a metal bucket filled with water. The largest could be grasped with tongs as shown above. Smaller pieces were scooped out with a flat blade shovel. (The steel plate base made this easy.) The quenching not only cools the material for handling, but more important tends to shatter the structure of the stone. This means easier work in the next phase - crushing for size.

Meghan did the lion's share of the ore crushing - a process that took all afternoon. In the image above Sarah works away with her (and processed about a quarter of the ore total). Individual pieces of roasted ore are pounded with hammers on steel plates to break them down to the sizes required. This is roughly between 'rice to pea' size, with the dust retained. Any rock fragments obvious are also removed and discarded. Over the course of about 6 hours roughly 45 kg of crushed ore was processed.

Along with setting up some of the overhead covers for the work areas, the existing smelter was examined and repaired for the upcoming smelt.
The furnace has been fired a total of three times at this point. Once in June of 2006, then two firings one day after the other in November. The smelter was covered over with a sheet of metal for the winter, but otherwise was exposed to the elements. In the spring it was found that the layout of our new work area was not ideal, and about 5 cm of standing water was pooled at the base of the smelter. Earlier in the year the work area was trenched out and a drainage pipe was installed. Despite this, the smelter structure was found to be in remarkably good shape. It has slumped forward slightly, but there is only minimal cracking. This is assumed to be because the repeated high temperatures from earlier firings have sintered most of the clay cobb to a more durable ceramic.

Most of the repairs required were at the base, around the tap arch. The base of the furnace was deepened slightly, mainly to make it possible to insert a number of standard bricks to support the leaning front edge and create a new tap arch. Patching was done using rough clay, in this case the local 'blue mountain' red. The gap between the irregular base and the replaced bricks was then sealed with clay on both inside and outside surfaces. Next the interior of the smelter was repaired. The last two smelts had eroded the interior wall around the tyere in the pattern seen in past experiments. The wall thickness above the tuyere especially had been reduced to about half, its original - to about 2 cm. With the original contour of the wall restored, the existing ceramic tuyere now only extends about 5 mm into the furnace. This is the same tuyere tube that was used for both of the November smelts, it shows little erosion and will certainly be able to withstand the next smelt.
The last step undertaken was to move both sets of side support bricks forward about 10 cm, This puts the sides just clear of the tuyere on that side and just at the edge of the tap arch on the other. The spaces were filled with the typical smelter debris of charcoal, ash, earth and small slag fragments from the floor in front of the smelter.