Group 2 - Max M, Morgan L, Zoe D and Peter H

Plan for a Modern Bloomery in Rhode Island

Intro and dimensions

During early March in Rhode Island, the area experiences frozen ground, prolonged and frequent precipitation, and low ambient temperature. Any sort of digging will be difficult to execute because of the hard ground and erode into a mud pit under spring showers. Likewise, shelter must be provided both for the fire and the furnace workers. A stack furnace fulfills these requirements most appropriately.

The stack furnace is to be a cylinder of two feet wide and six feet high. Height influences the rate of air flow, and a higher stack results in more drag losses of airflow and requires greater bellows input to maintain ventilation. Only four people are available to work the furnace, so the effect of input power needs to be maximized. Although larger furnace size enables a greater volume of iron production, it is at the expense of precision. The stack will be axisymmetric, tapering at the top and bottom, with double walls. The double walled design will trap a layer of stagnant air between the two walls, maximizing thermal insulation of the materials.

Materials/Thermo

The material technology of bloomery furnaces has seen little variation despite hundreds of years of cultural diffusion and technological innovation. Although the bloomery iron-making process is no longer a part of modern industry, the incorporation of new materials into bloomery furnaces would result in a drastic increase in their efficiency and durability.

The body of the stack or chimney must be resistant to thermal shock, as well as thermally insulating. The use of a homogeneous material like poured concrete will minimize thermal cracks that typically form on the interface of two different materials. Its homogeneity will also serve to minimize local thermal bridging, thus ensuring a uniform internal wall temperature. Although concrete has a very high compressive strength, ideal for building tall chimneys, vertical rebar should be added in order to increase its tensile strength under bending loads from wind.

Degradation of the tuyeres due to high temperature is often seen in bloomery excavation; therefore it is important to use a nonreactive material with high melting temperature in the construction of our furnace. Tungsten, which is often used in the aerospace industry for its favorable thermal properties, will provide a high thermal conductivity while being able to survive the extreme operating temperatures of the bloomery. Finally, the tuyere and the furnace itself should be made from materials that do not contain carbon since carboniferous materials prevent the ironworker from maintaining control over the composition of the bloom.

Preheating/Airflow

Air coming into the furnace will travel through a metal pipe that spirals downwards along the inside of the furnace. The entry point of the airpipe will be at an optimal height as to ease bellowing. As the incoming air moves from the cooler to the hotter part of the furnace, the air will be significantly preheated prior to its contact with the charcoal. The mounting of this pipe on the inside walls will create a spiral chute which will facilitate the upward movement of air inside the furnace as well as promote uniform circulation of the air. This pipe must be made of a stable metal with a high melting point.

Assembly

It is first necessary to mold the inner walls of the furnace with rebar-reinforced concrete, in halves. The airflow pipe would then be bolted to the inside of the wall before the two halves of the inner wall can be joined with more concrete. Then the outer wall of the furnace will be moulded in halves and connected around the inner walls. We will then bore a hole at the bottom of the furnace, out of which slag can be tapped. Then entire structure will be placed over a bowl-shaped depression in the ground.

Ore

Although mineral ores and bog ore are both readily available in Rhode Island, the feasibility of doing all of the heavy processing necessary to bring mineral ore to a smelt-ready state is questionable, and so bog ore seems like an easier material to utilize. We would collect it and then roast out as much of the gangue as possible. Finally we would mill it, by hand-hammering if necessary, down to the appropriate size for use in the bloomery. Bloomeries of very comparable size readily processed 30kg of ore per smelt (e.g. Senn et. al. 2010). This seems like a reasonable amount to start out with in our case.

Fuel Source and Ratio

Charcoal fuel will provide the high heat and fast burning that wood could not produce, without the unhelpful chemical inclusions in coal. We would prefer our charcoal to be in relatively large peices, so that it packs more loosely in the stack, facilitating even airflow. If no charcoal commercially available in Rhode Island suits our purposes we will produce our own. The high availability of hardwoods and softwoods nearby should make that fairly easy. Time and expertise permitting, the ideal would be to set up a distillation retort which would produce a higher charcoal yield than a pit or kiln and also produce salable bi-products such as acetate and wood alcohol (Gordon 1996). The charcoal/ore ratio is another important consideration. Since our bloomery is designed to have superior heat retention as compared to traditional materials, our fuel needs (and thus the ratio) would likely be lower. Since our dimensions were inspried by the idea of a scaled-down version of the American Adirondack bloomery described in Gordon and Killick, and the ratio used in that case was pushing the lower limits at 1.3:1 (1993, 261) as an experimental beginning it makes sense for us to choose of 1:1 ratio and fine-tune it on future smelts. The 1:1 ratio successfully used in a medieval-type stack furnace of approximately the same size as our design (Senn et. al. 2010) gives further support to this suggestion. Therefore the initial charge would be of 30kg of charcoal, although more would likely be needed for preheating main maintaining smelting temperature, so having as much as 100kg on hand would be ideal.

Works Cited

Gordon, R. 1996. Ore, Fuel, and Other Natural Resources, Ch. 2 in American Iron, 1607-1900, p. 27-55.

Gordon, R. and Killick, D. 1993 Adaptation of Technology to Culture and Environment: Bloomery Iron Smelting in America and Africa, Technology and Culture, 34(2): 243-270.

Senn, M. et al. 2010. Tools to qualify experiments with bloomery furnaces, Archaeometry, 52(1).