Blast furnaces seem alien, strange, and dangerous to us today. But in fact, they are a remarkably simple technology — albeit on a large scale — that is really no more mysterious than a fireplace or pottery kiln. They take iron ore, charcoal (or later, coal), and limestone to turn out iron for use by the community or for export and sale beyond the community. In order to understand the blast furnace, let's take a look at one built in Maryland in the 1760s. It was both advanced for its time and also representative of the traditional method of blast furnace construction. Looking at contemporary plans can let us understand the elements and design considerations that went into the structure. (Click on the images for a full-screen image.)

When determining where to put a furnace, the iron master needed a source of flowing water. Here you can see that the iron master has chosen to place it next to a stream he labeled "Guiny Falls." The falls gave the potential energy, or "head," of the water to drive the bellows that provide the blast.

On the furnace stack itself, he writes, "This being in the gully where the water stands 5' deep will not be much digging in the middle." In this he is referring to the typical need -- not needed here -- to excavate a great deal of earth to build the foundations of the furnace. Interestingly, although it is imperative to keep water from under a blast furnace's base, here he seems fine with building it in an obviously damp place. Either way, you can see at the bottom he notes it will be a 40'-wide furnace stack.

In designing the wheel, the iron master (or the millwright) wrote "Allowing 8 inches distance to [each] buckets [picture] an inch each. [I] think the Circumference 72 foot will take 96 [buckets]." "The slope of the above Buckets is 8 inches from the perpendicular at ye extreme part in ye Extreme part in the clere & 7½ at ye double[?] 7½" You can see therefore that the layout went from bucket to wheel, rather than the other way around.

When the millwright needed to build a waterwheel to run the bellows, he needed to channel water over it. If it was to be about 23 feet tall, he needed to dam the river upstream where there was at least 23 feet of drop from the top of the dam to the mill and furnace.

On this drawing, he has noted at no. 1 near the top, "the rock where the dam is to be" by Guiny falls, as well as a small side stream that enters the main river just below it. The water to the mill (or 'mill race' as it was called) therefore had to be piped or channeled over the side stream before it could run in a race cut into the ground. You can also see a small secondary dam that runs a race to the building numbered 3 which is labeled simply "The Mill" (presumably a grist mill) and the diagonal line on the right runs just a bit further to the furnace (off the map in this view.

Overall, the raceway to the furnace is noted as being 30 perches long in the trough over the side stream and 130 perches long to the furnace itself. A perch is an old-time unit of length measurement. Find out how long it was and then calculate the distances involved. Approximately what sort of acreage, therefore, does this mill and furnace complex cover?

This site plan shows the main square furnace in the middle, and the 22' diameter wheel drawn schematically at the top left. The L-shaped lines extending up and to the right from the furnace are the outlines of the wooden buildings typically built against the furnace stack.

The top area is labeled "Bellows house" where the wood and leather bellows that were run by the waterwheel (like at Saugus) would be kept inside out of the elements and blew air into the "Bellows arch". To the right is the "Casting house" where the molten metal would be tapped out of the "working arch" onto a sand floor and molded into useful objects of just small ingots known as "pigs."

In this closeup of the furnace, bellows and waterwheel, you can see the dark shaft coming off the waterwheel (here drawn flattened on its side) and the two projections on the shaft (called 'cams') that drive the bellows. Typically a bellows would be driving on the upstroke and then some heavy counterweights would compress them, forcing air into the furnace at an even rate. The two bellows would operate out of sync, so that a more or less steady rate of air flow would be achieved.

You can also see the footprint of the furnace, with the two large indents for the bellows and working arches so that the workers (and bellows nozzle) can get into the relatively small center area of the furnace itself. The sideways text reads, "A sketch of Guiny falls & Pertinations for the furnace &c."

The caption above relates to the cost of the race in the next picture and reads, "But as the race will require to be but narrow shoal vidz. at first and for some distance 3 foot wide & 2 deep or eighteen inches [it] will not be worth six pence per pearch so that article will come cheaper than Calder."

In this summation of the costs involved, the millwright has broken it down into categories:

• The Dam may cost £50
• Troughs at 13/6 per perch £20 5
• Digging the race and making good the same £20 5 [subtotal £90 5]
• 30 Peers of stone about 2 foot square to carry the troughs to the raising ground may cost about 10s per. £15 [total £105 5]

Remember that they are still pricing in pounds and shilling here. Find out how much £100 in 1765 is in today's dollars. Notice that he is being rough in his calculations.