Iron making evolved over a few thousand years. Using the ancient "bloomery" method, iron ore was converted directly into wrought iron by heating the ore while at the same time melting the ore's impurities and squeezing them out with hand hammers. This is also called the "direct process." By the 1100s water-powered hammers replaced hand hammers for forging out bars of iron.
In the late 1300s, some theorize that because of the ravages of the plague upon the labor forces in Europe, water power began to replace human or animal power to blow the air into iron making furnaces. Using water-powered bellows, a large and consistent volume of air created enough heat to completely melt the ore from which iron was made. The technology created two major developments in iron production. First, blast furnaces were now able to make cast iron for the production of hollowware such as pots and kettles. Second, in the new "indirect process" cast iron could be converted into wrought iron with a higher yield of iron from the ore than the direct process.
It is the indirect process that was brought to Massachusetts and spread through North America by the skilled metallurgists/iron makers that came to Saugus. With improvements, the direct process continued as well and a few of the second generation iron making plants after Saugus thrived in the rural economy using the older, bloomery method.
This page will guide you sequentially through the smelting, refining, forging, rolling, slitting, and blacksmithing processes that were performed at Saugus.
In 1646, the original blast furnace roared to life, lit with a 3000 degree fire that was kept burning 24 hours a day for months at a time. The blast furnace is where bog ore was smelted to create cast iron "pig" bars, so named because liquid cast iron was fed from a larger trench into smaller trenches as a mother sow to suckling pigs. To make cast iron, three raw materials were brought over the charging bridge and loaded into the chimney of the furnace.
Charcoal fueled a fire that burned hot enough to smelt the ore. Charcoal production was very labor intensive and required the work of many woodcutters, carters, and the colliers who oversaw the conversion of seasoned wood into charcoal.
Bog ore is an iron-rich sedimentary rock that was harvested locally from bogs and similar bodies of water. It was also found in fields and meadows that used to be bogs. Bog ore is often considerably less than 50% iron. The rest of the rock was made up of impurities that the workers had to remove.
Gabbro was used as a flux;a way to purify the ore. It was mined on nearby Nahant and transported up the Saugus River by boat.
A wood fire was started at the bottom of the furnace to dry out the mortar that was between the new lining stones and brick. Gradually at first, charcoal, iron ore, and gabbro were charged into the top of the furnace in layers by the furnace fillers. The "burden" as it was called, was carefully managed by the founder. The founder also was responsible for managing air flow from the bellows.
The burden was held in place above the crucible (where the molten iron was collected) at the bottom of the furnace by a narrowing of the furnace lining called the "boshes". Air was pumped into the furnace above the crucible but below the boshes.
Air, though invisible, was also a raw material and blew into the furnace using large, water-powered bellows. Oxygen in the air made the fire burn hot and (managed properly) created the appropriate conditions for carbon monoxide to remove oxygen from the iron ore. As the air passed upward through the burden, it first came across the charcoal. As the charcoal burned, the air converted to carbon monoxide. Carbon monoxide continued to rise upward. It latched onto the oxygen atoms in the ore and was carried further upward and out the furnace stack as carbon dioxide.
High heat that was generated by the fire caused the gabbro to melt and form a flux. Flux performed multiple functions. Since it melted at a lower temperature than the iron in the ore, it facilitated the flow of silicates and other impurities from the ore. The glassy flux also coated the iron as it melted. It formed a protective barrier between the liquid iron and the oxygen in the furnace gasses and prevented the iron from oxidizing away.
As a charge was transformed, the flux with its impurities descended past the boshes into the crucible. The liquid iron that was coated in flux trickled past the boshes, through the slag, and settled to the bottom of the crucible. The displaced liquid slag floated on top of the molten iron along with any unburned charcoal bits, ash, and other dross.
Iron was graded as gray, white, or mottled and was checked by fracture testing, that is, breaking the iron to visually inspect the way that carbon was interspersed through the cast iron. The crystallization that produced the various grades was deliberately controlled by the founder. With great knowledge and skill, he regulated ore, fuel, air, flux, and even cooling rate to create desired attributes in the iron.
The casting shed at the base of the furnace is where the cast iron and slag waste were removed from the furnace. Molds were specially prepared and awaited the molten metal.
Gray iron was poured into molds composed of a clay/sand mix "loam" to make cast iron cookware like pots, kettles, posnets and skillets. Molds had to be carefully dried to reduce the risk of exploding steam pockets when the moist mold came in constant with molten iron. Gray iron was also cast in the sand to make firebacks. When poured into the molds, it was necessary to separate the slag from the iron to keep the slag from being trapped in the iron. Cookware was finished by filing and cleaning before it was brought to the river for shipment.
Mottled iron was also cast into the sand in the shape of long bars. In this case the iron would be cast with slag and all and the slag would float to the top of the bars where it would break off. The iron then froze into heavy ingots or "pigs". Pig iron was an intermediate step in the making of wrought iron. Pig bars were dragged to the forge using oxen.
The slag waste solidified when it cooled and sometimes resembled glass. The slag was disposed of at the waterfront by dumping it over the bulkhead. Over time the slag pile grew. The slag pile remains today and when the archeologist was searching for the furnace, he traced the slag pile back to its origin at the furnace.
Refining into Wrought Iron
Workers in the forge converted brittle cast iron "pigs" and "sows" into malleable wrought iron by carefully removing excess carbon in two separate processes, fining and hammering.
Details regarding the original design of finery hearths have yet to be discovered. Typically, they were specially constructed with stone and lined with cast iron plates. It is possible that gray cast irons and white cast irons were both processed by positioning the iron plates and aiming the air draft from water-powered bellows. A charcoal fire built that was large enough to cover the end of a sow.
To refine cast iron into wrought iron, heavy pigs and sows were dragged from the furnace to the forge by oxen. They were positioned.in the finery hearth through an aperture in the side of the chimney. Rollers guided the sows into the fire where they were slowly melted. Long iron bars or "ringers" were used to manipulate the melted iron. The iron was melted iron was lifted up into the air blast over and over again until the carbon was sufficiently reduced. As the carbon content went down the melting temperature went up. Perhaps this was an indicator to the finer when the iron had reached the desired carbon content. The process produced more slag and it is possible that some slag may have been deliberately added to assist in the carbon reduction process.
The iron was removed from the finery hearth as a "loop". Excess charcoal was removed from the outside surface of the loop then the hammering began. The initial hammering was done with long-handled sledge hammers. Then it was dragged to the 500 pound helve-hammer for heavier blows.
Hammermen completed the wrought iron bars by forging them between the hammer and the anvil. The loop was hammered into a block or "bloom". From there the bloom was hammered systematically from its middle out towards one end. The bar would be re-heated numerous times in the "chafery hearth" to maintain a welding heat. The bar would be turned end-for-end in the tongs and the hammerman would draw out the other end of the bar, again from the middle, outwards.
In the loop stage, the iron was in the form of spongy mass of iron crystals with pockets of slag throughout. The hammering process welded and elongated the iron crystals. As in the blast furnace, slag acted as a flux to reduce oxidization while the iron was welded together. By working from the center outward, excess slag was squeezed to the ends of the bars. The result was the main product of the iron works, wrought iron merchant bars.
The majority of merchant bars were brought to the Saugus River for shipment to merchants or blacksmiths. Eventually it was the blacksmiths off site that would hammer the wrought iron into serviceable tools and hardware.
Making Flats and Nail Rod
Merchant bars were further worked to create other semi-finished products that blacksmiths could use. Contrary to the blast furnace and forge, little was recorded about the archeological foundations of the rolling and slitting mill in the early 1950s. Much of what is known about the rolling and slitting mill is based upon inventories and accounts of the original iron works and 17th and 18th-century engravings of similar machinery.
While the blast furnace and forge were wonders of chemical and metallurgical engineering, the rolling and slitting mill machines represent a relatively new application of precision in mechanical engineering. Sometime in the 1580s the use of gears (similar to what might be seen in grist mills or saw mills) was applied to rolling mills for the purposes of flattening iron.
The rolling mill consisted of a pair of cast iron rollers supported in a heavy-duty wrought iron framework. The machine was linked to waterwheels with iron couplings. The top and bottom rollers turned in opposite directions so that the bar iron could be pulled into the machine.
Wrought iron merchant bars were preheated in a cord-wood fired reverberatory furnace to bring the iron to a red/orange heat. When the iron was malleable, it was fed into the rollers. The torque of the waterwheels on the rollers created a high pressure and flattened the iron bars. There was likely a mechanism for adjusting the distance between the rollers so that flats of varying thicknesses could be made. Flat bar was shipped out so that blacksmiths would have the wrought iron stock to make wagon tires, axes, saw blades, and hinges.
Some flat bar might also be processed through the slitting machinery. Archeological finds provide evidence that the slitting mill made ¼" X ¼" iron rod for the purpose of making nails. The slitter machinery was made up of two square iron bars with cylindrical bearings. In the case of the ¼" slitters, ¼"thick steel [?] cutting disks and ¼" thick iron spacer disks were alternately stacked upon the square shaft and bolted together. A similar but interlocking set of cutters and spacers was assembled upon the other square shaft. These were also coupled to waterwheels and turned in opposite directions. Water was fed over the slitters to keep the precision cutters cool and properly heat treated. Iron flats were heated to a red/orange heat and fed into the slitters. The flat bars were pulled through the slitters and sliced lengthwise. Thus, a quarter inch thick flat bar passing through quarter inch slitters produced ¼" X ¼" slit rod.
It is possible that slitters may have been larger sized like 1" thick. A 1/4" thick flat that passed through 1" thick cutters would produce ¼" X 1" slit flats that could be useful for making horse or ox shoes.
Both flats and nail rods were semi-finished dimensional iron that helped a blacksmith save a lot of time. In a previous time, flat bar and slit bar would have been pounded into dimension by using a series of water-powered hammers in a "battery" or perhaps more commonly, using hand hammers.
Site of Joseph Jenkes Blacksmith Shop
The Joseph Jenckes blacksmith shop is where semi-finished product from the forge and the rolling and slitting mill were turned into finished products. Jenckes was an independent blacksmith that was tied directly to the iron works. He built his shop on the tailrace of the blast furnace and used its water power to run a hammer and wire drawing mill before the water returned to the river.
His hammer wheel was a small overshot waterwheel, that is, the water passed over the wheel. Cams were mortised into the waterwheel shaft and the cams struck the back part of a helve hammer that was supported in the middle of the helve. It is likely that it was a "tail helve" hammer that produced quick blows to take advantage of the heat in thin sections of iron. Between his hammer and anvil Jenckes hammered out axes, saws, scythes, and draw shaves.
To make an axe, flat bar from the rolling mill was first heated to the required temperature (indicated by visual observation to a bright orange) and forged either by hand or under his power hammer into a symmetrical butterfly shape. The butterfly wings were folded and hammer welded together. Since Jenckes was paid for "steeling axes" for the iron works, we know that he was welding a harder and more durable (imported) steel bit onto the wrought iron body of the axe. The edge of the steel axe would be forged to a wedge shape, ground, hardened, tempered, and sharpened. Hardening and tempering were specialized metallurgical processes that controlled the attributes of the steel. The tool was heated to a point where it was no longer magnetic (this could be done visually) and quenched in a special concoction of water or oil that may have been enhanced with other additives. The steel was now hard but very brittle. To control the brittleness, the axe (especially the steel) was tempered by slowly heating the body of the axe and watching the edge progress though a range of oxidizing colors. By taking the steel away from its heat source once the desired color was reached, the skilled smith controlled the hardness of his finished tool thus balancing hardness with durability for a particular function for instance cutting oak vs. pine wood.
Jenckes made hand saw blades and saw mill blades. He may have made them under his power hammer but it is more likely that e purchased rolled iron from the iron works. Until further analysis is done, it appears that Jenckes was using wrought iron to make his blades rather than steel. For a two-man hand saw, the ends of the blade would have holes punched that would allow riveted tangs to hold wooden handles. In the case of a mill saw blade, holes would be cut through the ends which the blade would be mounted in its water-powered, reciprocating frame. Jenckes' "new invented sawmill" may have been a way of cutting out the teeth in his blades. A triangle was cut from the body of the saw to form each tooth. Then, in either the hand saw or the mill saw, the teeth had to be "set." Each tooth had to be bent so that the cutting edge of the blade was just a little wider than the back of the blade. This would keep the back of the blade from binding in the "kerf" (slot) of the cut. Each tooth was then sharpened by filing. If a saw was re-sharpened, the blade was also re-set. The iron works paid Jenckes for making a "saw wrest", the slotted tool that was used to bend the teeth.
While in his 60s, Joseph Jenckes drew brass and iron wire at his Saugus shop. To draw wire thin strips of metal were rounded off and tapered at the end. The wire was passed through a "draw plate." The draw plate was made of steel with a series of progressively smaller holes and heat treated for hardness. The draw plate was held fast in a framework. The wire drawer used a special pair of tongs that were gripped by a leather strap and attached to a mechanism that pulled with great force. The harder the strap pulled, the tighter the tongs gripped. Brass wire was most likely pulled with a hand-cranked capstan or windlass to obtain mechanical advantage.
Jenckes' iron wire drawing equipment relied upon water power. An iron crank was attached directly to a water wheel. In half of the rotation, the crank rotated away from the wire drawer. In the other half of the rotation, the crank rotated toward the wire drawer. To draw iron wire was a matter of timing and rhythm. The wire drawer held the tongs and on the away rotation, grabbed the wire at its most distant point, close to the draw plate. The tongs bit into the wire and pulled the iron through the draw plate on the toward rotation. The drawer released the tongs when they went slack and grabbed the wire again. The movements were repeated over and over until the entire wire was pulled through the draw plate. The wire was progressively made thinner and longer by repeating the process through the successively smaller holes.
After one or two pulls through the plate, the wire would become "work hardened." This is similar to what happens when you repeatedly bend a coat hanger to break it off. The metal becomes brittle. To relieve the stresses, the metal wire was "annealed" to soften it. Iron wire was put into a charcoal fire and brought up to an orange heat. The fire was banked and the iron was allowed to cool slowly until the fire went out. The wire was the ready for a couple more pulls through the draw plate.
To draw iron wire, the iron had to be extra carefully made for the purpose. When iron was being refined, the process had to eliminate pockets of glassy slag for if slag was to get caught at the draw plate, the wire would snap. Jenckes petitioned the Massachusetts General Court for money to build a shed over his wire drawing operation. It is unknown whether he got the money. His intention was to use the wire for making fish hooks and parts for spinning wheels. In the archaeological investigations of his shop in 1952 the archaeologists found over 900 brass pins. To make a pin, two thicknesses of wire are needed. The heavier wire is used to make the shank. The lighter wire is wrapped tightly around the shank and a head is forged round with a very small set of precision "swages", each with a hemispherical cavity that is used to compress the head into a round shape. The pin then had a point ground on the end and it is likely that the pins were dipped in hot tin to keep them from corroding and to fuse the head to the shank. Perhaps one of Jenckes' biggest contributions In 1646 Jenckes petitioned the Massachusetts General Court to protect his intellectual property rights. He was about to build his shop on the tailrace of the blast furnace. The General Court recognized the value of having a blacksmith that could convert semi-finished goods into finished goods that would satisfy the needs of nascent New England industries such as farming (scythes), timber and ship building (saw blades and axes), and fishing (hooks).
Compiled and written by Curtis White, Former Supervisory Park Ranger at Saugus Iron Works National Historic Site, 2015.