Historical Markers
Colebrookdale Furnace Historical Marker
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Colebrookdale Furnace

Hershey/Gettysburg/Dutch Country Region


Marker Location:
Pa. 562 near SR 2040 SW of Boyertown

Behind the Marker

Photo of a cast iron Colebrookdale stove Plate which features the words "Colebrookdale Furnace".
A cast-iron stove plate made at Colebrookdale Furnace, c. 1765.
Colebrookdale Furnace set the mold for subsequent charcoal blast furnacesin Pennsylvania. As the first, it proved that British charcoal blast furnace technology could be used successfully in the colonies.

markerThomas Rutter built a bloomery forge in 1716 to make iron, but he knew that a charcoal furnace could make better quality iron more efficiently than a bloomery forge. Financed by Philadelphia merchant investors, Rutter built Colebrookdale Furnace about 1720 to supply iron to his nearby refinery forge and supersede the bloomery forge. He named the furnace after the leading English iron furnace in the early eighteenth century–Coalbrookdale. When he constructed the furnace he replaced his bloomery forge with a refinery forge that could work iron from the furnace. In 1725 Rutter reorganized and recapitalized the partnership, bringing in new investors. The owners in turn leased the furnace to Thomas Potts, who became the furnace's resident manager.
Diagram of a typical cold-blast, charcoal furnace.
Diagram of a typical cold-blast, charcoal furnace.

The Colebrookdale Furnace employed the basic technology found in all charcoal furnaces.   Until the late 1830s, a charcoal furnace consisted of a flat-topped pyramid or stack, usually about twenty-five feet square and twenty-five to thirty-five feet tall, with a hollow chamber inside. The heavy stack consisted of quarried stone or brick to support the inner chamber. The bottle-shaped chamber contained the iron smelting, so it was lined with sandstone that withstood high temperatures.  (Fire bricks later replaced sandstone as the lining). The upper end of the chamber, called the tunnel head or throat, was usually about two feet in diameter. As the chamber descended, it gradually widened to about nine feet in diameter near the middle. From this widest point to the crucible below was the bosh, the heart of the furnace where the smelting occurred. The chamber then narrowed to crucible at the bottom, about three feet in diameter and five to six feet high. The crucible extended from the bosh to a sandstone hearthstone that formed the floor of the chamber.

At least two arches distinguished exterior faces of the furnace. These arches spanned openings into the crucible. An arch on one side of the stack enclosed a pipe that ran from the exterior through a hole, the tuyere, to the crucible to admit compressed air into the crucible, essential to raising the temperature of burning charcoal high enough to smelt the iron ore.

During the 1700s water-powered bellows blew compressed air into the pipe. After 1800, blowing cylinders or tubs replaced bellows. On another side of some furnaces, a second tuyere arch provided an additional air blast. On the front of the stack, a work or cast arch spanned a large sandstone block or tymp stone that separated the upper part of the crucible from workers. A dam stone located below the tymp stone had two holes plugged with clay, which held back the molten iron and slag until it was time to tap the crucible. An open space or cinder notch between the dam stone and tymp stone allowed workers to remove large cinders that formed in the slag in the crucible.

This invoice letter head reads: K. Grim, President, C.L. Keely, Secretary, O.W. Sabold, Treasurer. Established 1835. Incorporated 1880. Colebrookdale Iron Company. Manufacturers if Hardware, Fine Quality Gray and Japanese Castings. Dated Pottstown, Pa., April 5th, 1897.  Special Line of SADIRONS and TAILORS IRONS, Mrs. Potts SADIRONS. All agreements are subject to Strikes, Accidents, or Delay from causes beyond our control.
Colebrookdale Iron Company letter head invoice.
Charcoal furnaces operated continuously after they were started or blown in. Workers loaded charcoal through the tunnel head to fill the inner chamber. Next they ignited the charcoal and as it burned and settled dumped more charcoal in until they had a large mass of glowing coals inside the furnace. They then loaded alternating layers of iron ore, charcoal, and limestone.

The iron in the ore became molten as the temperature rose to 1,100-1,500 degrees centigrade. Because molten iron is heavier than the charcoal or limestone, it settled to the bottom of the crucible. The limestone served as flux, which united with impurities in the ore to form slag that floated on top of the heavier molten iron. When the founder, who directed the operation, decided it was time to tap the furnace, workers unplugged the top hole in the dam stone, drained the slag off the top, and left the slag to cool and harden off to one side. They then drained molten iron through the bottom hole in the dam stone into molds shaped in an adjacent sand floor.

 Diagram image of a stone furnace stack and the water wheel-powered bellows that blows blasts of air through the hearth.
Diagram image of a stone furnace stack and the water wheel-powered bellows that...
Workers continued loading iron ore, charcoal, and limestone and tapping the furnace for as long as possible, often around the clock for weeks or months. They often operated a furnace for nine months a year, shutting down or blowing out during unbearably hot summer months or in winter when the water wheel froze in place. A charcoal furnacenoted one observer, "was an impressive sight when in blast. The intermittent roar of the forced blast could be heard a long distance away. From the top of the furnace stack a stream of sparks was occasionally emitted as the flames rose and fell. At night the almost smokeless flames cast a lurid glare upon the sky which was visible for miles around." 

Operating a furnace to make high-quality iron efficiently took considerable skill, judgment, and experience. The founder had to monitor the process and judge the quality of the iron by sight. For example, he evaluated the quality of the iron, impurities in the iron ore, and conditions in the bosh from the texture and color of the hardened slag. Workers also had to repair the furnace, sometimes closing down the operation unexpectedly. They routinely had to replace sandstone linings and fix or replace worn-out parts. If workers did not operate the furnace properly, they could damage it, leading to more repairs.

Colebrookdale Furnace continued to operate after Thomas Potts's death in 1752. By then, the furnace had consumed the nearby forest and was drawing lower-quality iron ore from the deposit that supplied it. The furnace was blown out for the last time just before the American Revolution. Although it came to an early close, Colebrookdale Furnace played a significant role in the early development of iron production in Pennsylvania.
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