Deciphering timber surface marks to help determine the age of a mill
In the absence of primary source historical records such as maps, deeds, land surveys etc., architectural historians and preservationists often use physical evidence in a building’s construction materials to determine its date of construction. In the case of a timber-framed mill, the timbers themselves provide clues. Since the technology of producing lumber from trees has evolved over the centuries, it is possible to narrow the time period of timber production based on evidence found on the surfaces of the lumber.
The earliest method of producing dimensional lumber, generally from the 1600’s up until the mid-1700’s, was hand-hewing. This involved chopping off the bark and sides of a log with an ax to create a roughly planar surface, then smoothing further with an adz or draw-knife. This was done on two sides to create log joists, or on all four sides to make a rectangular timber.
Simplified drawings of the hand-hewing, pit-sawing and sash sawing techniques, by Eric Sloan (1905-1985):
Hand-hewing remained in practice well into the later period of sawing methods because it allowed for longer timber lengths. A long, straight tree trunk could be worked by hand into a timber beam, 30 feet longer or more. Most sawing methods limited the timber lengths that could be produced to the length of a mechanized sawmill shed itself. The visible evidence on hand-hewn beams is the chop marks of the ax, and the slice-marks of the draw-knife, on its surfaces. (1600’s to 1750 and later)
The earliest method of timber sawing was “pit sawing,” which was used from circa the mid-1700’s to 1800 or later. A long trench or “pit” was dug in the ground, deeper than a man’s height, and saw horses were placed spanning across it, where the log was clamped on them. One man walked in the pit, with another man above, and they used a double handled saw to push and pull the saw through the log to produce a timber. Imagine what a nasty job it was to be the “pit-man”. I hope that they traded places often! The physical evidence on pit-sawn lumber is the random angled saw-marks with no uniformity, on the surfaces of the sides. (Circa 1750 to 1800 and later)
From the late 1700’s to circa 1850, most mechanically-sawn construction lumber was produced by a water-driven straight saw blade, mounted vertically in the center of a wooden rectangular frame similar to a window sash. A gear with cogs moved the log forward on rails on the floor, in equal steps, while the sash saw slid up and down in the same coordinated steps, cutting on both the up and down strokes. The resulting saw marks, which date this method, are vertical lines on the faces of the timbers, equal distances apart (usually about ¼” apart). These are typically called “water-sawn” or “sash-sawn” timbers and joists. (Circa 1780 to mid-1800’s)
The method of circular sawing began post-1850, initially water driven and later steam-engine driven. In this method, a circular blade of about 3-feet diameter was rotated by the power source to cut through a log as it moved steadily along rails. The physical evidence of this method is concentric circular lines on the timber faces. (1850 to 1900 and later)
So, the manufacturing date of dimensional lumber, and therefore roughly the date of a building, can be narrowed down to one of these approximately 50-year periods of sawing methods, based on the observation of saw-marks. However, due to the slow change in the technology in some regions, there could be an overlap in the sawing methods of lumber for up to a quarter of a century (25 years). It is often possible to see dimensional lumber from two or more of these periods of cutting methods in the same structure, also due to the recycling of earlier timbers from former structures. Thriftiness was a common practice of early Americans!
By Daniel T. Campbell, AIA, Preservation Architect, Chester County, PA www.danielcampbellarcht.com