The precursor to the blackboard was the school slate, a handheld tablet upon which students could write with chalk or slate pencils.1 Made of slate with a wooden frame, school slates had been used for hundreds of years in Europe and were starting to become more widespread in America in the late 1700s and early 1800s.
Blackboards arrived in America with George Baron, a military academy teacher from England. According to historian Stephen Ambrose, Baron began teaching math to cadets at West Point on September 21, 1801, and “illustrated his lecture by making marks upon a standing slate with a white chalk, thereby introducing the blackboard to America.”2 By the 1820s blackboards had spread to primary schools and colleges in Maine, Connecticut, and Massachusetts.
Not everyone cottoned to the newfangled blackboard. In 1830, during a math class, the fine youth of Yale College rebelled against having to describe a theorem depicted on a blackboard instead of using the traditional method of merely reciting from a book. The “Conic Sections Rebellion” resulted in the expulsion of forty-three of the ninety-six members of the class of 1832. Not content with simply expelling the mathematic malcontents, Yale administrators sent the students’ names to surrounding colleges alerting them to admit the reprobates at their own risk.
Teachers liked school slates because students could share and reuse them and teachers could bring them to exotic places. Some of the largest artifacts found at the Donner party’s final camp in the Sierra Nevada are pieces of slate, thought by archaeologists to be school supplies carried across the country by Tamzene Donner. In 1820 missionary Hiram Bingham helped introduce writing to the Hawaiian islands with school slates.3
Not all early blackboards were made from slate. Many were simply wood boards painted with black paint, later modified to a specially formulated application called liquid slating. Toward the end of the 1800s, a wood pulp and cement mixture known as hypoplate appeared. These blackboards suffered either from cracking; chipping; warping; absorption of oil, dirt, and water; or uneven wear, but they were inexpensive, at least in the short run. For the long run, however, nothing could match slate.
Blackboard slate and the smaller school slates came primarily from Lehigh and Northampton counties in Pennsylvania. Smooth, durable, and uniform, the slate took chalk easily and legibly, didn’t absorb water, and stayed straight and true. By 1905 the majority of blackboards sold in the United States were made of slate. Six years later, the Cyclopedia of Education reported on blackboards that “It is doubtless no exaggeration to say that [slate blackboards] . . . should be used [in] all brick, stone, or concrete buildings.”4
Although I did not know it during my youthful days of scribbling on the blackboards at Stevens School, I could not have used a better combination of materials for writing, at least from a geologic point of view. If I could have looked at chalk dust under a high-powered microscope, I would have seen a skeletal menagerie of countless single-celled marine algae. Known as coccoliths, they lived by the billions in the upper surface of the sea and were less than twenty micrometers wide. Some had shells shaped like steering wheels, others like pig-snouts surrounded by ruffles, and many resembled a hollow disc with radiating fingers. When these plankton died they accumulated in a white ooze on the seafloor and later lithified into chalk.
Pennsylvania’s slate also began as a muddy ooze deposited in an ocean, when rivers carried clay, silt, and sand out into a deep marine basin. The 450-million-year-old sediments first formed into shale, followed tens of millions of year later by metamorphosis to slate, under thousands of feet of rock. At present, up to seven thousand feet of slate beds make up the valleys and ridges around Pen Argyl, seventy-five miles north of Philadelphia.5
Chalk and slate are the peanut butter and jelly of the geology world. You can write on many surfaces with chalk, and other stones can write on slate, but no combination looks as good or works as well together as slate and chalk. It is a relationship that needs no special preparation or processing. Any chunk of chalk writes perfectly well on any slab of slate. No other pair of rocks has such an integral and elemental connection.
Slate has dozens of uses aside from blackboards, though none of the uses are grand or known for their exquisite beauty or elegance. Instead, slate became the most utilitarian of materials, the building stone you turned to because “it is non-absorbent and germ-proof . . . sanitary and easily cleaned,” as one early promoter described it. Slate doesn’t compress under a load so it works well as paving, floor tile, and steps. It also can be inscribed in great detail and resists erosion, making it ideal for grave markers. Hard and impermeable, it works well for desktops,wainscoting, and urinals.
We look to marble for its beauty. We turn to granite for its durability. We build with brownstone because the Vanderbilts did. Sometimes, though, we just need a rock that gets the job done. Nothing fancy or famous or beautiful. And for that, no rock surpasses slate.
If you had been born a hundred years ago, you would have rarely spent a day of your life without seeing slate. Your mother would have washed your clothes in a slate laundry tub, set you to rock in a cradle next to a slatemanteled fireplace, and chopped food on her slate countertop. Your father would have used slate if he worked at a leather factory, brewery, or printing shop. When friends visited, they could have tied their horse to a slate hitching post, stepped over a slate curb, walked over a slate sidewalk, and up slate steps to enter your slate-floored residence. As electricity in the home became widespread in the 1920s, your family would have purchased slate electrical panels, because they could be drilled cleanly, didn’t warp, resisted fire, and didn’t conduct electricity.
The years around the turn of the twentieth century were the halcyon days of slate, when it was the modern equivalent of plastic: ubiquitous and practical. You’d be less likely to encounter slate today, in a modern home. Slate floors and countertops have had a resurgence of popularity, but few people seem to want to go back to the slate laundry tub or slate refrigerator shelf.
Of all slate’s many applications, roofing is by far the stone’s most widespread use. In 1830 an estimated 50 percent of the homes in New York City and one-third of the homes in Baltimore had slate roofs. During the peak years of slate production in the early 1900s, 75 percent of all quarried slate went to roofing.6
No other natural roofing materials possess qualities as appropriate for roofing as slate. Fireproof, resistant to rain and snow, and able to withstand high winds, a slate roof doesn’t rot or get eaten by insects. Slate shingles don’t deteriorate in the sun. They don’t curl and lose their water resistance with age. They also look good and can last for centuries.
Some of the earliest archaeological evidence for slate roofs comes from Roman-era England. The eighteen-by-twelve-inch shingles resemble the bottom end of a necktie, with a flat top and triangular bottom. A single iron nail held the slate in place. Although archaeologists suspect that the use of slate roofing faded when the Romans left, by the 1300s slate roofing was on the rebound.
Slate shingles kept the elements out of the great estates of the landed gentry and the cottages of peasants. Court rolls and manor accounts reveal that slate had become so popular by the fourteenth and fifteenth centuries that the more desperate stole cartloads of it, possibly for resale, but also for covering their own homes. Other, more industrious thieves attempted to excavate slate illegally, usually from land owned by the local manor owner.
By the time America’s earliest colonists started to abandon England for the New World in the 1600s, slate quarrying and slate roofing were well established in Europe. No doubt most people who ventured across the Atlantic had seen a slate roof. Many may have known how to put on a slate roof and many more would have known of slate’s fire resistance and durability.
In 1662, in what may be the earliest building code in the colonies, Virginia governor William Berkeley drafted “An act for building a towne.” It stipulated that towns should be built with thirty-two houses, each made of brick two feet thick at the foundation, eighteen
feet tall, and covered in a roof of slate or tile. At least some Jamestown residents complied; archaeologists have found roofing slate from buildings constructed in 1663. Seventeen years later, after a fire destroyed eighty buildings and seventy warehouses in Boston, the General Court enacted a similar resolution requiring slate or tile roofing.7 Canadian statutes of the early 1700s also recommended the use of slate to cover buildings.
Officials forgot one little factor—few, if any, people could afford slate. Only a handful of slate-roofed houses are known prior to about 1750. They include Thomas Hancock’s stone home, well-known for its early use of granite and brownstone, and the Slate Roof House in Philadelphia, built around 1690, which achieved notoriety for its singular use of slate.8 In Canada about the only group who could afford the stone were the Jesuits, who clad a church, a college, and a convent in Quebec with French slate.
Slate for roofing in seventeenth- and eighteenth-century America generally came from Wales or England. Old World slate had several advantages over the domestic supply. Welsh slate cost the same or less and builders thought it looked better and was of higher quality. Transporting Welsh slate was also easier because it could be shipped from seaport to seaport, whereas a lack of trains or canals often made transport Of American slate challenging.
According to slate-roofing historians, the first commercial slate quarry in the country opened about sixty miles east of Philadelphia, near the towns of Delta and Peach Bottom, Pennsylvania. A Welshman discovered slate there in the 1730s, but operations didn’t start officially until 1785. To honor this quarry and its history, the Maryland Historical Society has erected a sign just south of Delta, on the Maryland side of the Mason-Dixon Line, highlighting this eminent fact. The sign also reports that Peach Bottom Slate was judged “Best in the World” at the “London Crystal Palace Exposition of 1850.” Never mind that the London exposition took place in 1851 and the only slates to win an award in London in 1851 came from Wales and Sardinia.9
After the initial Pennsylvania discovery, as well as ones in Virginia, New York,Vermont, and Maine, the American quarries lumbered along with little growth. Not until the 1840s did the slate industry start to grow. Slate historian and consultant Jeff Levine has cited three factors in the switch from Welsh slate to American slate. First was the spread of railroads, which lowered the cost of shipping. Second was the introduction of architectural pattern books by people such as Calvert Vaux and Andrew Jackson Downing, who advocated ornate roofs as a design element and stressed the beauty and colorfulness of slate. Vaux in particular promoted the use Of American slates over Welsh.
Levine’s research into the history of the U.S. slate industry showed that the most significant factor was Welsh immigrants, who became the backbone of the industry. They began to arrive in large numbers in the 1840s, driven by poor working conditions, poor pay, strikes, and food shortages in their home country. The Welsh immigrated to all of the American quarry regions, which is why Pennsylvania, Maine, and Virginia have towns with Welsh-derived names such as Pen Argyl, Bangor, Arvonia, and Bethesda.
Slate quarrying, like all other stone quarrying in the 1800s, was labor intensive and dangerous, but it required a new twist, one related directly to the geology of slate. All slate quarried on the East Coast originated geologically as shale that later metamorphosed into slate. Central to the story was the Iapetus Ocean, which spread east from North America around 550 million years ago. Adjacent to the continent, the shallow water teemed with life, which led to deposition of limestone. Farther east lay a deep marine basin, out of which rose a volcanic arc. Fine-grained mud washed off the mountains out into the basin through submarine canyons, periodically interrupted by earthquakes that sent coarser sediments into the water and deposited beds of sand atop the mud. Deposition occurred slowly, on the order of one inch every three thousand years, and lasted from about 540 to 420 million years ago.
Environmental conditions did not remain stable throughout the millions of years of deposition. During warmer periods, sea levels rose and the water became stagnant. Little oxygen reached the deep sediments, which slowed decomposition of organic matter and facilitated the accumulation of material such as plankton. Rich in carbon, these sediments turned gray to black. During cooler periods, ocean circulation improved. More oxygen mixed into oceanic waters and lightly oxidized the iron in the sediments, turning them green and purple. Animals burrowed into and churned up the mud, leaving behind well-stirred, homogenous beds with few depositional features.
Nor did the volcanic arc remain stationary. Plate movement carried the islands toward North America. As the arc approached it created a bulge in the seafloor, analogous to what happens when you push a carpet into a wall. Sediment deposited on the bulge accumulated in an oxygen-rich environment, which turned the mud brick red.
Not all of the slates along the eastern seaboard followed this exact plotline. Red slate occurs only along the New York–Vermont border, where the best beds of green and purple slate also are located.10 Slate quarried in Pennsylvania,Virginia, and Maine is almost exclusively black or gray, but within the thousands of feet of these sediments further variations exist.11 Thin bands of sand, called ribbons, appear periodically. Some layers are richer in quartz, which makes the layers harder and less suitable to use for blackboards. Others have richer accumulations of iron-bearing minerals, which can weather and change color after the stone has been quarried. (Slaters refer to these varieties as “fading” or “weathering.”)
To change to slate, the shale had to pass through the geologic equivalent of a trash compactor, getting squeezed and compressed. The walls of the trash compactor were the North American continent and a series of volcanic island arcs. As the arcs pushed into the beds of shale, the sedimentary beds began to fold, like when you hold either end of a piece of paper and move your hands together. Arc collisions happened 450 million, 410 million, and 370 million years ago. Squeezing not only compressed the rock by folding, it also drove out excess space filled by water in beds of shale. The new metamorphosed rock, slate, was harder and more dense than the original, unmetamorphosed shale.
Again, not all of the slates experienced the same level of metamorphism. The aforementioned “Best in the World” Peach Bottom Slate is harder than other slates because it was metamorphosed twice and at lower pressure and temperature, sort of like what happens when you slow cook bread and it becomes dense and tough. Its hardness led to its downfall because it cost too much to cut and shape. Quarriers determined that the best way to sell Peach Bottom slate was to grind it and use the granules on asphalt roofing shingles. You can easily determine hardness by wrapping a slate shingle with your knuckle; higher quality shingles ring instead of thud.
Squeezing also generated two additional and critical features of slate. Most important is the alignment of flat minerals such as mica within the beds. As the vise flattened the beds of shale, micas that were askew to each other began to rotate and align, perpendicular to the direction of squeezing. In addition, the heat and pressure from metamorphism dissolved parts of minerals, so that the grains’ longest axes now ran perpendicular to compression. Think of mineral realignment as creating a rock with an internal structure akin to a deck of cards. Geologists refer to this alignment of minerals as slaty cleavage.
Cleavage makes slate an unusual building stone. For other sedimentary rocks, such as limestone and sandstone, the critical factor is bedding, the layers of sediments that formed during deposition, because quarrymen exploit bedding to split or cut stone. In contrast, quarrymen rely on cleavage for giving slate its principal quality, the ability to be split into sheets. Cleavage also gives slate its name; slate is a corruption of the French word esclater, to split. No other rock relies on cleavage for splitting.
Bedding and cleavage affect the quarrying of rock in a similar way: Quarrymen exploit a zone of weakness to fashion a block of stone. The central difference between bedding and cleavage is that cleavage generates an almost unlimited number of planes with which to spl
it rock, because the split occurs between aligned minerals, whereas with bedding planes the split occurs between layers, which can vary greatly in thickness. If the bed is four inches thick, for example, it is hard to split the rock into two-inch layers. A quarryman can make a two-inch-thick layer but it requires cutting, a more machine-intensive process than splitting.
Bedding and cleavage reflect a fundamental geologic difference. Bedding is a first-order process. It develops during the original deposition of the rock. Cleavage is a second-order process. It develops after the rock originally formed, during a subsequent change induced by pressure and temperature. Metamorphosis also generates a secondary mineral alignment that affects quarrying.
During folding, elongate minerals such as quartz get reoriented and become aligned in the direction of the tectonic push, comparable to what happens if you slide your hand into a pile of toothpicks and they get pushed and turned parallel to your fingers. Known as sculp, this alignment gives the quarrymen a plane of weakness running roughly perpendicular to cleavage, as well as at an angle to the bedding planes of slate.
When the Welsh arrived in America in the 1840s, they knew better than anyone how to take advantage of cleavage, sculp, and bedding to work slate. They knew that in tightly folded rock such as slate, they needed to find a good bed, called a clear run, and follow it. Because the tectonic trash compactor pushed horizontally, beds folded vertically and clear runs often dove steeply into the ground, resulting in the narrow, vertical holes that characterize slate quarrying. To reach the rock, the Welsh introduced a hoist and trolley system that would allow deeper penetration than the prevailing dredge and crane. A trolley with a hoist ran along thick steel cables stretched between derricks on either side of the quarry. Men would be lowered into the vertical quarry on a wooden platform. The deepest known quarry plunged nine hundred feet into the ground at Pen Argyl, Pennsylvania. An apocryphal story goes that a quarryman once said that the pit was so deep that “you could see stars during the day from the bottom.”
Stories in Stone Page 23