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Company Keenan Brothers
Company Keenan Brothers
This mill at the Bangor Superior Quarry is full of orders and working steadily 10 hours a day. The mill is turning out 10,000 ft of blackboards and structural work a month.
This mill at the Bangor Superior Quarry is full of orders and working steadily 10 hours a day. The mill is turning out 10,000 ft of blackboards and structural work a month.
Engineering and Mining Journal, Volume 67, 1899
Roofing was the largest commercial product being produced by the slate industry by a significant margin, but it was not the only product being made from slate . . .
Renowned chemist, geologist, and longtime slate manufacturer Albert Masters of Pen Argyl was responsible for installing electronically operated chalk boards at Princeton for Albert Einstein.
"Slate suitable for blackboards must be soft and fine grained. Such slate is obtained from what is known as the soft vein region of Lehigh and Northampton Counties Pa. The soft vein is the northern slate belt which includes the region in and about Bangor East, Bangor, Pen Argyl, Danielsville, Slatington and Slatedale. This comparatively small area, not over 22 miles long, comprises the best if not the only good blackboard slate deposits in the world. Because of their smoothness uniformity permanence and attractiveness slate blackboards are superior to all other types of blackboards now in use."
"Slate suitable for blackboards must be soft and fine grained. Such slate is obtained from what is known as the soft vein region of Lehigh and Northampton Counties Pa. The soft vein is the northern slate belt which includes the region in and about Bangor East, Bangor, Pen Argyl, Danielsville, Slatington and Slatedale. This comparatively small area, not over 22 miles long, comprises the best if not the only good blackboard slate deposits in the world. Because of their smoothness uniformity permanence and attractiveness slate blackboards are superior to all other types of blackboards now in use."
Oliver Bowles, The Technology of Slate, 1922
Oliver Bowles, The Technology of Slate, 1922
Mill Stock
A common general term for slate which was used for structural purposes was "mill stock". This included such diverse types of uses as is listed below. In fact the industry was always trying to find new and creative ways to use slate to a point where they offered awards by placing promotional contests in magazines like Popular Mechanics.
Slate could easily be milled into common rectilinear forms in the mill (hence the term mill stock) which were strong and very decay resistant, and any use that could be discovered was exploited. Aside from roofs, slate also found extensive use in the form of heavier slabs for building in such uses where strength and density were important, but probably the most important property was resistance to abrasion. Although ribboning was sometimes seen as decorative, slate that was exposed to severe wear, as in floors or steps was most often best if hard and uniform, a benefit found at the Chapman quarries. The presence of hard veins, or ribboning, was undesirable because they would cause uneven wear but slate from the Slate Belt region was ideal because the beds were so thick that ribboning was easily avoided.
A promotional section in Popular Mechanics magazine from August 1924
On the left a book from 1882 entitled Jack's Slate about a mischievous boy who likes to draw. Slate production for school slates was in full production at the time and dominated the school writing system to the point where books were written about the topic.
Below A Non-Exhaustive List of Common Commercial Uses
1) billiard table
2) beds and mantels
3) fire-boards
4) register frames
5) radiator tops
7) platforms
8) tiles
9) wainscoting
10) moldings
21) blackboards
22) mangers
23) curriers
24) slabs
25) imposing stones
26) grave boxes
27) grave covers
28) headstones
29) grave markers
30) vault doors
31) water tables
32) belting courses
33) counter tops
34) brewers vats
35) greenhouse shelves
36) chimney tops
37) dough rising boxes
38) flour troughs
39) well caps
40) soda water fountains
"The Westinghouse Company is reported to have acquired a tract at Slatedale in the heart of the Lehigh slate region for the erection of a large factory for the manufacture of switchboards. Slate switchboards are an important factor in the electrical industry. A factory now located at Slatedale saws these boards sends them to Chicago for boring and then they are shipped back to Reading Pa. for finishing. Under the plans of the Westinghouse Company the entire process from the rough slate to the finished product will be accomplished in the new Slatedale factory. "
"The Westinghouse Company is reported to have acquired a tract at Slatedale in the heart of the Lehigh slate region for the erection of a large factory for the manufacture of switchboards. Slate switchboards are an important factor in the electrical industry. A factory now located at Slatedale saws these boards sends them to Chicago for boring and then they are shipped back to Reading Pa. for finishing. Under the plans of the Westinghouse Company the entire process from the rough slate to the finished product will be accomplished in the new Slatedale factory. "
“Notes From the Slate Quarries.” Stone Magazine, Mar. 1915: 146. Print.
“Notes From the Slate Quarries.” Stone Magazine, Mar. 1915: 146. Print.
Electrical Uses
Slate was found to be ideal for electrical uses because it was strong but also non-conductive. the most common use of slate in electrical applications was for switchboards which were similar in function to today's breaker panels. Although marble was actually a better mineral insulator, slate had advantages over marble due to its superior toughness, ease of workability as well as lower cost. Not all slate was created equal though. The requirements for electrical slate, aside from strength and workability (drilled and sawed without scaling) was uniform composition (usually determined by a fairly uniform color) and most important of all, high electrical resistance which depended on the absence of magnetite and metallic veining. Electrical slate was limited mainly to the products of those deposits which had high insulating values. A slate of high absorption is not apt to prove satisfactory for this purpose unless it is thoroughly dried and treated with some preparation to prevent the entrance of moisture. Strength and toughness are important properties, but the final test is that of electrical resistance, and this test it of little value unless applied to the entire panel. The producers of switchboards often had appliances for conducting such tests. To test the slate for conductivity a special apparatus conceived by Robert Notvest, who worked for the Structural Slate Company, was often used. This consisted of a transformer, a milliameter (an instrument for measuring electric current in milliamperes) and wires with brush like terminals. A wire was applied to one side of the slate slab while a second wire was brushed over the surface. The Milliameter was then used to identify any current that was being carried in the stone.1
Marbleizing
There was a certain irony to marbleizing slate although it was cost effective. Due to its dense small grain, slate was a material that was easier to work than actual marble, and since it was inexpensive, it was an ideal material to modify to look like other materials. The process used a baked enamel paint which created a durable exterior surface finish and increased the number of uses of slate considerably. Slate was seen as a plain utilitarian stone but with the application of marbleizing, items traditionally reserved for finer quality stones could be manufactured from marbleized slate such as columns, clock cases, church alters and even candelabras. The process of marbleizing was done in two ways. One was by hand painting requiring a fair amount of artistic skill, but the other was though the using of a "float". Different oil paints were sprinkled onto the surface of a vat of water to create the desired effect and the slate was then immersed into the water transferring the paint onto the stone.
"In the marbleizing process it is susceptible of considerable ornamentation which makes it more desirable still for many of the above uses and also extends the list of its uses as follows: Table tops, stand tops, card receivers, checkerboards, door plates, signs, and paper weights. Just a few of the types of stones that could be imitated with the marbleizing of slate were gray granite, Mexican onyx, fossil limestone, Devonshire marble, Tennessee marble, blue agate, red serpentine as well as both real and petrified woods."2
For mantels, table-tops, etc., and all kinds of marbleized work, the softer varieties of slate were preferred; they could be sawed with both circular and band saws, and were easily planed and rubbed. For tiling and other uses in which the slate was subjected to considerable wear the harder varieties were employed, with the slate from the quarries at Chapman especially applicable for those purposes.
Listed as "All original and completely intact 19th century American Eastlake style marbleized slate interior residential fireplace mantle with richly colored polychromatic finish" on the Urban Remains website.
Local Vernacular Uses
Slate was used locally, in a comparatively rough state, for sidewalks curbstones hitching posts underpinning cellar walls and doorsteps, as well as to build houses.
Image Source: Pennsylvania Geological Survey
Vernacular use of slate in Northampton county as fence posts.
Image Source: The American Contractor 1918
Walls for bathroom stalls were a common use of slate.
Pool Table Tops
The J.M. Brunswick Company, the largest manufacturer of pool and billiard tables in the world, originally obtained all of their slates from Pennsylvania but have not done so in decades. The video above shows the modern process of quarrying slate and finishing it for table top use.
Blackboards
For blackboards, slate had few competitors until the advent of dry erase boards in the late 50's and early 60's, as well as green chalkboards, generally made of porcelain enamel on a steel base, which also first appeared in the 1960's.
Making blackboards was similar to making roofing slates in that both were split in similar ways. Blackboard manufacturing required much more care though in the process since the slabs were generally larger, some of which could be considerably larger. Once they were split, the slabs were sand rubbed on a rubbing bed and then polished with a buffer. An earlier practice of hand planning was also used. For this a sharp blade with long cutting edge was used.
Once planed, the slabs were then laid out for rubbing using water and sand. the rubbing bed was a flat circular piece of cast iron from 8 to 10 feet in diameter which revolved horizontally on a shaft. Sand was added to a trough above the rubbing bed which had one end slightly lower than the other. The lower end was placed close to the central revolving shaft, and by adding water to the trough the sand was deposited onto the rubbing bed near its center. Due to the centrifugal force of the revolving rubbing bed, the sand would be gradually worked outward to the edge of the bed where it would be washed off as silt. The slabs of slate would be placed on these revolving beds and held in place by stationary arms or timbers which extended across the bed. Slate for the chalk boards often required a finer polish so the slabs would be rubbed using find sand or emery afterwards.
While most slate blackboards have in recent years been either replaced by whiteboards or eliminated all together, prior to that there had been a method of enameling sheet metal which also eroded the industry. There is no question though of how important the slate blackboard was to the United States and the world as a whole. For perspective, it was noted in the September 1908 issue of Stone Magazine that the largest single order of slate blackboards ever recorded, had just been shipped out of the Lehigh region through one dealer consisting of 12,000 square feet. Typical large, single sheet blackboards like those used in classrooms throughout the country, were 4 feet by 10 feet or the equivalent of 40 square feet. This one shipment alone would have provided enough slate for 300 blackboards!
One of the slate blackboard factories in Slatington was located on Factory Street and can easily be see in this 1913 Insurance Map. The structure still exists.
School Slates
When referring to slate, industrial journals of the day would often refer to slate in two principle categories. Roofing slate was the biggest market and was seen as its own category while the remaining slate fell under the listing of "other uses". Of these other uses, blackboard material, including school slates, made for nearly half that subset for the entire United States with production of school slates amounting to more than 4,000,000 a year, most of which was exported. The Slate Belt had almost exclusive control over the entire blackboard and school slate industry, which was not due to marketing and monopoly, but instead was a result of the quality and type of slate available in the region. School slates required a soft gray color with smooth cleavage and extremely uniform color and composition, and the slate from the Soft Belt was perfect.
Certain beds of the Slatington-Slatedale district furnished the most desirable material for the use of chalkboards and the qualities that fit, made it unfit for roofing slate because the original black color was not permanent, instead fading when exposed to sun and rain. To make a school slate, the typical sizes being 4X6 inches and 9.5 X 14 inches, each piece was first sent through a machine called a beveller which creates an incline on the edges. Following this the slate was then placed in a second machine known as a shaver where it was forced between two knives that were set apart to the desired thickness. For the next step the slate was passed between two paper covered cylinders which rotated quickly in opposing directions and work as buffers. Finally the slate was grooved to provide lines if desired (these lines were treated with red lead to allow them to stand out). Finally the slates were washed and framed and them packed and shipped, many being sent to Europe and beyond. School slates had become such an important part of the industry that by 1878, there were at least 9 school slate factories in the area of Slatington which combined were producing 3,300,000 individual school slates a year!
The above table shows the different steps taken to finish slates for school slate use after they were split, as well as the total number of people required for one set of machinery capable of turning out about 20,000 school slates per day.
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