Swimming the Plate

"Swimming the Plate" from Pilklington: An Age of Glass by TC Barker, 1994.

Every once in a while, as you are answering reference questions at the Rakow Research Library, you stumble across a word or phrase which, while well-known to a handful of collectors, historians, or artists, will be a mystery to those of us who generally skim the surface of the vast field of glass history. As a collector of crazy words and quirky phrases, I love those moments!

I ran across some of those peculiar terms the other day, when exploring plate glass manufacturing. Perhaps this is not the most exciting-sounding topic you’ve ever heard of, but, like many glass subjects, it is one that draws you in once you start taking a closer look.

Rolling the Plate, Pittsburgh Plate Glass Company Trade Catalog, (about 1900).

And where else can you read about swimming the plate, treading, pugging, teeming and burgy banks? Some of you may be wondering, “What IS plate glass exactly?” Plate glass is basically sheet glass that has been produced at a very high quality and then ground and polished to a smooth finish. It gained popularity in the 19th century for store fronts, tabletops, windows for wealthy homeowners and for businesses, as well as for use in mirrors. It was strictly a luxury item, due to the laborious and expensive process required to make it.

Grinding the Plate, Pittsburgh Plate Glass Company Trade Catalog (about 1900).

In the late 19th century, the process was this: molten glass was poured onto large iron tables, rolled flat, and then placed in an annealing oven to cool. Upon emerging from the annealing oven, the glass was shifted to a grinding wheel to remove all roughness. Finally, the glass was polished to a smooth finish, cleaned and packed for sale.

It sounds fairly routine, but when you delve deeper into the process, you begin to understand why plate glass was so expensive to make. First, consider that the glass had to be melted in pots made from a material that wouldn’t break off or melt into the glass batch and mar the plate glass. And the pots needed to be large—typically holding about a ton and a half of glass. A 1916 booklet published by the Hires Turner Glass Company describes the pot-making process.

5,000 pots at 3,000 lbs apiece, stored in a single factory for "seasoning" from Glass: history and its universal application (1923). Pittsburgh Plate Glass

Trade Catalog for the Hires Turner Glass Company of Rochester, NY ( date unknown)

First several different clays are mined and left to weather, often for longer than a year. When the clay is ready to be formed, the different types are mixed together and kneaded or “pugged” in a mill. Then the clay has to “ripen” for up to another year’s time before it can be worked. Next comes the part I love the best—after it has ripened it requires “treading,” which is exactly what it sounds like—workmen walk on the clay until it develops the plasticity necessary to form the pots. According to the Hires booklet, “No machinery has thus far been invented [which can replace] the primitive treading by the bare feet of men.”

Once the pot is formed, it needs to dry anywhere from several months to a year. Doesn’t it sound like the process for making wine? Unbelievably, then, the pot is only usable for a period of twenty-five days! Pots took 3 years to make, so factories stored acres of them in order to insure continuous production of the glass. Talk about an expensive manufacturing process…

Moving the Plate Glass, Pittsburgh Plate Glass Company Trade Catalog (about 1900).

Once the glass was melted in these laboriously-created pots, it was lifted with 20 foot long tongs and placed in a “pot wagon,” which was pushed and carried by about 17 men to the casting table.  Most of the 17 men were practically riding the tongs to counterbalance the weight of the giant pot of glass.  At the casting table, the pot was lifted by a “teeming” crane and swung over the iron casting table where “the pasty” or cooling glass was poured and rolled flat. Workmen would then use a “stowing tool” to push the glass into the annealing oven to cool. The cooled plate was hoisted and placed on the giant grinding wheels which had been coated with water and Plaster of Paris to prevent the glass from being chipped or marred during grinding. Some larger plates weighed over 1,400 pounds, requiring many men to carry. Once placed, it was squished down into the plaster by a group of workmen who would walk round and round on its top in a process is known as “swimming the plate.”

Royal Visitors Travel around on a Grinding Plate at the Cowley Hill Plate Glass Works in 1913. Pilkington Bros Limited Edition Souvenir Booklet.

Now the glass was moved to the polishing table where it was polished with iron peroxide (rouge) and water. The rouge would quickly coat everything, so that the polishing machines themselves appeared to be red. The plates were necessarily washed many times during the production process as they were caked, first by Plaster of Paris, and then by rouge. By the end of the process, up to half of the glass would have been removed and a whole lot of waste material created. Called “burgy,” this waste from the grinding and polishing had to be dumped somewhere outside the factory, so the surrounding area of the factory would have hundreds of acres of “burgy banks,” piled high with glass waste.

Continuous Feeding Polishing and Grinding Machine, from British Glass. Pilkington Bros (1924)

For all of these reasons, plate glass was a luxury product, and its elaborate production was of great interest to the general public. One well-known company, the Pilkington Brothers, even received a royal visit from the King and Queen of England to their plate glass factory on July 8th, 1913, and published a souvenir booklet in celebration of the event.

Not everything was manually accomplished—steam was used to power the grinding machines beginning in the late 19th century and eventually the process was even further automated once electricity became available. By the 1920s, the molten glass would flow from a spout onto the casting table, rolled flat into plates, then fed through the annealing oven to a grinder and polisher in one continuous process, requiring less physical effort from workmen but a substantial amount of factory space. Plate glass factories seem to stretch for miles when you look at old drawings and photographs.

Val Saint Lambert depot in Lyon, France. Emmanuel Lejuene, Opérator photographe (1900-1920). Val St. Lambert archive.

So when you see those old photographs of beautifully decorated art nouveau shop windows, imagine the incredible journey that one plate glass window took from a dust-covered factory floor to its pristine resting spot and marvel at the equally elaborate technology that made that possible. And don’t even get me started on all of the crazy glass terminology that comes with beveled plate glass windows…

If you’d like to read more about plate glass history and manufacturing, see Kenneth Wilson’s chapter on plate glass in the book, Twentieth-century building materials : history and conservation (1995), Float: Pilkingtons’ Glass Revolution (2009)  by David J. Bricknell, or the classic book by Pearce Davis, The Development of the American Glass Industry (1949).

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As a member of the Rakow Library’s public services team, Regan Brumagen answers reference questions, coordinates e-reference and provides expertise and leadership in the identification, assessment, and recommendation of emerging technologies and electronic resources that enhance and expand library services and instruction. Before joining the Museum staff in 2004, Brumagen worked as a reference librarian and instruction coordinator at several academic libraries. She received an M.A. in English and a M.L.S Library Science from the University of Kentucky. Currently, Brumagen is a member of several American Library Association divisions. She has served on numerous committees for these divisions over the past 12 years.