The Pilkington Process & Float Glass

Instructor: Sunday Moulton

Sunday recently earned a PhD in Anthropology and has taught college courses in Anthropology, English, and high school ACT/SAT Prep.

In this lesson, we learn about float glass and the innovative process developed by Alastair Pilkington to create it. Specifically, this lesson addresses why it was needed, how Pilkington developed the process, and how float glass is made.

Seven Years of Bad Luck

You've likely heard the superstition that breaking a mirror causes seven years of bad luck, but do you know why? Well, the Romans actually started that superstition as a scare tactic to discourage carelessness around mirrors. At that time, mirrors were extremely expensive and fragile. The silver backing cost was the least of the expense, as the glass production took much more time and effort. As time went on, glass production innovations led to stronger glass, but it was still very expensive. Yet, today, we can buy a mirror for a very low price at the corner store, why?

Back Luck

Sir Alastair Pilkington's Great Idea

The answer to that comes from the innovation of Sir Alastair Pilkington, a man who earned his knighthood from Queen Elizabeth II for the invention of float glass. First, however, we should establish a little background on glass production prior to Pilkington's breakthrough.

Glass production was still expensive up the end of the 1950s, not for any lack of materials, but because truly clear, uniform glass required a time-consuming process. There were two different methods of making glass at the end of the 1940s. The first method pours the molten glass into a mold where it was pressed between plates to make it nice and flat. Unfortunately, surface contact with the plates marked up the sheet of glass, requiring extensive polishing and inspection to ensure a perfect sheet ready for customers. If you've ever heard the term plate glass, it refers to the end product of that process. The other method, resulting in sheet glass, extracted the molten glass as a ribbon from a furnace, but that glass was full of imperfections. Sheet glass was still useful where clarity was not important, like in barns and greenhouses. It was also much cheaper to make.

Sheet Glass Production
Sheet Glass Production

Alastair Pilkington, from the family credited with inventing plate glass, wanted to find a way to make glass as perfect as plate glass at the lower cost of sheet glass. As legend goes, he found his inspiration while washing dishes in 1952. He noticed one of his dinner plates floating in the sink and realized that if he could take the ribbon of sheet glass out of the furnace and float it on a liquid surface, it would come out perfectly smooth without needing pressure plates.

Plate Glass Production - Pilkington Factory 1944
plates

The problem with his idea was finding a suitable liquid base. Water would boil and steam under the hot glass, distorting the surface and possibly shattering the glass from the rapid temperature change. What he needed was a hot liquid that could float the glass ribbon long enough for it to solidify. After seven costly years of experimentation, Sir Alastair Pilkington found the answer, molten tin. The ribbon flowed out of the furnace onto the molten tin and cooled very slowly. This allowed surface irregularity to melt away, making the glass perfectly flat. Once the glass cooled enough, it could be removed from the tin without the rollers leaving a mark on it. The finished product had a perfectly smooth surface and no distortions, eliminating the polishing process altogether.

Float Glass on Rollers after Tin Bath
float glass

Step-by-step Process

The Pilkington company, still operating today, describes this process in six steps.

Pilkington Truck
Truck

Step 1: Melting and Refining

Fine grains of the ingredients are measured and mixed before being heated to 1,500 degrees Celsius in a melter. The glass is melted, refined, and mixed with other batches in the furnace over a 50-hour period.

Step 2: Float Bath

Once finished, the glass flows onto a bath of molten tin. The glass removed from the furnace starts at 1,000 degrees Celsius and cools on the tin until it reaches 600 degrees Celsius.

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