Long before window glass became flat, it was a huge bubble at the end of the glassblower’s pipe, who had to spin it quickly into a very big disk called “crown” to straighten out the bubble. After it cooled off, the deformed disk would be cut into small panes. Comparatively, if the bubble was swung over a pit, it could be stretched and elongated into a cylinder. After the glass cylinder was cool, it was cut on the length, reheated and then flattened. This was the procedure used to make larger panes. Yet, the glass would still be distorted and the work just as tiring.
A glass pane however is not at all a complicated structure. If anyone having little to no hands-on knowledge about window glass making were to create a method to produce it, it would be very unlikely for them to come up with anything like the method currently used by professional glass makers. Yet, glass blowing is quite common. However, to a specialist knowledgeable of its manufacturing method the very term “blowing” communicates a rather unsound notion. Although blowing is a very important part of the manufacturing process, it would never be possible to obtain a flat window pane only by blowing alone.
How is it really made?
There are three ways to produce flat glass tables or plates, one of which does not require blowing at all. Blowing-made glass is commonly known as plate glass, while the glasses obtained by the other two methods, which involve blowing, are referred to as “crown glass” and “cylinder glass” respectively. Does it ring a bell? It should, as their names resemble the different characteristics of their manufacturing techniques (mentioned above). Let us quickly walk through each of them so you get the picture.
1. CROWN GLASS
In this particular case, the workman takes a large amount of glass on the pipe end. Then he rolls it on the marver in the exact same way as for making bottles. This operation though requires greater care and more finesse as the risk of unevenness is higher because the mass of plastic glass is a lot greater. Once the ball shape formed on the marver is re-shaped into a pear. After it is slightly expanded by blowing, it is softened in the furnace and rolled on the marver one more time to eliminate any unevenness caused by the last heating and also collect the greatest mass of glass concentrated at the extreme end, which is known as the “bullion”. The bulb thus formed is expanded again by powerful blowing. During this procedure, the workman rests the pipe on a horizontal bar known as the “bullion bar”. The process ends when the bulb at the end of the pipe resembles a sphere-like shape. In some cases, the bullion bar is replaced by another device. Its purpose is to assist the workman with spinning the bulb in order to keep it as globular in shape as possible. The next step is the bottoming hole. This is a round hole in the furnace screened by a masonry wall extending far enough to protect the workman from the high heat. At this stage, he also needs to keep his face protected with a mask specially designed with openings protected by glass plates to allow him to see his work.
The pipe with the expanded bulb is placed across a hook, which fits into the masonry wall. The bottom of the bulb or the bullion is now exposed to high heat. The centrifugal force generated by rapid spinning causes the bulb to spread out to the sides until it becomes almost flat. At this point, an assistant inserts an iron rod (referred to as the “pontil” or “punty”) in the center of the side facing the pipe to keep it in a snail stand position on the rotation axis. The workman then removes the pipe by touching the neck of the hot glass with a cold iron bar wetted in water.
The side where the pipe has been attached to is heated in the furnace while the workman continues to spin the expanded glass, which expands even more and the hole left by the detached pipe widens causing the bulb to become fully flat. This could not be achieved without first exposing the bulb to the heat in the “flashing furnace”, though, which is similar to the bottoming hole only larger. Now the glass has become a flat circular plate with a so-called “bull’s eye”. The bull’s eye is nothing else than the mark left by the punty when it was attached. This is kept to hold the glass in position while rotating so the shape is preserved until the glass is cool enough to support itself. Then, the glass is taken with a big iron fork and the punty cracked off. Next, the glass plate is taken to the annealing furnace where it is left for 24 hours. After that, it can be cut into smaller size plates and packed.
Moving forward, next comes:
2. CYLINDER GLASS
Unlike with crown glass, the blowing abilities of the glass maker are not so heavily involved in the making of cylinder glass. Specifically blowing expands only over the shaping of a cylinder with apertures at both ends. In this particular case, blowing is combined with other handling techniques to deliver the desired result. The furnace used to produce cylinder glass is designed differently. Its holes are slightly elevated, with platforms projecting out from the furnace base, one corresponding to each hole or pot, which serve as a stand for the glass maker. These platforms are of appreciable length and with deep enough pits dug in between to allow the workmen to move the bulb back and forth during the elongation process. The glass is being handled pretty much in the same way as for making crown glass up to the moment of expansion. Blowing into the plastic glass produces expansion in that part of the mass resembling a pear shape usually called the “neck”. The workman holds it above his head and the weight of the denser part of the glass mass presses onto the expanded portion until it resembles the top of a bottle once used by druggists of old times to keep their potions and tinctures. The expanded part of the mass is now allowed to cool, which, given that it’s thin, it does in a short while.
As it cools off, it becomes rigid, allowing the glass maker to resume his work. He blows through the pipe while continuing to spin it to maintain the cylindrical shape of the expanding part swinging the ball in one of the pits at the same time, thereby causing the cylinder to elongate.
As it thins out and cools off, the glass hardens. The hardest part of the job is practically expanding the glass to the specified size at the exact time when it becomes cool enough to preserve form. When the cylinder walls are equally thick and required length achieved, the farthest end from the closed pipe is hemispherical in shape. This end is heated quickly at the mouth of the furnace. Blown into through the pipe, it bursts open. The pipe is rotated and the part, which is now open, expands due to the action of the centrifugal force to the exact same size of the cooler parts of cylinder. Then, the cylinder is placed into a frame and the pipe is removed.
The end where the pipe has been detached from has now the shape of the upper part of a druggist’s bottle. That part has to be removed! To do this without damaging the semi-final product, the glass maker takes a small plastic glass wad from the furnace using little iron rods. He then separates the two rods thus drawing out the glass into a heated plastic cord, which he rolls around the cylinder in that area where it begins to contract, usually towards the neck. The cylinder being heated all over, it cracks off in the direction of the heated line in contact with cold water.
At this point the cylinder has to be opened. To do so, the workman places a hot iron bar on the length of the cylinder. Due to sudden cooling, it cracks open from one end to the other. Now it is transferred to the annealing furnace and then on to the flattening kiln. The design of this furnace is unique. It is composed of a vault sheltering a revolving iron frame. This rotating frame, in turns, supports platforms, which are called flattening stones. Around the furnace there are openings where glass makers stand.
How is this furnace used? A cylinder is placed horizontally in the first opening with the cracked side up. It is allowed to heat until it becomes flexible. As soon as this is achieved, a second cylinder is placed in with the aid of a winch. The partial rotating movement causes the heated cylinder to travel all the way to the next opening facing it. Here, another workman spreads it out with an instrument consisting in a wooden block on an iron handle. So it becomes a plate. At this stage another workman evens it out. Then it is passed through several chambers and subjected to gradually decreasing temperatures until annealing is completed.
Last step – cutting into panes and packing.
So, this is how window glass is made. However, glass making requires a lot of skill and finesse perfected in years and years of practice.