The Process of Making Windows

How Windows Are Made

The windows you look out of every day went through an intricate process to reach your home. Learn about how they were made.

Before the Industrial Revolution, glassmakers used a technique called broad glass. This method involved gathering molten glass on a blowpipe and blowing it to an elongated balloon shape. The glass was then cut along its length and reheated to flatten.


A window is a frame that holds glass panes. It sits inside a larger frame called a casing, which is installed into the walls of your home.

Modern windows are made of uPVC, which stands for polyvinyl chloride. It is melted into long sections that are cut to the correct size and joined together with heat based welding techniques.

In the past, glass for windows was shaped into a flat disk using a technique called crown glass. This produced uneven thicknesses, so glassmakers set aside the thicker edges and used them for less expensive windows. In the mid-16th century, a new method created much flatter, smoother glass for windows. This became known as broad glass. The new technique was also much cheaper, so it allowed glass windows to become more affordable for homeowners. The resulting broad glass was more transparent than previous types and also very durable. The process of making glass for windows continues to be refined.


In the early days, stained glass windows were hand-made. Talented artists would first develop full-size sketches (called cartoons) that portrayed the overall composition of a window, including the shapes and colors of individual glass pieces to be used. They then used special tools (like dividing irons and grozing irons) to cut glass to the precise shapes needed for each panel in a window.

Next, they placed the glass into a bath of molten tin. The tin is ideal for this part of the process because it mixes well with the glass. The glass floats on the surface of the tin and transforms into a sheet. The temperature is lowered, and the sheet is climbed onto rollers.

This forms the frame of your uPVC windows. Workers then place the finished gas filled glass panes into their frames. This ensures a perfect fit and energy efficiency. The frame is sealed and caulked at this point too. The result is a window that will keep the warm air inside and the cold air out.


Before glass became commonplace in homes, people used paper, wood and animal horn to create windows. In Europe, stained glass began to appear in churches around 680 A.D. The metal oxides that were added to molten glass created different colors and allowed the windows to be decorated with gorgeous illustrations.

Stained glass is now only produced in specialized factories. Today, a flat sheet of glass is made by pouring molten glass over liquid tin. The resulting “float” glass is left to cool. Modern technology allows the glass to be thinner than a millimeter.

Older double-hung sash windows were supported by weights held in boxes on either side of the frame. Newer sash windows use spring balances rather than traditional counterweights. Foldup windows have two equal sashes that can be offset to simulate a double-hung, or in-line. They can also swing inward or outward depending on the window type. These are popular for screen rooms and kitchen pass-throughs.


Windows have various hardware components that make them work. Double-hung windows have a top and bottom sash that open by sliding up and down in the frame. To keep the sash in an open position, they use a system of counterweights attached to chains or rope to help lift it up and down.

Glass for window panes starts with base ingredients such as silica sand and minerals. These are heated until they are liquified. Workers then force the liquified glass into a bath of molten tin, where it floats on the surface and transforms into a sheet.

The sheet is annealed or tempered (reheated and then slowly cooled) to increase strength and prevent shattering. It can also be glazed or coated with heat-absorbing tints to improve insulating value. Before the late 1700s or early 1800s, this type of glass was called crown glass because it was more transparent than broad glass and fairly expensive. When a method was developed from which much larger panes of flat glass could be made, windows became more common and affordable.

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