When we talk about windows and window design, it is easy to focus on the many types of window styles, sizes, interior and exterior finishes, even hardware. But we’re there’s one item that every single window has: glass.
Glass, in its most basic form, is made by heating sand to an extremely high temperature. In nature, a lightning strike to a sandy beach can form glass from the melted sand. Found near volcanoes, obsidian is a black glass formed by cooling lava.
In manufacturing, glass is made by heating sand, lime, and soda ash together to form an amorphous solid. In amorphous solids, the molecules move very slowly around instead of remaining static, or in one place. Modern glass is called float glass because the molten glass is floated over a layer of molten metal, making the glass very thin, smooth, and uniform. The glass is annealed, or slowly cooled down, to relieve any internal stresses in the material.
The wavy glass you see so often in old homes and buildings was handmade. It’s thicker than modern glass, and includes air bubbles, waves, and other inclusions that give it character. (A definite step up from the first windows, made by the Roman Egyptians around 100 AD—their glass was so thick you couldn’t see through it!) Our team sources salvaged vintage glass to use in projects where it’s important to keep that old-time character.
This vintage glass has inclusions and bubbles.
Stained glass is colored using metallic salts during the manufacturing process. The colored glass pieces are assembled and held together using leading. Historically, stained glass is used in churches and other religious structures. We’ll devote a separate article to this 1,000-year-old window art form soon!
Tempered glass is about four times as strong as conventional glass. Tempering, like annealing, is a thermally controlled process that puts the outer surface of the glass into compression and the inner surface into tension. Tempering not only results in stronger glass, but when the glass breaks you get granular chunks rather than jagged shards. Tempered glass is required by code where there’s possible high impact, there are very large panes of glass, windows are close to the floor, or other safety considerations.
Laminated glass is another kind of safety glass. Instead of treating the glass with heat, layers of glass are fused together with a polyvinyl butyral (PVB), ethylene-vinyl acetate (EVA), or thermoplastic polyurethane (TPU) between the glass. In the event that the glass is broken, the plastic layer holds it together.
This safety glass holds shards together.
Window glass can also be coated to make it more energy efficient. One of the most common coatings results in low-e or low-emissivity glass. Emissivity is the measure of how much a surface emits thermal radiation. Low-e coatings are microscopically thin and minimize the amount of infrared and ultraviolet light transmitted through the glass. The amount of visible light is unchanged. In the summer, low-e coatings keep interiors cooler by reflecting the heat off the exterior of the glass. And because low-e coatings also reflect ultraviolet light, furnishings fade less than with conventional window glass. In the winter, low-e windows reflect the interior radiant heat back into the building rather than transmitting it through the glass. We’ll do an even deeper dive into low-e and other energy-saving components to windows and doors in a later piece on high performance.
Eight over eight divided lite windows.
A window sash may consist of one or more planes of glass called lights or lites. If there’s just a single plane of glass in each sash, the glass pattern is one over one. If there are two lites in each sash, the pattern is two over two. You can have one lite in each sash or twelve or more. The number of lites in each sash can differ as well (eight over one, for instance).
The lites are separated by mutins, sometimes called grilles, which are strips of metal or wood that separate and hold the lites in a grid. A true divided-lite is when the mutins physically hold the panes of glass in place.
A simulated divided-lite is when the mutins are affixed to the surfaces of the window to appear as if the glass is actually divided. When the glass is insulated or there are multiple panes of glass, an optional spacer bar may be used between the panes to make the windows appear to be authentically divided.