What are The Black Dots Around a Windshield?

Frit Band

Sometimes our brains can look past the things we see the most, without ever questioning what purpose they serve. We’ve all noticed them, but nobody seems to know what purpose the black little dots around a windshield have. So what function can such a unassuming, common thing have? Actually, quite an important one.

Frit around the rear view mirror mounting point.

What is it?

The black band around a car’s windshield is called frit. Frit is an enamel coating baked into the glass during the windshield’s manufacturing process.

Traditionally, to secure a windshield to an older vehicle, a gasket was placed in the frame, then the windshield, and it would all be clamped down underneath a piece of chrome trim that ran around the entire frame and provided a finished look.

Older vehicles required the windshield be physically clamped down under a piece of trim.

When a windshield is mounted on a modern vehicle, it can’t be simply clamped down, it must be bonded to the frame with a specialist adhesive. This adhesive holds the windshield down, and provides a perfect seal from the outside, stopping any fluids or air passing through into the cabin.

This presents a problem however, as adhesive needs a rough surface to bond to, and glass is notoriously smooth.

Enter frit.

What Does it do?

The baked in frit band gives the glass the rough surface it needs to adhere to the glue. Frit also increases the adhesive’s lifespan by blocking ultra violet (UV) rays from the sun. UV rays are degrading to adhesives.

So what are the dots for?

The dots have two uses.

During the manufacturing process the windshield is heated up to high temperatures. Due to the different heat absorption properties of glass and frit, there can be warping where the two meet. This distortion can interfere with light passing through and cause strange visual effects in the glass.

The smooth gradient of black dots allows the heat to be more evenly distributed, greatly reducing glass warping.

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The smooth gradient of dots blending into the clear glass, shown here.

Finally, the dots serve an aesthetic purpose. They help to make the abrupt transition from black band to glass smoother, making it easier on the eyes.

This relies on a visual concept called halftone. Halftone is a technique where dots gradually changing in size and spacing can, from the correct distance, appear to be one continuous image to the eyes.

Halftoning tricks the eyes into believing the dots are a continuous gradient.

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So, frit does many things; it can’t mow your lawn, but it’ll help glue stick, protect against harmful rays from the sun, aid manufacturing processes and ease the entire assembly on the eyes.