Here's What Those Little Black Dots On Car Windows And Windshields Are For

If you've ever noticed the black dots on a car's windshield and wondered what purpose they serve, you're not alone. In most cases, these dots are accentuated by black bands or stripes around the glass' periphery. One might think these dots are printed for aesthetic purposes. In reality, these dots are an integral component of the design process — one that keeps the whole windscreen assembly attached to the car's frame. 

So, how exactly does black paint play a role in structural integrity? At the most basic level, it's about surface tension — creating a tight grip. These are technically known as frits. An ideal design for a car would require the glass plate to be held assuringly in its place. To achieve that, a strong adhesive is required to keep the glass and car's frame tightly locked, resulting in a secure grip. 

The frit is essentially an enamel that is baked right onto the margins of the glass. It provides a surface for tighter adhesive bonding between the glass plate and the car's frame. But there are a few more benefits to these black stripes and dots.

The frit, as explained above, is essentially a contact point. "The purpose of the frit is to provide an etched surface that allows the adhesive to bond to the glass," explains DeDona Tint & Sound, a North Carolina-based automobile service provider. Without this enamel, there's a risk that the adhesive could degrade, weakening the bonding, which in turn could result in the glass shield loosening.

But how will the adhesive go weak if the enamel has done its job of tightly sealing the glass and car frame? Well, the culprit is sunlight, or more specifically, the ultraviolet (UV) rays. "In general, the exposure of a polymer to UV radiation causes breakage of the chemical bonds in the material, a subsequent reduction of its molecular weight and a loss in its mechanical features," says a research paper published in the Polymers journal.

The black frit strip also plays a key role by blocking the UV rays from breaking down the urethane adhesive. In scientific terms, this process is referred to as photo-oxidative degradation. But in doing so, we face yet another technical challenge. If the black lining absorbs the UV radiation, where does it go? As glass and heat don't really play well together, that's where the dots come to the rescue.

You see, if a glass sheet simply just had a plain band of UV-absorbing enamel, it would get hotter while the rest of the region would be at a lower temperature under sunlight. So, what we have here is a sharp separation between hot and cold glass on the same surface. That's not a good sign, as it could lead to deformation and become prone to damage over time. Hot glass can also create an optical distortion effect, which is also referred to as lensing.

To tackle that, a buffer zone is needed, one that serves as a nexus between hot and cold glass. The dots serve just that purpose. They create a zone where the light exposure is scattered and heat is distributed. In doing so, they essentially create a temperature gradient, avoiding the sharp transition between hot and cold glass.

They also serve a cosmetic purpose. These dots are bigger near the black enamel stripe, and get smaller as they move outward. Moreover, this opaque enamel also hides the usage of adhesive underneath, which would otherwise look quite unsightly if left exposed.

This pattern, in some cases, is also deployed to reduce glare from sunlight. "This group of dots, called a "third visor frit," blocks the sun from beaming through the space between sun visors," notes Kryger Glass, a Nebraska-based company that offers glass repair and replacement services for automobiles as well as architectural needs.

The frit dots, aside from being a mainstay on the windshield, are also applied on the rear-view mirrors. "Frit bands often found around rear-view mirrors serve a similar purpose, preventing glare and direct sunlight from causing visual discomfort or hindering the driver's view of the road," says Beam Auto Glass, a member of the Auto Glass Safety Council and the National Glass Association. A number of modern cars have frit patterns right ahead of the rearview mirror, to block sunlight.

But there are a few other technical benefits, as well. The frit also helps with weight and stress distribution between the glass and adhesive, reducing the chances of a weakened bonding and breaking. Additionally, the frit band also keeps the corners and edges of the glass from chipping, while also protecting the polyurethane adhesive from debris and moisture accumulation, both of which could contribute to structural weakness.

There's another side of glass frits on automobiles that most people don't know about, and that's forensic analysis. According to this research paper published in the ​​Journal of the American Society of Trace Evidence Examiners, frit analysis can help with matching parts of glass fragments at the site of a crime or accident.

Moreover, they can also help with traceability, since the chemical composition differs from one manufacturer to another, and even between car models. In a nutshell, it can act as a unique identifier. Moreover, variability in aspects such as gloss, color, and texture can further help with investigations.