By definition, "tempering the glass panel all the way through" is not possible.
The interior and exterior parts of the glass must have different properties.
Tempered glass is made by cooling the exterior and interior at different times:
- The glass panel is heated to a uniform temperature hot enough to soften it.
- The exterior surface is cooled by air until it becomes hard.
- The interior part remains soft enough to flow.
- The entire panel is allowed to slowly cool back to room temperature.
The essential part happens during the final cooling period. As it cools, the glass shrinks, but the outer surface is already cooler, so shrinks less than the interior. By the time the entire panel has cooled, the outer surface is pulled tightly together by the interior, which has shrunk more than it has. At the same time, the interior is pulled outward by the exterior layer.
This stress (compression of the surface layer and tension of the interior) is what gives the glass its strength.
On the other hand, while the compressed outer layer is extremely resistant to damage, when it does experience damage, it is catastrophic.
A hard blow by a sharp object can create a crack in the outer layer.
This will relieve some of the internal stress and cause the crack to spread rapidly.
This will distort that part of the glass, which will create extra stress on the surrounding areas, which will themselves crack and propagate the destruction throughout the entire panel.
This explosive shattering into small pieces is what makes tempered glass much safer, as it does not leave any large jagged pieces that could cause severe injuries.
Side windows on cars are made of tempered glass for this reason.
More interestingly, it is possible to break the interior of tempered glass without breaking the exterior layers, such as by a single hammer blow to a nail directed into the edge of a glass table top. Though it can look beautiful, the result should not be used, as it will no longer have its original strength nor its safety features. It should be discarded or safely framed as "art".
The "Time Warp" TV show featured an example of interior shattering: