Mirror and Safety
Mirror is annealed glass that has silver applied to the reverse side.
Annealed glass is produced without stress so the glass can be cut, drilled, polished and worked.
Toughened glass is heated to point of softening - around 620 °C (1,148 °F). The entire panel is at this temperature all the way thought its thickness. it is then cooled quickly. As glass does not conduct heat well, this puts a lot of surface stress into the glass as this shrinks as it cools before the warm core. As a result, it is not flat and its distortions are visible when silvered. This means that the stored energy and stress gives it a strength approx 4-5 times that of annealed glass; but with the feature that it cannot be cut or worked as the stress would release across the glass as it would shatter into very small pieces with dulled edges.
Owing to the heat used in the toughened process, the mirror backing would burn off during toughening.
If you were to ‘silver’ toughened glass it would not yield a nice reflection as the stresses would be visible.
To provide safety, we spec the thickness of the glass appropriately, ‘glaze’ it so that it has no stress, vibration or pressure points in use, and finally, should it succumb to the worst then we have a safety backing which is adhered to the back of the glass and holds it all together should it break.
The point of this is that non toughened glass can form dagger like shards. A safety backing to the glass aims to hold the panel together and instead of pieces when broken, and holds together showing only cracks. The large pieces are secured from behind on the backing. The panel needs to be secured from the front and then the panel replaced after damage.
In rougher environments, toughened glass (often clear but can be tinted) can be laminated to the front of the mirror. This gives a toughened glass working surface and holds together dulled edges if they do ever break. This has increased strength over the mirror alone as the working surface is toughened glass.
mirrored furniture has the strength of only annealed glass, and when bevelled
becomes very thin. We are asked to repair this frequently as it is so fragile,
but it is often uneconomical to repair and questionable if it is fit for
purpose. It is adhered to backing so you cannot fall through the glass, but we
question if it has the strength for its intended use given the number of
repairs we are asked to quote for.