by A&O » Tue Nov 14, 2017 12:35 am
Float glass when cut will leave really sharp edges. Then a fine sand paper is used to smooth the edges. When broken float will break into shards.
Toughened glass usually has nice smooth edges (usually but not always). That is because it starts live as normal float glass, cut to desired sized then is heated to almost melting point then cooled a number of times. The slightly rounded edges comes from sanding the edges before heat treatment, but because the glass gets so hot, the extreme edges do melt making the sides of the glass a lot smoother. This is the fun part, toughened glass takes a bit to break, because what has happened during the heat treatment the outer layer constricts ever so slightly putting the centre of the glass under immense pressure, giving it the strength. Think of it this way, if you stand on a air hose with no air pressure it deforms easily, but add 100psi, not so easy to squish, keep increasing the air pressure and it becomes harder and harder to deform the hose. Same principle for toughened glass. The outer layer of glass has contracted enough to put massive pressure internally. So hit the middle of a toughened pane of glass with a hammer and it bounces off. Hit the edge or use a very fine sharp instrument, you release the internal pressure of the glass by fracturing the outer layer, and the pane will shatter into millions or little pieces. It’s the sudden release of pressure that causes the whole pane to break.
Laminated glass is 2 pieces of float vulcanised together with plastic. You can see when looking side on the line in the glass. Laminate can be toughened.
Borosilicate glass is moulded through high pressure injection into multi piece mounds or is hand blown into shape. You will see casting lines and marks in the glass. It is usually put through a hardening process after the item has been polished to remove casting marks.
So can you tell what is what. Nah not really. A glassier that has been in the trade for a long time might be able to pick it by seeing some subtle tell tale signs, but you don’t really know what it is until you wack it with a hammer.