Due to rapid cooling, the outer surfaces come under compression, and the middle half of the glass is under tension.
This heating and molecular-structure alteration give the glass its strength, five or six times stronger than annealed glass of similar thickness.
Glass cannot be cut, drilled, filed or altered in any way once tempered.
What I didn't know in my younger years is that had I simply run a file along the edge of the glass, it would have "exploded," just like your shower door did.
Delayed breakage sometimes occurs with tempered glass when the edge is damaged during handling, or due to vibration within its frame.
When the blemish is big enough, the whole thing lets go, rare as it is, in spectacular fashion and with total randomness.
The small pieces that come out of broken tempered glass are not sharp and generally do little damage to people or possessions.
Side windows in cars are tempered. Looking at your Buick with polarized sunglasses, you may see patterns — this is not a '60s flashback, but the patterns left by the array of cooling air nozzles used during manufacturing.
As an aside, car windshields are made of laminated glass, not tempered. Both are considered safety glass. Wire-reinforced glass seen in many shower doors is not safety glass; this is a fire-rated glass.
Heating oil worries
For those concerned about heating oil this winter, I have good news. No, the news is not about that inevitable spike in your bill, but about an emerging alternative fuel. Biodiesel, a fuel made from vegetable oil (soy, canola or others), has been in the news for its use in machines that run on petroleum-based diesel.
This completely renewable, almost nonpolluting, nontoxic, grown-in-America fuel has converted many owners of diesel-fueled vehicles, myself included.
But without a diesel-fueled vehicle, many have had no use for biofuels. Diesel fuel, however, is virtually identical to heating oil.
Starting this winter, a few local companies, including Laurelhurst Oil in Seattle, will sell bio-heating oil in 10-percent, 20-percent and 30-percent mixtures with traditional petroleum-based fuel. Check with your local oil company to see if they plan to sell biofuel.
Adding just 20 percent bio-fuel decreases the particulates, carbon dioxide, sulfur oxide (down 83 percent!) and nitrogen-oxide emissions, and improves the unpleasant odor, according to a Massachusetts Oil Heat Council study by Energy Research in 2003.
Tom Marier of Laurelhurst Oil is calling this move to bio-fuels "customer pulled, not manufacturer pushed." People are "literally clamoring" for these products, as a vehicle fuel and to heat their homes.
Biofuel is mixed with petroleum product rather than straight, due to issues with seals in older furnace/boiler burners. Newer burners will be compatible with straight biofuel. In diesel vehicles, it can be run straight or mixed.
Darrell Hay is a local home inspector and manages rental properties. |
