They use special purpose-built ovens for this because of the size of the panes possible being so large and with height being un-needed. And you don't want the liability issues of a DIY job coming back to haunt you if it gets broken. Code inspectors now want to see the proper tempered certification stamped or etched into the glass before they will give you a 'pass' where it is required, and that isn't a DIY thing. Tempered glass isn't all that expensive once you know what's going on behind the scenes. There's a fairly high percentage of scrap involved as any flaw in the glass being tempered will make the finished product unacceptable if it survives the tempering unbroken. Tiny air bubbles become big and visible, unseeable stresses in the glass can crack it, and once tempered there's no re-cutting so errors there add up too leaving large chunks of wasted time and money behind. Plus it's usually as permanent as a concrete block so you only pay once forever.
There is usually some way of saving money in doing anything; the wisdom comes in when the time, effort, and hassle of that approach are figured in which often makes "the standard practice" look much more viable than it does at the first glance. If there were a better way you can be sure the Pros in construction would all be doing it that way because we're in the business to make money, not waste it, and we've got to compete with some pretty sharp folks doing what we do if we're to still be doing business tomorrow. We order our tempered glass simply because that's the best way to get the job done.
Phil
