Just when we thought we were pretty smart, we learned another kiln-glass lesson. It’s a corollary to Lesson No. 1, which is: NEVER think you know what’s going to happen in the kiln.
In fact, we sooooo know that we don’t know, that we always test before doing major projects.
For instance, we’d never fire up over 150 pounds of sheet glass without first test-firing a sample stack. This particular sample for this particular project – a SOFA-bound 3″ thick glass bench – was composed of strips of Opaline White intermixed with clear in an internal stepped pattern.
Opaline is a fascinating glass that fires from clear in the cold sheet to a misty white at full fusing temperature. The glass is very time and temperature sensitive, the white becoming increasingly dense the longer it fires and the higher the temperature it’s taken to.
So, of course, we fired our test stack using exactly the same firing cycle we intended to use in the final project. We were also careful to select all the sample glass from the same production run that we’d use for the final project.
We liked the results, so we fired the two final benches.
Only we didn’t fire them in the same kiln used to fire the tests.
And we didn’t consider the effect of the increased mass – a major heat sink – on the rate of heating and cooling. The sample stack weighed about 20 lbs. The two benches together weighed over 230 lbs. That’s 10 times the volume of glass in our finished firing versus our test.
The white in the finished bench slab was noticeably more opaque. The Opaline had opacified considerably more than the same glass used in the test block – clear evidence that despite the firing programs and glass styles being identical, the firings were NOT equivalent.
A fundamental lesson: what you program isn’t always what you get. Never be fooled into believing that your controller is totally in control. The glass is in control. Always.
The resulting bench was just different enough from the sample – i.e. denser than we wanted – that we considered both benches rejects and had to remake the entire project.
Thanks to a major hustle by the studio team, the benches were remade in record time and are being loaded onto our SOFA truck tomorrow. Come by and sit on them in booth 607.
If you’re like me, they’ll make you feel really stupid.
The upside: this is a reject. You’ll need to come to SOFA to see the heroes.