Sunday, January 19, 2014

Now that was a fail Part 2: The Glass Strikes Back…

Uhhhhh there’s no redemption this time. Everything started off on a good note. I was able to re-fuse a piece that had broken off during ramp up. I had decided to incorporate a pot melt that I didn’t particularly care about just to try out this fancy shmancy mold. 
There was a little bit of overhang but I figured it would get drawn in as a glass slumped (and it did for the most part).
Here’s the results:
D’OH!, D’OH!
After doing some research I figured I’ve had just about everything bad happen that could happen (well not really…but I feel better thinking that way because I feel like I’ve gotten all the bad mojo out of my system in one fell swoop).
Here’s what went wrong.
  1. The gaping holes in the glass: I don’t think they were air bubbles because the glass didn’t fully slump into the mold so there was plenty of places for air to escape.  I don’t think the holes were caused by thermal shock because they really look too organic.  It looks like the glass was torn as it was slumping.  Why? The border I added consisted of scrap blown glass and although it was fused together at a fairly high temperature (20 minutes at 1500 degrees) some of the bonds between the scraps may have been weaker to some of the colored glass coatings that were used.  Perhaps if I fired those kinds of borders at a higher temperature (1530 – 1550) that might create a stronger bond. Either that I should limit the size of those borders for 1/2 to 1 inch wide. I’ll need to experiment some more.
  2. The stretch marks on the underside of the glass: I’m not sure about this.  The glass I was using was scrap system 96.  Here’s the slumping schedule:
250 250 15
250 900 30
300 1100 10
75 1250 30
FULL 960 60
75 800 0
125 700 0
200 100 0
Here’s what I’ve come up with:
  • The glass had been fired too many times.  Due to the thermal shock that occurred during ramp up I had to fire the piece two additional times.  So from beginning to end the pieces was fired a total of 7 times.  That might have caused the stability of the glass to degrade.
  • My ramp up is too fast relative to the thickness of the glass and the kind of mold I was slumping into. The overall thickness of the pieces was about 7.5 mm and I thought I had a fairly safe ramp up but maybe not. In this situation slower is better; perhaps a 150 for the initial ramp to the annealing point and then 50 to the slump target temperature.
  • The target temperature was too high.  Maybe I should do a longer hold at a lower temperature at, lets say 1150 to 1200 degrees.
  • My blank was too big.  There was about a quarter inch overhang around the mold and It’s possible that the extra glass caused some resistance as it was being dragged into the mold creating the stretch marks (and contributing to the tears). Next time I’ll try making the blank a little smaller than the mold and see if that helps.
I was able to get a cross section picture of the piece after I broke it up and it looks like the stretch marks are a series of vertical cracks up extending about one half two thirds the thickness of the glass.
    3. And finally…It didn’t slump into the mold completely.  I’m assuming it was partly due to problems #1 and 2.  Maybe for a mold like this it’ doesn’t need to slump to a “airtight” seal so to speak. When all was said and done it looks like it was probably slumped enough.
Well that’s enough wound licking for now.

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