Bob Leatherbarrow's research shows 6mm drapes more slowly than 3mm. It seems the thicker glass takes longer to begin the slump. Glass behaves in a similar way for a drape. My experience of draping 6mm/0.25” and 3mm/0.125” in the same firing confirms that 6mm takes longer. Or, it needs a higher temperature. I know this goes against common sense, but tests and experience show it to be true.
If you try to drape
3mm/0.125” and 6mm/0.25” pieces at the same time, the 3mm will reach the
desired shape before the 6mm. You then have the choice of an under draped 6mm
piece or an over draped 3mm piece. This indicates
that draping different thicknesses in the same firing will be unsuccessful. To a lesser extent, the size of the drape
will influence the speed of the drop. So,
you are unlikely to achieve completely desirable results with significantly
different sizes of drape in the same firing either.
Observation is essential in all draping operations. You
cannot know how long it will take for a piece to drape or drop to your
requirements. To be sure of your result you need to observe the
progress of the drape. There is rarely a
safety net of a form to drape onto as in slumping. To observe, set your top temperature with a
long soak/hold. Start peeking at frequent intervals from the time top temperature is reached.
When the glass has
reached the desired shape, advance to the next segment. Your controller manual will give you
instructions on how to do that.
Different thickness and sizes of glass require different
firing conditions.
Hi Stephen, Initially I thought that thicker projects would slump more quickly than thinner ones bea=cause of the extra mass. However it is the opposite... thicker projects slump more slowly, or at a higher temperatures. But remember that glass is a poor conductor of heat so the thicker the glass the longer it takes for heat to penetrate to the middle of the layer. The slump doesn't start until that middle part is hot enough to flow. I find that the onset of slumping is about 20F (11C) hotter in 6mm thick compared to 3mm thick projects.
ReplyDeleteI find slumping schedules are the most difficult to all schedules. The variation in onset of slumping for the range of pieces that I work with is about the same as the range in temperatures between a tack and full fuse. Visual observations are critical for successful slumps.
Thanks, Bob. After reading your book on Firing Schedules for Kilnforming, I reviewed my practice and did a little experimenting. I found that I heated glass thicker than 6mm much more slowly at one rate to the slumping temperature, which may explain why I find the thicker slumping about the same or at a lower temperature than the 6mm. I put that down to the heat work put into the glass all the way up to the top temperature soak.
DeleteAs you see from the entry above, I say that thicker glass will take longer to slump if it is in the same firing as thinner glass, and I don't recommend putting different thicknesses into the same firing. One or the other will be under or over done.
I appreciate you taking the time to respond and agree with your observations given similar ramp rates to the top temperature.
Best wishes,
Stephen