Did this happen because i slumped to fast

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toolfan88
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Joined: Tue Jun 04, 2013 8:23 pm

Did this happen because i slumped to fast

Post by toolfan88 »

I got a new mold and tried my normal slumping scheduled. After firing in the mold the edges pulled inward. Did this happen because i used a pretty fast ramp up speed to get to 1250F. I went about 400 degrees from 1050 to 1250. Im guessing a slower ramp up speed will allow the glass to sag at a more even rate and i wont end up with this bow tie looking plate.iv seen some schedules that go as low as 150 degrees. am i right or do you have any suggestions?
Morganica
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Re: Did this happen because i slumped to fast

Post by Morganica »

Lots of variables here and without a picture can't say for sure. I suspect, though, that the problem is the shape of the mold, and that it's causing what's called "dogboning."

When you have square glass in a square-ish mold with steep-ish sides, the glass is moving down into a smaller footprint--it needs to compress to fit. It's easier to do that in the middle of the side of each mold, because you've got one side of the glass with a relatively large space. In the corners, though, you've got two sides of the glass coming together AND needing to compress, so there's simply more glass that needs to slide down the mold. As a result, the glass in the corners won't go down as far as the glass in the middle...and you get dogboning.

You can reduce the tendency to dogbone by watching the slump carefully and stopping the minute it touches down rather than letting it sag all the way down. And you can sometimes get better results by first slumping the piece into a shallower mold of similar shape, letting the glass redistribute into the new mold shape and THEN slumping a second time into the final mold.

Depending on the mold, though, you'll probably still get some dogboning. The only way I know to prevent it is to change the shape of the blank itself, so that there's less glass in the corners and more glass in the middle. It takes some trial and error to figure out what shape you need to cut, and you have to deal with distorting patterns and such, but it can alleviate dogboning if you get it right. Like this:
dogboning.gif
dogboning.gif (7.62 KiB) Viewed 5725 times
Cynthia Morgan
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S.TImmerman
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Re: Did this happen because i slumped to fast

Post by S.TImmerman »

Gosh, once again, morganica, what a wonderful reply!! You have such vast knowledge and wonderful spirit helping others!
S
toolfan88
Posts: 10
Joined: Tue Jun 04, 2013 8:23 pm

Re: Did this happen because i slumped to fast

Post by toolfan88 »

that helped so much!!


In theory if i fired a square till the footprint changes just a little so the edges flow out, it could help prevent dog boning on molds that are prone to it, right?
Morganica
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Re: Did this happen because i slumped to fast

Post by Morganica »

toolfan88 wrote:that helped so much!!


In theory if i fired a square till the footprint changes just a little so the edges flow out, it could help prevent dog boning on molds that are prone to it, right?
In theory, yes. In reality, getting it to work would make me nuts. First, your bowls would be thicker and heavier (remember the quarter-inch rule--glass must be thicker than that to spread, so your square would need to be 3 layers or more). Second, different colors of glass flow at different rates, and if your piece is multicolored you'd either have to let it process for a long time to even out, or you'd get some colors that flowed and thinned while others stayed thick. And if you heated it that much you might overheat the glass and have other problems. So you'd get it working for one bowl but then things would change as soon as you changed the color/pattern.

Most likely you'd wind up trimming the glass to the right shape anyway. And if you're going to do that, just cut it that way in the first place.

To get the cutting pattern you need to tinker a bit until you get it right. I've cut a piece of paper the size of my square blank, then fit it into the mold and looked at where it bunched up to get an idea of how much dogboning that I'm liable to get with that mold. I still needed five tries to get the final shape. Once I have it, though, I make a clear plastic template and store it with the mold so I can always reproduce it:
http://www.morganica.com/bloggery/2012/ ... tool-tips/

If you've got a flat lap grinder and some practice, you can also grind sides/edges of the piece down flat. I think that's what a lot of people do--but it's not as easy as it looks.
Cynthia Morgan
Marketeer, Webbist, Glassist
http://www.morganica.com/bloggery
http://www.cynthiamorgan.com

"I wrote, therefore I was." (me)
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