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Hi all!

Did make a contra-mold from (only) plaster in order to make a mold.
And from the made-mold I wish to make a ceramic mold (for more firings)

I lay 1 piece of Float on the plaster mold and just open the kiln:
The glass has in the middle a crack. (see the image, link below)

Due to..................................??
I assume there was not enough glass to fully fill the mold ???

Most important thing: how can I repair it ?

Will this work:
Place another piece of floatglass on this "product" and then slump again.
Will it fill the crack ?

Or what is a better way to go?

Thank you for you help!

Jan

http://www.pixum.nl/viewalbum/?id=1201895[/img][url]

Did want to enclose the image, but no result!

Jan

From the picture, it is hard to tell what happened. Did it crack on heatup or cool down. Soft edges would indicate heatup crack and sharp edges mean cool down crack.

You should be able to get that shape of mold to work in ceramic or plaster. BTW if you want a plaster mold to not crack, you need to add other materials like silica or grog...

There is no fix. start over and do it again.

What kind of element configuration does your kiln have, side or roof elements?

Bert,

Soft edges.. so heatup crack, you think.
It is not possible that the amount of glass was too low????? It had no possibilty to flow enough ? Or what will happen then ?
Just want to know for projects in the future!

Yes, for a better mold I have to use plaster + silica, but this contra-mold was just meant for 1 time (to make the definitive mold of clay)

My kiln had only roof elements.

Thank you for your help!

Jan

Jan de Jong wrote:Bert,

Soft edges.. so heatup crack, you think.
It is not possible that the amount of glass was too low????? It had no possibilty to flow enough ? Or what will happen then ?
Just want to know for projects in the future!

Yes, for a better mold I have to use plaster + silica, but this contra-mold was just meant for 1 time (to make the definitive mold of clay)

My kiln had only roof elements.

Thank you for your help!

Jan

Jan

I'm wondering if water in the plaster played a role in the heat shock. You shouldn't have had much difficulty slumping that.

It is hard to tell but it looks to me like the glass folds over the outside rim of the mold. This is probably why it cracked. Sometimes the glass can split when slumping if it is asked to stretch too much before it gets hot enough to stretch easily. Usually we just get the glass to bend without stretching it much at all.

Just cut your glass so that it is the same diameter as the mold and can slide down in the inside a little.

It would be good to know your firing schedule when answering questions like this.

ch

charlie holden wrote: Just cut your glass so that it is the same diameter as the mold and can slide down in the inside a little.

It would be good to know your firing schedule when answering questions like this.

ch

Charlie, thank you !
I did place more floatglass and slump and now I can use the mold for making a mold of clay.

Did fire with a firing schedule I did use with my old (ceramic) kiln (sided elements), so I wonder if it was okay for my glass-kiln (roof-elements) ?
So here my schedule, and please comment if it is not right!(for projects in the future)
565 degr. C.(about 1049 F.) in 2.35 hrs
740 degr. C. (about 1365 F.) in 1.30 hrs.
840 degr. C. (about 1545 F.) AFAP
840 degr. C. hold 1 hr.
565 degr. C. (about 1049 F) AFAP
398 degr. C. (about 750 F.) in 2.25 hrs

Thank you very much, Jan

Jan de Jong wrote:

charlie holden wrote: Just cut your glass so that it is the same diameter as the mold and can slide down in the inside a little.

It would be good to know your firing schedule when answering questions like this.

ch

Charlie, thank you !
I did place more floatglass and slump and now I can use the mold for making a mold of clay.

Did fire with a firing schedule I did use with my old (ceramic) kiln (sided elements), so I wonder if it was okay for my glass-kiln (roof-elements) ?
So here my schedule, and please comment if it is not right!(for projects in the future)
565 degr. C.(about 1049 F.) in 2.35 hrs
740 degr. C. (about 1365 F.) in 1.30 hrs.
840 degr. C. (about 1545 F.) AFAP
840 degr. C. hold 1 hr.
565 degr. C. (about 1049 F) AFAP
398 degr. C. (about 750 F.) in 2.25 hrs

Thank you very much, Jan

Jan

Your schedule is confusing to me as it is significantly different than what I would do to slump 6mm (1/4") float glass.

I would take 1.5 hours to go from 100ºF - 1000ºF
ramp up 40 minutes up to 1350ºF and soak for 10 minutes
AFAP down to 1000ºF soak for .6 hours
ramp down to 900ºF taking .6 hours
ramp down to 700ºF taking .4 hours
ramp down to 300ºF taking .4 hours

That is how my program reads. The power turns off somewhere during the last segment and it cools at it's own rate.

For Bartlett control users that is
600 dph to 1000
525 dph to 1350 soak 10 minutes
9999 dph to 1000 soak 36 minutes
167 dph to 900 no hold
500 dph to 700
1000 dph to 300 off

hi ,

from the picture I would assume the crack happened because of uneven mass

the plaster mold has bigger mass on the circumference than the center.

nelson