I am a sculptor and have taken to casting my relief pieces in glass. I am using the 7 to 10 formula of silica and plaster. My pieces are from 16" x 12" to 25" by 15", all about 1 inch thick. The major difficulty I am having is that the pieces seem to crack in the cooling down stage and they allow glass into the crevice formed right where the figure is and so many of the figures are distorted with a kind of long slender fin.
I am putting the molds in the kiln wet, the foundry claims is doesn't make a difference. Could this be a problem? If so, how long would be appropriate to let them dry given their size?
I am also wondering about how effective it would be to add fiberglass? Would that help prevent the crack? I hate to, as it is such a pollutant.
Another artist suggested using chicken wire, another responded with the the fact that it creates other problems such as hot spots.
Is anyone out there doing something similar? I would appreciate any help that could be offered. I am now getting a lesser ratio than one to two in good castings. An expensive learning!
Thanks for whatever help can be offered.
Carole
molds cracking in casting cooling
Moderator: Brad Walker
I am a sculptor and have taken to casting my relief pieces in glass. I am using the 7 to 10 formula of silica and plaster. My pieces are from 16" x 12" to 25" by 15", all about 1 inch thick. The major difficulty I am having is that the pieces seem to crack in the cooling down stage and they allow glass into the crevice formed right where the figure is and so many of the figures are distorted with a kind of long slender fin.
So, these are 1" thick, open faced castings?
I am putting the molds in the kiln wet, the foundry claims is doesn't make a difference. Could this be a problem? If so, how long would be appropriate to let them dry given their size?
In the past, a 3 day air drying was recommended, before a long pre-firing, before the actual use. Now, it's okay to load a wet mold, but build a 6 hour hold at 200 into your firing schedule.
I am also wondering about how effective it would be to add fiberglass? Would that help prevent the crack? I hate to, as it is such a pollutant.
Forget the fibreglass.
Another artist suggested using chicken wire, another responded with the the fact that it creates other problems such as hot spots.
Chicken wire is better. Forget the hot spots.
Is anyone out there doing something similar? I would appreciate any help that could be offered. I am now getting a lesser ratio than one to two in good castings. An expensive learning!
Most of your problems are going to be in your plaster/silica mix, or your firing schedule. Try 50/50 plaster silica, and buttress your mold against thre kiln walls with anything that'll take the heat, brick. kiln shelves, kiln furniture, refractory material.
Thanks for whatever help can be offered.
Carole
_________________
http://www.CaroleMurphy.com
So, these are 1" thick, open faced castings?
I am putting the molds in the kiln wet, the foundry claims is doesn't make a difference. Could this be a problem? If so, how long would be appropriate to let them dry given their size?
In the past, a 3 day air drying was recommended, before a long pre-firing, before the actual use. Now, it's okay to load a wet mold, but build a 6 hour hold at 200 into your firing schedule.
I am also wondering about how effective it would be to add fiberglass? Would that help prevent the crack? I hate to, as it is such a pollutant.
Forget the fibreglass.
Another artist suggested using chicken wire, another responded with the the fact that it creates other problems such as hot spots.
Chicken wire is better. Forget the hot spots.
Is anyone out there doing something similar? I would appreciate any help that could be offered. I am now getting a lesser ratio than one to two in good castings. An expensive learning!
Most of your problems are going to be in your plaster/silica mix, or your firing schedule. Try 50/50 plaster silica, and buttress your mold against thre kiln walls with anything that'll take the heat, brick. kiln shelves, kiln furniture, refractory material.
Thanks for whatever help can be offered.
Carole
_________________
http://www.CaroleMurphy.com
My memory is so good, I can't remember the last time I forgot something . . .
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Bert Weiss
- Posts: 2339
- Joined: Tue Mar 11, 2003 12:06 am
- Location: Chatham NH
- Contact:
I pretty much agree with Brock. I think your problem is in the plaster recipe and maybe the firing schedule.
The water is either being driven out too fast or it is too dense to let it out without cracking. It is possible to fire without cracking. The hotter your final temp, the weaker the mold will get.
My understanding of 50/50 plaster silica is 50/50 by volume. It comes closer to 40% plaster/60% silica by weight.
If you still have problems, look in to the technique using a 50/50 splash coat with grog added to the rest of the plaster mix.
Another great technique is the Bullseye box casting method. Vermiculite board does not crack at all.
The water is either being driven out too fast or it is too dense to let it out without cracking. It is possible to fire without cracking. The hotter your final temp, the weaker the mold will get.
My understanding of 50/50 plaster silica is 50/50 by volume. It comes closer to 40% plaster/60% silica by weight.
If you still have problems, look in to the technique using a 50/50 splash coat with grog added to the rest of the plaster mix.
Another great technique is the Bullseye box casting method. Vermiculite board does not crack at all.
Bert
Bert Weiss Art Glass*
http://www.customartglass.com
Furniture Lighting Sculpture Tableware
Architectural Commissions
Bert Weiss Art Glass*
http://www.customartglass.com
Furniture Lighting Sculpture Tableware
Architectural Commissions
-
Lauri Levanto
- Posts: 270
- Joined: Tue Mar 11, 2003 7:33 am
- Location: Halikko, Finland
Another sculptor interested in relief casting!
My pieces have so far been max 50x30 cm, most often smaller. I have made much shallower pieces, trying to keep
the thickness 7 to 15 mm.
I make ceramic molds, openface, no undercuts, and bisquit fire them before use. They last for several firings.
The shape of the relief figure is critical. The glass contracts
more in cooling than ceramics or plaster molds.
If your figure has sharp outlines, the mold gets trapped,
and tears the glass apart. With softer lines, the
glass can move.
___|-----|____ NO
___/-----\_____ YES
----|____|-------YES
----\____/....... SAFE
HAve you tried slumping sheet glass to relief molds?
It gives some interesting results.
-lauri
My pieces have so far been max 50x30 cm, most often smaller. I have made much shallower pieces, trying to keep
the thickness 7 to 15 mm.
I make ceramic molds, openface, no undercuts, and bisquit fire them before use. They last for several firings.
The shape of the relief figure is critical. The glass contracts
more in cooling than ceramics or plaster molds.
If your figure has sharp outlines, the mold gets trapped,
and tears the glass apart. With softer lines, the
glass can move.
___|-----|____ NO
___/-----\_____ YES
----|____|-------YES
----\____/....... SAFE
HAve you tried slumping sheet glass to relief molds?
It gives some interesting results.
-lauri
Brock,
Yes, these are open faced molds. I am not sure of how long the timing is for 200 degrees as the glass foundry I use at this point is doing the timing for me. I will check into it and make sure it is for 6 hours at 200.
You say to forget the hot spots, don’t they cause problems with the crystallizations of the glass, leaving places where there are whiter patches on the surface?
I will try buttressing the mold. That sounds like it might help.
Thanks so much.
Carole
Yes, these are open faced molds. I am not sure of how long the timing is for 200 degrees as the glass foundry I use at this point is doing the timing for me. I will check into it and make sure it is for 6 hours at 200.
You say to forget the hot spots, don’t they cause problems with the crystallizations of the glass, leaving places where there are whiter patches on the surface?
I will try buttressing the mold. That sounds like it might help.
Thanks so much.
Carole
Bert,
Your comment on the water being driven out too fast because of the denseness seems to make sense. These are high relief pieces so some of the areas have 2 inches of mold material below them and other areas have 4 inches. I can’t see how that can be helped given the piece itself. How high should the final temp be?
I have measured the amount of plaster/silica by weight, not volume. I will adjust that to volume.
Grog? What is grog?
Vermiculite board? Really? Isn’t that that soft material that plant startings are set into? Is that really something that would be strong enough to hold it?
Thanks,
Carole
Your comment on the water being driven out too fast because of the denseness seems to make sense. These are high relief pieces so some of the areas have 2 inches of mold material below them and other areas have 4 inches. I can’t see how that can be helped given the piece itself. How high should the final temp be?
I have measured the amount of plaster/silica by weight, not volume. I will adjust that to volume.
Grog? What is grog?
Vermiculite board? Really? Isn’t that that soft material that plant startings are set into? Is that really something that would be strong enough to hold it?
Thanks,
Carole
I just don't think a small volume of chicken wire is going to cause a hot spot. The mold itself is a great insulator. BrockCarole wrote:Brock,
Yes, these are open faced molds. I am not sure of how long the timing is for 200 degrees as the glass foundry I use at this point is doing the timing for me. I will check into it and make sure it is for 6 hours at 200.
You say to forget the hot spots, don’t they cause problems with the crystallizations of the glass, leaving places where there are whiter patches on the surface?
I will try buttressing the mold. That sounds like it might help.
Thanks so much.
Carole
My memory is so good, I can't remember the last time I forgot something . . .
Lauri,
Ceramic firing is not a possibility as there are many undercuts in each piece. Thanks for the thought though.
I haven’t tried slump glass yet. This is a new medium for me. Consider me a novice as I have cast in bronze and cold cast bronze, clay and hydrostone only up to this point. Mostly bronze and cold cast. What would be the advantage of slumping?
Carole
Ceramic firing is not a possibility as there are many undercuts in each piece. Thanks for the thought though.
I haven’t tried slump glass yet. This is a new medium for me. Consider me a novice as I have cast in bronze and cold cast bronze, clay and hydrostone only up to this point. Mostly bronze and cold cast. What would be the advantage of slumping?
Carole
Brock,
So when putting the chicken wire it, it would be added to the bottom of the mold I take it? One flat cut of it running the length of the mold? As far away from the glass as possible, right? Sounds like it might be best to make the mold and then add another small pouring on top with the chicken wire laid first?
Carole
So when putting the chicken wire it, it would be added to the bottom of the mold I take it? One flat cut of it running the length of the mold? As far away from the glass as possible, right? Sounds like it might be best to make the mold and then add another small pouring on top with the chicken wire laid first?
Carole
It would be ideal if it was imbedded in the center of the mass of the mold. BUT, it you totally buttress your mold, you can get away without the chicken wire. Even if the mold cracks, it can't move. Ergo, no fins! You'll probably have better results when you do your own castings. BrockCarole wrote:Brock,
So when putting the chicken wire it, it would be added to the bottom of the mold I take it? One flat cut of it running the length of the mold? As far away from the glass as possible, right? Sounds like it might be best to make the mold and then add another small pouring on top with the chicken wire laid first?
Carole
My memory is so good, I can't remember the last time I forgot something . . .
-
Jerry
Carole,
I think I'd fire the foundry that said firing a wet mold doesn't make any difference. Maybe if the mold is wet sand, but wet plaster heated to casting temperatures, well, the laws of physics are what they are.
An easy way get your mold dry is to let it set out in your kiln area, preferably while the kiln is hot, for several days. Then, put it in the kiln, ramp to 250 slowly and hold there. Depending on the size of the mold you might be there for a lot of hours, so program in, maybe, a 24 hour hold; probably you'll need half that. Now, here's the neat part; find a piece of small mirror and hold it over one of your peep holes. You'll see condensation on the mirror if the mold is still damp. It takes about an hour at 250 for the mold to start kicking out the water fast enough to see it on the mirror, but when there's no more condensation, the mold is physically dry. Now, take it up a hundred degrees and hold there for about 4 hours to get rid of the chemically bound water.
One more step. Go ahead and put your glass in the mold after it cools enough to handle it. Then remember that the plaster/silica doesn't cure until around 1200 degrees. Ramp up at about 300 an hour to 1200 and you should be just fine.
One other tip is to add talc at up to the same percentage as your plaster/silica. It does make the mold material harder but is slower to mix because the talc likes to float on the surface longer. Another possibility is to double the amount of silica. That's on the "Mixing with the best" CD from Europe.
Good luck,
Jerry
I think I'd fire the foundry that said firing a wet mold doesn't make any difference. Maybe if the mold is wet sand, but wet plaster heated to casting temperatures, well, the laws of physics are what they are.
An easy way get your mold dry is to let it set out in your kiln area, preferably while the kiln is hot, for several days. Then, put it in the kiln, ramp to 250 slowly and hold there. Depending on the size of the mold you might be there for a lot of hours, so program in, maybe, a 24 hour hold; probably you'll need half that. Now, here's the neat part; find a piece of small mirror and hold it over one of your peep holes. You'll see condensation on the mirror if the mold is still damp. It takes about an hour at 250 for the mold to start kicking out the water fast enough to see it on the mirror, but when there's no more condensation, the mold is physically dry. Now, take it up a hundred degrees and hold there for about 4 hours to get rid of the chemically bound water.
One more step. Go ahead and put your glass in the mold after it cools enough to handle it. Then remember that the plaster/silica doesn't cure until around 1200 degrees. Ramp up at about 300 an hour to 1200 and you should be just fine.
One other tip is to add talc at up to the same percentage as your plaster/silica. It does make the mold material harder but is slower to mix because the talc likes to float on the surface longer. Another possibility is to double the amount of silica. That's on the "Mixing with the best" CD from Europe.
Good luck,
Jerry
Brock,
The only thing the foundry is doing is setting the digital timer for me, so I am pretty much doing my own casting. I am looking for a kiln though, second hand, they seem hard to find. I am not sure if they will have enough buttressing material for me to use, but I do think that a good plan and will try to use it. Changing the mold combo to more plaster and less silica, well that one sounds good too, it should offer more strength. Perhaps all of that with the addition of lengthening the time at 200 degrees would help. Perhaps it will dry the mold more completely before moving on, though since the mold does not crack until it is cooling, perhaps that is not he issue.
Okay, I will place it in the center if I ended up using it. I will try the buttressing first.
Thanks for all your time, Brock. I really appreciate it.
Carole
The only thing the foundry is doing is setting the digital timer for me, so I am pretty much doing my own casting. I am looking for a kiln though, second hand, they seem hard to find. I am not sure if they will have enough buttressing material for me to use, but I do think that a good plan and will try to use it. Changing the mold combo to more plaster and less silica, well that one sounds good too, it should offer more strength. Perhaps all of that with the addition of lengthening the time at 200 degrees would help. Perhaps it will dry the mold more completely before moving on, though since the mold does not crack until it is cooling, perhaps that is not he issue.
Okay, I will place it in the center if I ended up using it. I will try the buttressing first.
Thanks for all your time, Brock. I really appreciate it.
Carole
You're welcome. If your mold isn't cracking until it's cooling, you shouldn't have a problem with the glass running out. If you have glass migrating out of the mold, it cracked at or before top temperature. BrockCarole wrote:Brock,
The only thing the foundry is doing is setting the digital timer for me, so I am pretty much doing my own casting. I am looking for a kiln though, second hand, they seem hard to find. I am not sure if they will have enough buttressing material for me to use, but I do think that a good plan and will try to use it. Changing the mold combo to more plaster and less silica, well that one sounds good too, it should offer more strength. Perhaps all of that with the addition of lengthening the time at 200 degrees would help. Perhaps it will dry the mold more completely before moving on, though since the mold does not crack until it is cooling, perhaps that is not he issue.
Okay, I will place it in the center if I ended up using it. I will try the buttressing first.
Thanks for all your time, Brock. I really appreciate it.
Carole
My memory is so good, I can't remember the last time I forgot something . . .
Jerry,
A number of things you are saying are confusing to me. What I am hearing from other people on here is that running the kiln at 200 for a number of hours works well for drying it out so it makes the drying a non-issue. Your experience with this has been different? Or have your tried drying it in the kiln?
The advice I am getting from other people here is to add more plaster, less silica as it tends to strengthen the mold. Is your experience different from this? I have never heard of adding talc to a mold, has anyone else?
Thanks for your time, Jerry,
Carole
A number of things you are saying are confusing to me. What I am hearing from other people on here is that running the kiln at 200 for a number of hours works well for drying it out so it makes the drying a non-issue. Your experience with this has been different? Or have your tried drying it in the kiln?
The advice I am getting from other people here is to add more plaster, less silica as it tends to strengthen the mold. Is your experience different from this? I have never heard of adding talc to a mold, has anyone else?
Thanks for your time, Jerry,
Carole
You're welcome. If your mold isn't cracking until it's cooling, you shouldn't have a problem with the glass running out. If you have glass migrating out of the mold, it cracked at or before top temperature. Brock
Brock,
The foundry owner is the one telling me that the mold isn't cracking until it begins to cool, I am not there at that point. He says that it looks fine all the way through with no evidence of cracking until the cooling begins. You are thinking that it couldn't crack just as it begins cooling? Perhaps then he doesn't know what he is talking about. I just know that I don't know. So far all of his suggestions have not stopped the cracking so that's why I am asking here.
do you have a website for your art?
Carole
Brock,
The foundry owner is the one telling me that the mold isn't cracking until it begins to cool, I am not there at that point. He says that it looks fine all the way through with no evidence of cracking until the cooling begins. You are thinking that it couldn't crack just as it begins cooling? Perhaps then he doesn't know what he is talking about. I just know that I don't know. So far all of his suggestions have not stopped the cracking so that's why I am asking here.
do you have a website for your art?
Carole
do you have a website for your art?
Carole
Not really, but you can see some of my work on my teaching partners site:
http://www.averyanderson.com
Go to Teaching Schedule and click on my name.
Check out Avery's work also! Brock
Carole
Not really, but you can see some of my work on my teaching partners site:
http://www.averyanderson.com
Go to Teaching Schedule and click on my name.
Check out Avery's work also! Brock
My memory is so good, I can't remember the last time I forgot something . . .
-
Jerry
Carole,
I've tried drying molds with the glass in them and it seems to be a function of how much time I've already spent on the piece if that will work or not. The more time I've already got invested in modeling and casting, the more likely the mold will fail because I hurried. My process is to let the plaster/silica mold sit out for a couple of days until it is warm to the touch. Then it's into the kiln at 250 until the condensation stops. That works for me but it's up to you to decide.
There are more formulas for plaster/silica stuff then there are people doing such stuff. I learned my process several years ago and have modified it based on the advice I"ve gotten from people whose work I've come to admire. Again, it's your choice. Experimenting is the best way to decide. As far as the ratio of plaster to silica goes, remember why you are even bothering with the silica; plaster breaks down under heat but silica gives it the strength to survive. Use less silica and you lessen that high temperature strength (isn't that what you said was happening; cracking?); again, experiment.
The talc idea I got from Delores Taylor, a very accomplished pate de verre crafter. She believes that the talc does two things; strengthens the mold and adds to the detail. I've been using talc for about a year with great results but maybe your projects don't require talc. You are working with shallow models so you might not need it. However, you said you are getting cracking and "fins" where the glass runs into the crack, Yes? Well, strengthening the mold seems to be the right answer.
Anyway, those are just some musings from a glass crafter stuck out in the willy-wags of Maine. There's not much kiln activity out here so I rely on this board for advice and do a lot of testing.
Good luck,
Jerry
I've tried drying molds with the glass in them and it seems to be a function of how much time I've already spent on the piece if that will work or not. The more time I've already got invested in modeling and casting, the more likely the mold will fail because I hurried. My process is to let the plaster/silica mold sit out for a couple of days until it is warm to the touch. Then it's into the kiln at 250 until the condensation stops. That works for me but it's up to you to decide.
There are more formulas for plaster/silica stuff then there are people doing such stuff. I learned my process several years ago and have modified it based on the advice I"ve gotten from people whose work I've come to admire. Again, it's your choice. Experimenting is the best way to decide. As far as the ratio of plaster to silica goes, remember why you are even bothering with the silica; plaster breaks down under heat but silica gives it the strength to survive. Use less silica and you lessen that high temperature strength (isn't that what you said was happening; cracking?); again, experiment.
The talc idea I got from Delores Taylor, a very accomplished pate de verre crafter. She believes that the talc does two things; strengthens the mold and adds to the detail. I've been using talc for about a year with great results but maybe your projects don't require talc. You are working with shallow models so you might not need it. However, you said you are getting cracking and "fins" where the glass runs into the crack, Yes? Well, strengthening the mold seems to be the right answer.
Anyway, those are just some musings from a glass crafter stuck out in the willy-wags of Maine. There's not much kiln activity out here so I rely on this board for advice and do a lot of testing.
Good luck,
Jerry
Jerry,
Since I am such a novice, it is difficult to figure out just what advice to take. It's just that two other people on here are recommending just the opposite. It is very confusing. What you are saying makes a lot of sense if the silica is the one that offers the strength at high temps.
The time I am taking on making the mold in the beginning is quite ridiculous really. I am pouring a casting in wax in the silicone rubber mold and then making a box around the wax, sealing it all, fixing whatever imperfections in the wax casting and then finally pouring the mold. It is taking a couple of days at least for each mold. Then there is the cost of firing, $60 to $100. per firing. It is not only time cosuming but getting very expensive, especially given the poor results. I have galleries waiting for the pieces and I have shows I want them ready for.
Talc, eh? Hmmm.
Thanks Jerry,
Carole
Since I am such a novice, it is difficult to figure out just what advice to take. It's just that two other people on here are recommending just the opposite. It is very confusing. What you are saying makes a lot of sense if the silica is the one that offers the strength at high temps.
The time I am taking on making the mold in the beginning is quite ridiculous really. I am pouring a casting in wax in the silicone rubber mold and then making a box around the wax, sealing it all, fixing whatever imperfections in the wax casting and then finally pouring the mold. It is taking a couple of days at least for each mold. Then there is the cost of firing, $60 to $100. per firing. It is not only time cosuming but getting very expensive, especially given the poor results. I have galleries waiting for the pieces and I have shows I want them ready for.
Talc, eh? Hmmm.
Thanks Jerry,
Carole