Has anyone else used talc in their reciepes? Has anyone else increased the amount of silica while reducing the plaster with good results?
Carole
molds cracking in casting cooling
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Bert Weiss
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Carole
There are 2 kinds of water. The kind that is driven off at 200º is one. The other is chemiocally bonded water. It is driven off at a much higher temperature. There is probably something about that in the WGBB archives. Look in the old archive as well as the new.
I suggest that you read Boyce Lundstroms book Glass Casting and mold making book #3. He gives a lot of background info about how different recipes work. There are a lot of talented glass casters in your neck of the woods. I'd find some and get advice. It should improve your odds.
There are 2 kinds of water. The kind that is driven off at 200º is one. The other is chemiocally bonded water. It is driven off at a much higher temperature. There is probably something about that in the WGBB archives. Look in the old archive as well as the new.
I suggest that you read Boyce Lundstroms book Glass Casting and mold making book #3. He gives a lot of background info about how different recipes work. There are a lot of talented glass casters in your neck of the woods. I'd find some and get advice. It should improve your odds.
Bert
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charlie holden
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This is my take. I'm a little confused about the thickness of your molds and glass. You say that the glass is about one inch thick, but that the molds are two inches thick at some points and four inches thick at others. It seeems to me that your glass would have to be three inches thick in places?
You understand that the water has to be burned out of the mold slowly. If it steams out too fast it will start cracks. (It may be that the cracks are forming on heat up but not spreading until the mold starts to contract on cool down. I've never heard of this happening, but it doesn't seem impossible. Is it leaving a depression in the top surface of the glass?) You can help yourself a lot by letting the molds air dry for several days before they go in the kiln. Jerry is exactly right about watching for moisture before cranking it up. I ramp up at about 50 degrees per hour then soak at 400 F, instead of 250, for about 5 hours, for a one inch thick mold. I often burn out my molds overnight then load the glass the next day. If you did that you could look for cracks.
One thing for sure, you don't need two inches of mold material under an open faced mold at its narrowest point. You could probably get away with a half inch. The glass pushes out on the walls of the mold the most where it is deepest. It only pushes down on the bottom of the mold. That means you need thick walls for thick glass, not a thick bottom. (Of course it's good to match the bottom thickness to the wall thickness for annealing purposes.) When you have four inches of mold, which sits between the kiln shelf and the glass, it is very hard to heat it through and drive the water out. I would probably hold a mold at 400 for 15 hours if it was 4 inches thick in places.
If you read any of the books, they measure dry ingrediants by weight, and the starting point for most mixes is 50/50 pottery plaster/silica. (Weight is the only way to be accurate. You can't look at a certain volume of powder and tell if it is packed tightly or loosely.) Start with that mix. I cast open face molds with 50/50 mix all the time, no chicken wire, occasional bracing with bricks.
You can help yourself a lot if you mix the dry ingrediants very well before you add them to water. Clumps of mostly plaster will expand differently than clumps of mostly silica. The casting cd "Mixing It With the Best", which is the summary of a fairly scientific testing of casting molds recommends a 33/33/33 mix of plaster/silica or olivine/ and a modifier -- talc is one modifier, ludo and grog are others. Ludo is just ground up, already fired molds (free except for grinding labor). The modifier is to introduce different sized particles, so the mix packs more tightly. Olivine is a safer alternative refractory to silica.
You don't say how you are loading the glass into the mold. It should be piled into the center with at lease a half inch empty space between the glass and the mold walls. If the glass is packed in too tightly it will push out on the mold walls as it heats, and crack the mold.
You could add fiberglass as long as it is borosilicate fiberglass. Soda-lime fiberglass will just melt. Bert's right about Lundstrom's book. Also look at the the Kervin and Fenton book or get the cd from somebody in Portland.
You understand that the water has to be burned out of the mold slowly. If it steams out too fast it will start cracks. (It may be that the cracks are forming on heat up but not spreading until the mold starts to contract on cool down. I've never heard of this happening, but it doesn't seem impossible. Is it leaving a depression in the top surface of the glass?) You can help yourself a lot by letting the molds air dry for several days before they go in the kiln. Jerry is exactly right about watching for moisture before cranking it up. I ramp up at about 50 degrees per hour then soak at 400 F, instead of 250, for about 5 hours, for a one inch thick mold. I often burn out my molds overnight then load the glass the next day. If you did that you could look for cracks.
One thing for sure, you don't need two inches of mold material under an open faced mold at its narrowest point. You could probably get away with a half inch. The glass pushes out on the walls of the mold the most where it is deepest. It only pushes down on the bottom of the mold. That means you need thick walls for thick glass, not a thick bottom. (Of course it's good to match the bottom thickness to the wall thickness for annealing purposes.) When you have four inches of mold, which sits between the kiln shelf and the glass, it is very hard to heat it through and drive the water out. I would probably hold a mold at 400 for 15 hours if it was 4 inches thick in places.
If you read any of the books, they measure dry ingrediants by weight, and the starting point for most mixes is 50/50 pottery plaster/silica. (Weight is the only way to be accurate. You can't look at a certain volume of powder and tell if it is packed tightly or loosely.) Start with that mix. I cast open face molds with 50/50 mix all the time, no chicken wire, occasional bracing with bricks.
You can help yourself a lot if you mix the dry ingrediants very well before you add them to water. Clumps of mostly plaster will expand differently than clumps of mostly silica. The casting cd "Mixing It With the Best", which is the summary of a fairly scientific testing of casting molds recommends a 33/33/33 mix of plaster/silica or olivine/ and a modifier -- talc is one modifier, ludo and grog are others. Ludo is just ground up, already fired molds (free except for grinding labor). The modifier is to introduce different sized particles, so the mix packs more tightly. Olivine is a safer alternative refractory to silica.
You don't say how you are loading the glass into the mold. It should be piled into the center with at lease a half inch empty space between the glass and the mold walls. If the glass is packed in too tightly it will push out on the mold walls as it heats, and crack the mold.
You could add fiberglass as long as it is borosilicate fiberglass. Soda-lime fiberglass will just melt. Bert's right about Lundstrom's book. Also look at the the Kervin and Fenton book or get the cd from somebody in Portland.
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Jerry
Carole,
I recently did an oriental dolphin that was 12" and 4" thick at it's thickest. I did a pot melt pour over 36 hours which included a lot of curing time for the plaster. The cooling was over 48 hours. I used my plaster/silica recipe with lots of talc, NO REINFORCEMENT and did just fine. There's a lot of information in the archives about all this, but the most popular formula is equal parts of #1 pottery plaster and silica; I use 325 but 275 is popular as well. If you didn't do anything else, that one would work. Just remember to get it dry and cured.
Every mold formula has problems. When I do shallow molds, a bit smaller than the ones you describe, I often get two uses out of each mold! So, if I were you, I'd do some experimenting and figure out what I like. When I get confused over a process, I take a lot of notes, develop a plan to try each element and work it out. Like you, I've heard the confilicting information here and have developed a process that works well for me, but I did send a lot of stuff to the land fill!
Keep at it; you'll get there.
Jerry
I recently did an oriental dolphin that was 12" and 4" thick at it's thickest. I did a pot melt pour over 36 hours which included a lot of curing time for the plaster. The cooling was over 48 hours. I used my plaster/silica recipe with lots of talc, NO REINFORCEMENT and did just fine. There's a lot of information in the archives about all this, but the most popular formula is equal parts of #1 pottery plaster and silica; I use 325 but 275 is popular as well. If you didn't do anything else, that one would work. Just remember to get it dry and cured.
Every mold formula has problems. When I do shallow molds, a bit smaller than the ones you describe, I often get two uses out of each mold! So, if I were you, I'd do some experimenting and figure out what I like. When I get confused over a process, I take a lot of notes, develop a plan to try each element and work it out. Like you, I've heard the confilicting information here and have developed a process that works well for me, but I did send a lot of stuff to the land fill!
Keep at it; you'll get there.
Jerry
Charlie, Jerry and Bert,
Okay, okay, I think it is confession time. I have not been mixing the dry ingredients before adding them to the water. It sounds like that could be the whole problem. Somehow it never occured to me. I have been carefully sifting it and following the receipe to the T, just didn't know about mixing the dry.
Good point about the thicknesses variation. I should have explained the size of the pieces better. The base that the sculpture is made on is 1 inch thick, the figures raise up out of that to height of at the most 2 inches. So there are spots where there are actually 4 inches of mold material. That is a lot of mold space, you are right. It would take an awfully long time to have that dry out, even in a kiln. I will up that time.
I am using a 50/50 by weight formula at this point. I think I will continue with it with these changes and see if it solves it. I will also bolster the sides with bricks and whatever I can find that works in a kiln.
If that doesn't work, I will try a modifier and see how that goes. I'll also check out Olivine and see about the possibilities with that.
When loading the kiln, I am placing the glass in the center and leaving plenty of space on the sides. thanks for the thought. I think I will pass on the fibreglass at this point, just cause of the pollutant aspects of things. I feel bad enough about making molds that are not usable again. I didn't know that any of the used material could be added to the mixture, I like that idea a lot. I was told it could not be cause the chemical reaction had already taken place and it would do nothing in the second use. Is that not true?
Since there are so many undercuts in my sculptures, using a mold more than once is not an option. If it did not break before hand, I would have to break it to get out the glass.
When you are talking about 275 or 325 are you talking about the mesh? If so, I use a 200 mesh, you think that 325 if better for these purposes?
I am so thrilled that so much help is being offered and by so many. So many thanks are sent.
Carole
Okay, okay, I think it is confession time. I have not been mixing the dry ingredients before adding them to the water. It sounds like that could be the whole problem. Somehow it never occured to me. I have been carefully sifting it and following the receipe to the T, just didn't know about mixing the dry.
Good point about the thicknesses variation. I should have explained the size of the pieces better. The base that the sculpture is made on is 1 inch thick, the figures raise up out of that to height of at the most 2 inches. So there are spots where there are actually 4 inches of mold material. That is a lot of mold space, you are right. It would take an awfully long time to have that dry out, even in a kiln. I will up that time.
I am using a 50/50 by weight formula at this point. I think I will continue with it with these changes and see if it solves it. I will also bolster the sides with bricks and whatever I can find that works in a kiln.
If that doesn't work, I will try a modifier and see how that goes. I'll also check out Olivine and see about the possibilities with that.
When loading the kiln, I am placing the glass in the center and leaving plenty of space on the sides. thanks for the thought. I think I will pass on the fibreglass at this point, just cause of the pollutant aspects of things. I feel bad enough about making molds that are not usable again. I didn't know that any of the used material could be added to the mixture, I like that idea a lot. I was told it could not be cause the chemical reaction had already taken place and it would do nothing in the second use. Is that not true?
Since there are so many undercuts in my sculptures, using a mold more than once is not an option. If it did not break before hand, I would have to break it to get out the glass.
When you are talking about 275 or 325 are you talking about the mesh? If so, I use a 200 mesh, you think that 325 if better for these purposes?
I am so thrilled that so much help is being offered and by so many. So many thanks are sent.
Carole
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Jerry
Carole,
Sounds like you are sorting things out pretty well. I apologize for not telling you about mixing your dry ingredients. Some people don't do that at all and say that their work is fine, but it seems contridictory to me. I pre mix, usually a 100 pounds at a time so I have plenty to work with.
You asked about the mold wall thickness; well, 2" is good for the bottom but it may be too much for the sides. I'd opt for 1" walls; anything less is going to be weak and crack. Either way you have to be aware that the plaster is an insulator and will prevent the heat from getting to the glass uniformily or allowing it to cool. So the thicker the mold, the more time consideration for those issues. You didn't say anything about how you are mixing your material so I assume you have a handle on that. If you are unsure, ASK!
The book I'd recommend is Dan Fenton on Patte De Verre. Boyce tends to confuse me, but I'm dumber than the average bear. Dan is straight forward, clear and provides lots of formulas. Buying both wouldn't be a bad idea, however, and yes, I DO own both. Actually, I took a class with Dan a couple of years back on casting and a lot of what I do now is a result of that class.
Keep at it. Good things come to glass casters with patience.
Jerry
Sounds like you are sorting things out pretty well. I apologize for not telling you about mixing your dry ingredients. Some people don't do that at all and say that their work is fine, but it seems contridictory to me. I pre mix, usually a 100 pounds at a time so I have plenty to work with.
You asked about the mold wall thickness; well, 2" is good for the bottom but it may be too much for the sides. I'd opt for 1" walls; anything less is going to be weak and crack. Either way you have to be aware that the plaster is an insulator and will prevent the heat from getting to the glass uniformily or allowing it to cool. So the thicker the mold, the more time consideration for those issues. You didn't say anything about how you are mixing your material so I assume you have a handle on that. If you are unsure, ASK!
The book I'd recommend is Dan Fenton on Patte De Verre. Boyce tends to confuse me, but I'm dumber than the average bear. Dan is straight forward, clear and provides lots of formulas. Buying both wouldn't be a bad idea, however, and yes, I DO own both. Actually, I took a class with Dan a couple of years back on casting and a lot of what I do now is a result of that class.
Keep at it. Good things come to glass casters with patience.
Jerry
Since I mix the receipe with an electric mixer for about three minutes before pouring, perhaps that is not the problem. I will try it though to see if it makes a difference.
Thanks Jerry, I will try a mold with lesser thickness on the sides and see what happens. Such a list of things to try. It is very confusing. Also costly and time consuming. I had hoped to be turning out good castings by now. Awe well, as you said patience is the key.
Carole
Thanks Jerry, I will try a mold with lesser thickness on the sides and see what happens. Such a list of things to try. It is very confusing. Also costly and time consuming. I had hoped to be turning out good castings by now. Awe well, as you said patience is the key.
Carole
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charlie holden
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I'm saying that two inches is too thick for under the piece. The thickness of the walls has to reflect the depth of the glass. I know people that build one inch thick molds for glass that is eight inches deep, but they can bury their molds in sand when the fire. Compromise and try a mold with one inch walls and one inch bottom at its narrowest. And dry your molds for several days before they go in the kiln just to be sure.Carole wrote:Since I mix the receipe with an electric mixer for about three minutes before pouring, perhaps that is not the problem. I will try it though to see if it makes a difference.
Thanks Jerry, I will try a mold with lesser thickness on the sides and see what happens. Such a list of things to try. It is very confusing. Also costly and time consuming. I had hoped to be turning out good castings by now. Awe well, as you said patience is the key.
Carole
Another thing that has occured to me is that you should put your mould on top of some sand in the kiln. This makes it easy to level and allows moisture vent out cleanly. You may already be doing this, I don't know.
Ludo, if you add it, doesn't react with the other ingrediants. It just helps to pack the particles of plaster and silica into a tighter matrix, because it is going to have different sized particles. Imagine trying to stack a bunch of billiard balls. You could pack the bottom layer together and still fit marbles in between their corners. If the silica is all one size particle, the plaster crystals have to grow to bridge the gaps between them. If there is a range of particle sizes, the gaps are smaller. Mixing the dry ingrediants well gets all of the silica well coated in plaster.
ch
Charles,
Okay, I will make the molds one inch at their thinnest point. Both sides and base. I will dry them for three days before bringing them in.
Sand sounds like a great idea, I am not sure how I would hold that in there though. Do you mean just place it on the shelf?
Someone at the place I buy the ingredients at suggested changing to a 250 mesh instead. She said that it is irregular and that it would offer the same effect as the ludo. Do you agree?
That makes sense, that the silica would get coated in plaster. Thanks.
Again, I appreciate your kind assistance,
Carole
Okay, I will make the molds one inch at their thinnest point. Both sides and base. I will dry them for three days before bringing them in.
Sand sounds like a great idea, I am not sure how I would hold that in there though. Do you mean just place it on the shelf?
Someone at the place I buy the ingredients at suggested changing to a 250 mesh instead. She said that it is irregular and that it would offer the same effect as the ludo. Do you agree?
That makes sense, that the silica would get coated in plaster. Thanks.
Again, I appreciate your kind assistance,
Carole
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Bert Weiss
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This is exactly what I just explained over on the sand casting thread. Developing an understanding of particle sizes and what sticks and what doesn't is essential to figuring out how to design these techniques.charlie holden wrote:
Another thing that has occured to me is that you should put your mould on top of some sand in the kiln. This makes it easy to level and allows moisture vent out cleanly. You may already be doing this, I don't know.
Ludo, if you add it, doesn't react with the other ingrediants. It just helps to pack the particles of plaster and silica into a tighter matrix, because it is going to have different sized particles. Imagine trying to stack a bunch of billiard balls. You could pack the bottom layer together and still fit marbles in between their corners. If the silica is all one size particle, the plaster crystals have to grow to bridge the gaps between them. If there is a range of particle sizes, the gaps are smaller. Mixing the dry ingrediants well gets all of the silica well coated in plaster.
ch
Glass particle sizes are also essential to understand.
The best book that explains this stuff is Lundstrom "Book 3 Glass Casting and Mold Making"
Bert
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Architectural Commissions
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charlie holden
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Yep, just pile sand onto the shelf and press your mold into it. Put a level on it and rock it back and forth until it is level. You can imagine how much eaisier it will be for steam to vent out through the sand than between the shelf and the mold.Carole wrote:Sand sounds like a great idea, I am not sure how I would hold that in there though. Do you mean just place it on the shelf?
Someone at the place I buy the ingredients at suggested changing to a 250 mesh instead. She said that it is irregular and that it would offer the same effect as the ludo. Do you agree?
I don't know about variations in mesh sizes, or shapes of silica particles. I don't even know which mesh silica I've been using for the last four years. I've heard that particle sizes vary in a given grit of Olivine, and the Olivine I have tried seems much finer than the advertised grit. I'm planning on switching to Olivine for safety sake, as soon as I have used up my last 50 pounds of silica. So I may soon be working through the same types of issues you are, until I figure out new formulas and schedules.
ch
Charlie,
There is just sooo much to know in this medium. Bronzing and cold cast bronzing just aren't as complicated, or so it seems to me. I don't know that I would have gotten involved in it if I had known ahead of time, but I am hooked now that I have seen some of my pieces in it. It's too late, I am captured by it.
Okay, sand it is. I think I will try also the 250 silica. I am going to check out what Olivine is made of and see if I feel okay about making molds with it, you never know what kind of pollutant it is.
Bert,
Another book. I will try to pick it up and will look through all three books recommended. Hopefully the library has some of them.
Thanks for the cue on the postings for sand.
It has been most helpful. I do appreciate the assistance from all of you in trying to help this novice find her way through the difficulties. I am grateful.
Carole
There is just sooo much to know in this medium. Bronzing and cold cast bronzing just aren't as complicated, or so it seems to me. I don't know that I would have gotten involved in it if I had known ahead of time, but I am hooked now that I have seen some of my pieces in it. It's too late, I am captured by it.
Okay, sand it is. I think I will try also the 250 silica. I am going to check out what Olivine is made of and see if I feel okay about making molds with it, you never know what kind of pollutant it is.
Bert,
Another book. I will try to pick it up and will look through all three books recommended. Hopefully the library has some of them.
Thanks for the cue on the postings for sand.
It has been most helpful. I do appreciate the assistance from all of you in trying to help this novice find her way through the difficulties. I am grateful.
Carole
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Jerry Barnett
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Brad Walker
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Thanks so much, I don't think I will have a problem finding it now.
I'll keep you folks posted as to the outcome of all of this help. I am making a mold frame tonight, tomorrow the mold pouring and then we'll see.
It's been delightful to know there are so many people willing to help so freely.
Carole
I'll keep you folks posted as to the outcome of all of this help. I am making a mold frame tonight, tomorrow the mold pouring and then we'll see.
It's been delightful to know there are so many people willing to help so freely.
Carole
Hey there all,
I went to the PNS meeting last night and met another sculpture who casts in glass. She was claiming that hydrostone or hydorcal, 50/50 with silica was a really strong mold and held up well in a kiln. Has anyone had any experience with it? My hesitation comes in the de-molding. I have used it for casting once and found it very strong. So strong that I would think that there would be a lot of difficulty getting it out of undercuts without breaking the glass. Any thoughts?
Carole
I went to the PNS meeting last night and met another sculpture who casts in glass. She was claiming that hydrostone or hydorcal, 50/50 with silica was a really strong mold and held up well in a kiln. Has anyone had any experience with it? My hesitation comes in the de-molding. I have used it for casting once and found it very strong. So strong that I would think that there would be a lot of difficulty getting it out of undercuts without breaking the glass. Any thoughts?
Carole