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debbie

plaster molds

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I would like to make a plaster mold of ceramic listellos and chair rails but when I mold them they do not release from the mold. I have no problem making molds from clay items which peel right out but when I try to use hard objects they do not release from the mold. I have tried murphy's oil soap as a release but the chair rail held firm and would not release from the mold. I looked at the latex molding materials online but am not sure if they will stand up to clay. If I can just get a clay model I can make a plaster mold from it.

 

 

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One option is to make a clay impression of the items and then make a plaster cast. The clay impression will be a negative impression; the plaster cast a positive impression. You can then make a plaster negative impression from the plaster cast.

 

Or you could bisque fire the clay impression and use that as a bisque mold.

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Latex can be used to to take an impression which can be transferred to plaster. This is the method used by large industrial ceramics companies. They take a latex mold of a piece then cast a positive from the latex in plaster. Then create many plaster molds from the positive transfer.

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Thanks for the suggestion. How do you keep the plaster from sticking to the plaster mold? Do you push the plaster positive into the poured plaster mold from the top or put it on the bottom and pour plaster over it like I do my clay pieces?

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Guest JBaymore

If the original piece is not releasing, are you SURE that there is not a subtle undecut on the form you are casting or you missed the half-way point a hair on a round form? It does not take much to make things not release.

 

best,

 

............john

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What I have tried twice is a ceramic fired glazed chair rail coated with murphy's oil soap as I read in a suggestion. I pressed it slightly into clay to adhere it to my container and poured plaster on top. When I released the mold from the plastic container I can't seem to get the chair rail to release from the mold. I make plaster molds this way with clay and have no issues because the clay is soft and easily removed. I just don't know how to make frame pieces and thought I could make a mold.

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Guest JBaymore

You can't have any undercuts with a hard item. So for example if you wanted to make a mold of a round wooden dowel, you would have to embed the "model" into a clay slab PRECISELY to the half-way point on the dowel (lengthwise). Otherwise the piece locks itself into the set plaster because the plaster mold has an opening that is just slightly SMALLER than the diameter of the dowel that it has to get pulled back through that opening.

 

Clay models are easy to work with, because they "move" when you pull them out. This lets you get away with a lot less precision than what you need for hard objects.

 

Maybe this is the issue. All it takes is a TINY undercut... and you are sunk.

 

best,

 

............john

 

(edit) to make it clearer.

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Sometimes a hard object doesn't want to release, and you have to crack it to get it out of the mold. I check the places where the object seems stuck, and scrape gently at the spot with a narrow paint scraper. If I missed an undercut I remove as much plaster as necessary, then see if I have to recast the piece. Murphy's has worked well for me as a release agent. Use two coats, and let it dry before pouring.

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If you are using glazed listels and tiles, before you do this again check the sides of these pieces, the overlapping glaze application can cause undercuts. Or there may be unglazed ends that are causing the plaster to lock on to them, there may be spacers on the sides that are causing the difficulty. If the models are textured this can be a big problem with undercuts. Even the draught of the commercial tiles may have been cut slightly off 90Ëš when they were manufactured causing undercuts.

 

With a little patience and work you can try this: wet the mold down with hot faucet water on the working side, try to avoid putting hot water on the model. Sometimes the hot water alone may release it. If it doesn’t, use a pen knife or something like it, wear protective gloves and goggles and if you have a workbench with ‘bench dogs’ this will help to steady the mold so that you can work on it. Slowly scrape away the plaster from around the edges of the model. This will alter your mold.

 

If the listel has a bull nose side you may have to go down rather deeply past the undercut, this will also alter your mold.

You may need to re-wet the mold as you work the model loose. Try lifting the model after you have worked on it awhile from an end with the pen knife. You may further harm the mold doing this.

 

If you want a flexible model it can be done with the flexible mediums such as the urethanes which come in many 'hardnesses,' and also in thixotropic. They are all two part mixes. There are a few steps and you must use the manufacturer's recommended release for them.

 

It is best however, to make your own models from water based or oil based clay when making plaster molds. You can then make allowances for the clay shrinkage upon firing.

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When I make plaster molds I throw a slab of clay on an old bat and seat the item to be cast into the soft clay. I make sure to remove any undercut from the object by using slip or coils to seal around the object. I then seat a bucket or container of the appropriate size that I've removed the bottom off of into the soft slab and seal it with slip. I then spray on some mold release onto the object and use some petroleum jelly to coat the bucket to make releasing easier. I then mix my plaster and begin to cast on top of the object by coating the object and making sure there is no trapped are by vibrating the bat or bucket by gently thumping it with a stick. I wait until it's set and still wet and the release the object while it's still warm and soft. That way I can patch or repair the mold if necessary.

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Debbie, you can also use plastercine (play dough) to remove any undercuts from your original that you wish to cast. And use dental tools and a finger damp with water to smooth the play dough. Use a table lamp and your wheel to turn the item around and make sure that there is no shadows that could indicate undercuts.

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When I make plaster molds I throw a slab of clay on an old bat and seat the item to be cast into the soft clay. I make sure to remove any undercut from the object by using slip or coils to seal around the object. I then seat a bucket or container of the appropriate size that I've removed the bottom off of into the soft slab and seal it with slip. I then spray on some mold release onto the object and use some petroleum jelly to coat the bucket to make releasing easier. I then mix my plaster and begin to cast on top of the object by coating the object and making sure there is no trapped are by vibrating the bat or bucket by gently thumping it with a stick. I wait until it's set and still wet and the release the object while it's still warm and soft. That way I can patch or repair the mold if necessary.

 

 

 

How long does it take for the plaster to get to the "warm and set stage"?

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Usually about 15-20 minutes. The initial hardening goes rather quickly, but the final drying takes weeks. Molds for slip casting dry for at least 8 weeks before being used.

 

WOW, never! You can use a dryer (fan in a box type of thing to dry molds) or use a biscuit dryer (if you a huge production factory). Prop your molds up on wooden bits so that the plaster does not have a lot of contact with the shelve, make sure there is some draft. And if the sun is out, we normally put them on a low overhang of the roof. Even in winter it does not take me more than 4 days to dry huge plate molds.

 

Wind is your friend if your molds is wet.

 

And if it is a small mold that could fit into your microwave, you can put it there for 10 minute increments on defrost.

 

But what ever you do, never ever expose it to temperatures above 60 - 80 Centigrade.

 

(I used to date a professional moddeler and mold maker, German trained.)

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Usually about 15-20 minutes. The initial hardening goes rather quickly, but the final drying takes weeks. Molds for slip casting dry for at least 8 weeks before being used.

 

 

WOW, never! You can use a dryer (fan in a box type of thing to dry molds) or use a biscuit dryer (if you a huge production factory). Prop your molds up on wooden bits so that the plaster does not have a lot of contact with the shelve, make sure there is some draft. And if the sun is out, we normally put them on a low overhang of the roof. Even in winter it does not take me more than 4 days to dry huge plate molds.

 

Wind is your friend if your molds is wet.

 

And if it is a small mold that could fit into your microwave, you can put it there for 10 minute increments on defrost.

 

But what ever you do, never ever expose it to temperatures above 60 - 80 Centigrade.

 

(I used to date a professional moddeler and mold maker, German trained.)

 

Well, that's how long my friends who make molds for slipcast mugs and such like to let their molds dry. They are often about 12" square and mostly solid. How much production do you run through your molds?

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My molds are typically dry enough to use in two weeks or less. I do leave them on another piece of plaster inside my studio. There is a small heater in my studio and I set the mold near the edge of my metal table which is right next to the heater. I flip the mold everyday, and change out the plaster underneath. My molds are made from Hydrostone which is a plaster/cement and very durable. The initial hardness is much greater than No.1 pottery plaster. Once they are set and dry, the mold will have a crush strength topping 10,000 lbs/in^2 and impact strength of 1320 lbs/in^2. These are not slip cast molds, but press molds, slump molds and hump molds. I purchased mine dry in a bag from a local fiberglass distributor who sells mold making supplies and a number of other materials. For you Portland residents I've listed a link to the company I purchased my materials from. This company was referred to me by my dentist who purchases the dental grade plaster from them to produce dentures.

 

http://www.fiberlayoregon.com/

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I live in South Africa, and we often have to use what we can get plaster wise :-)

I still use molds made 3 years ago with a white stone ware clay (300 casts easily). I never wipe the insides with a wet cloth - but I use white vinegar if the pores become clogged; but my molds that I use for my black clay have a shorter life span of about 90 - 120 casts. I always date my molds, and the ones that need replacement, was made 25 Dec 2010. (yea yea yea, I have no life :-)

 

Honestly, plaster molds dry much quicker than mentioned above. I never place wet molds flat on the work surface, but always store them on the edge of a wooden bat, so that as much as possible surface is exposed to air.

 

If I can remember and take my camera into the studio, I will take some pics of my set up and my molds.

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