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Plaster Vs. Bisque Bats And Molds

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32 minutes ago, RonSa said:

What is a good release to prevent the plaster from sticking to the metal springform, plexiglass or even wood?

Plaster won't stick to plexiglass, comes off very cleanly with no release. 

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Thanks Everyone.

 

37 minutes ago, Min said:

Plaster won't stick to plexiglass, comes off very cleanly with no release. 

Good to know

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ron, just read all this and realized you have not been advised that pottery plaster comes in 50 pound bags.  and it dies after 6 months so find out the date your supplier got it and use it all up asap.  there is really no way to hold onto it.  make a damp box with the leftover if there is any.

if someone tries to sell you a smaller amount, do not take it.  no telling how long it has been around.  plaster is cheap and you can always fill small styrofoam cups with  any excess.   they can be made into press molds with some carving into the surface.  something to do inside in awful weather.

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I just bought a 50lb bag this month. I knew it has a shelf life but wasn't sure how long. It was 2 months old when I got it. Thanks

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Plaster will last a long time in the desert .The best ones are in Chile and Africa. Everywhere else it collects moisture . The more moist your climate is the quicker it will get lumps in it. Lets see any snow or cold moist air in PA this time of year?The best way is to put it as soon as you buy it in a plastic trash sack and seal that up.I tend to use two bags as that seems to last a bit longer if one gets a hole in it. Once you open the bag it really sucks up the moisture in the air. . That will happen even in an unopened bag as well. The plastic bag will slow down this process.

Now if you keep that bag in the rafters above your wood stove in the living room all winter then it will have a small chance of keeping dry.I doubt many do this-and if you do keep that fire burning as wood heat drys out the air,If you are like most its in lesser location. Once you open that bag keep that (outer plastic bag sealed ) and as Oldlady said use it soon or toss it. It WILL get lumpy . For me plaster costs about 16$ a bag around here(#1 pottery) and I always just buy a bag when needed-I try to keep it going in summer but winter is just to moist to keep it here.

There has been many a post on plaster here  if you need some late night material to read.

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11 hours ago, Mark C. said:

Now if you keep that bag in the rafters above your wood stove in the living room all winter then it will have a small chance of keeping dry

I guess now I'm going to have to install a wood stove in my studio to keep my plaster dry. :D

A 50# bag is $20 where I got it, the shipping's the killer. There is a ceramic store near some friends we were visiting near Philly and we stopped in on the way there in to pick up a few things, about 200# worth.

So when its lumpy is when it bad, I'll keep an eye out for that. In the meanwhile I have an airtight container that I store the plaster in. Thanks

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I wonder about the life of plaster. I have been given old plaster, because the user thought it was useless. It was 1-2 yrs old, lumpy, ect. I sifted it with a flour sifter, used it with no noticeable issues. I'll be with alittle labor involved, i.e. Sifting, which is a pain by the way, but it was free. I have not noticed anything off about the mold I used it on. I will say that the plaster set quicker than normal, but on a small piece that wouldn't matter. I have recently made some bats out of some older, lumpy plaster, and not noticed any problems. The molds we small, a took the time to mix the plaster, working out most of the lumps, no issues to report.

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On January 19, 2018 at 2:17 PM, Min said:

Plaster won't stick to plexiglass, comes off very cleanly with no release. 

I have been using plexiglass to create a smooth surface for my table tops, i.e. Wedging tables. I pore them in blocks, upside down, so the finish side is the bottom. Using a sheet of plexiglass for the bottom, I have wood for the sides, clamp it all down. Makes for beautiful slab. I have a old reciprocal palm sander, that I use sometimes, when I'm feeling finicky, use to vibrate the bottom of the form. Alittle vibration, no bubbles, easy.

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3 hours ago, Johnmicheal said:

 I have been given old plaster, because the user thought it was useless. It was 1-2 yrs old, lumpy, ect. I sifted it with a flour sifter, used it with no noticeable issues

I hope you were wearing a mask

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What I have found is old plaster sieved is much weaker than new plaster-it never sets off right and is softer its whole life.

Its good for a damp box bottom in a plastic tub but more molds it wears out to fast and chips easily for bat making .

Edited by Mark C.

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You can use plaster no 1, which is a totally fine gypsum product to use but it does take some care in making and using. Like others have said, having a nice flat surface when you make the bat is step one. If you have an uneven surface, if/when you slam down a hunk of clay it could cause the bat the crack(seen it many a times), which is exacerbated if your using a product like bat mate to get your bats to adhere to the wheel head. If you throw a flat disk of clay on your wheel head first, and use that to stick your bats to, there is less stress on the bat. Clay is more flexible than a metal wheel head.

    There are other gypsum products which are harder and more durable than plaster but less absorbent/porous. I like commercially made hyrdobats from the ceramic shop in Philadelphia; made using hydrocal. They cost a little bit, but have rubber "grommets" (for lack of actual term) inserted into the base for use with bat pins (which I do use) which keeps the bat holes from getting blown out. Ultracal is even harder, but even less porous. Mix according to USGS specs and pour just like you would No 1 plaster.

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Thanks for all the info posted in this thread. I can confirm, plaster of paris doesn't work, especially if you use vaseline on the working surface while curing.

Edited by Armand

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Do you burp the plaster to get the air bubbles out?

ihave not used plaster in pottery.  Whenever we poured plaster for sculpture for 5 mins after pouring the plaster carefully to avoid bubbles, we had to slap the sides to encourage the bubbles to get out.  

 

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11 hours ago, preeta said:

Do you burp the plaster to get the air bubbles out?

ihave not used plaster in pottery.  Whenever we poured plaster for sculpture for 5 mins after pouring the plaster carefully to avoid bubbles, we had to slap the sides to encourage the bubbles to get out.  

 

I vibrate it a minute or two just after pouring if I get bubbles-its possible to pour bubble free. The tricks are mix in a bucket and then vibrate bucket a minute BEFORE pouring-pour slowly watching for bubbles over the lip and pour into side of whatever mold you are using.Also use a bubble free mixer like a Jiffy mixer.

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On 6/4/2018 at 4:46 PM, preeta said:

Do you burp the plaster to get the air bubbles out?

ihave not used plaster in pottery.  Whenever we poured plaster for sculpture for 5 mins after pouring the plaster carefully to avoid bubbles, we had to slap the sides to encourage the bubbles to get out.  

 

I recently took a class on mold building and was taught a different method for plaster preparation and filling molds.

When the water is filled in the bucket you add plaster until  a small heap of plaster is emerging in the middle.  If this heap remains for some seconds, there is a sufficient amount of plaster. After that you wait some minutes so that the plaster can absorb the water. After that you put your hand into the bucket to the very bottom and leave it there.  And then you carefully circle your hand but only on the bottom without moving your arm up and down. You stir as long as you feel that there are no more lumps and the plaster getting slightly thicker. This way you incorporate as little bubbles as possible.

When ready you start pouring.  First, you take only a little plaster just to cover the object for the mold and the bottom. This way, the object is secured to the bottom and the plaster can stiffen and will not leak through to the outside. You wait until the plaster hardens a little. With a cup you add then the remaining plaster until the object is covered about an inch above the highest point of the object. Recommendation was not to pour the plaster directly from the bucket into the prepared mold as it is better to add the plaster in small amounts. 

Once all the required plaster is in the prepared mold you do not burp the mold but you constantly tap with the back of your hand/fingers on the surface of the plaster. This will activate the bubbles to come to the surface. And you do this until the plaster hardens. Advantage with this method is that the mold is moved as little as possible.

I have used this method since taking the class several times and am surprised at the much better quality of my molds compared to my previous ones where I poured all the plaster in at once and used burping. What I have to work on though is on better eyeballing the amount of plaster/heap  to be used. In the past I have used the ratio as shared here in the table but weighing and measuring plaster and water is not my favorite task.

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45 minutes ago, Waltraud said:

I recently took a class on mold building and was taught a different method for plaster preparation and filling molds.

When the water is filled in the bucket you add plaster until  a small heap of plaster is emerging in the middle.  If this heap remains for some seconds, there is a sufficient amount of plaster. After that you wait some minutes so that the plaster can absorb the water. After that you put your hand into the bucket to the very bottom and leave it there.  And then you carefully circle your hand but only on the bottom without moving your arm up and down. You stir as long as you feel that there are no more lumps and the plaster getting slightly thicker. This way you incorporate as little bubbles as possible.

When ready you start pouring.  First, you take only a little plaster just to cover the object for the mold and the bottom. This way, the object is secured to the bottom and the plaster can stiffen and will not leak through to the outside. You wait until the plaster hardens a little. With a cup you add then the remaining plaster until the object is covered about an inch above the highest point of the object. Recommendation was not to pour the plaster directly from the bucket into the prepared mold as it is better to add the plaster in small amounts. 

Once all the required plaster is in the prepared mold you do not burp the mold but you constantly tap with the back of your hand/fingers on the surface of the plaster. This will activate the bubbles to come to the surface. And you do this until the plaster hardens. Advantage with this method is that the mold is moved as little as possible.

I have used this method since taking the class several times and am surprised at the much better quality of my molds compared to my previous ones where I poured all the plaster in at once and used burping. What I have to work on though is on better eyeballing the amount of plaster/heap  to be used. In the past I have used the ratio as shared here in the table but weighing and measuring plaster and water is not my favorite task.

The heap pile in the middle was how I was taught in collage buy a professor who did not know how to mix plaster by weight. Which is the right way to do it. USG has a great set of tables on mixing water and plaster by weights. The Crater lake method you speak of works but is a crap shoot at best.Always varies and is just rough guess at best. Why crater lake -well the island in the middle is the plaster mountain with lake around it.

Did you learn this at Humboldt State? as they are still backwards in some areas

Also not actuating the mold is also because the mold is not strong enough for that I'm assuming . Some agiation is best and molds should be able to handle some.

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Waltraud that is exactly how we mix the plaster.  Our molds were plastic stuff that we cut away. That’s why I guess we tapped the  mold for 5 mins.  

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4 minutes ago, Mark C. said:

 

Did you learn this at Humboldt State? as they are still backwards in some areas

Interesting.

This was a class thought by one of the former chief constructors of one of the most famous Austrian porcelain manufacturers and he is using this methods successfully since ages.

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9 minutes ago, Mark C. said:

Also not actuating the mold is also because the mold is not strong enough for that I'm assuming . Some agiation is best and molds should be able to handle some.

You are not correct with your assumption. The molds would be strong enough for that.

Apart of that, take it or leave it.
It is only a different method we were thought and it worked for me so far.

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Like I said I was tight that method in Art Collage myself and used it for about 15 years until I was taught the right way by a master mold maker. You can write USG and get there plaster specs and a handy slide rule of weights to water for free. 

I am not saying it does not work only adding that there is a more precise way to get better repeatable results with plaster. My prof also learned this at one of the best schools in the USA Alfreds in the 60's. Industry has a better way for refabeatable results. One thing to know is there are things we learn in school that later in life you will learn a better way-this is one of them.

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10 minutes ago, Mark C. said:

Like I said I was tight that method in Art Collage myself and used it for about 15 years until I was taught the right way by a master mold maker. You can write USG and get there plaster specs and a handy slide rule of weights to water for free. 

I am not saying it does not work only adding that there is a more precise way to get better repeatable results with plaster. My prof also learned this at one of the best schools in the USA Alfreds in the 60's. Industry has a better way for refabeatable results. One thing to know is there are things we learn in school that later in life you will learn a better way-this is one of them.

Mark, I understood you the first time and I appreciate learning from the experiences others have made.

Would just be fine if you could acknowledge as well that different master mold makers apply different methods. Our class was taught by a master mold maker for industry as well. 

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