Making a slip or press mold

ketoi

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I've been wanting to make one of these for a while so I might as share the experience.

1st you need to make a positive mold, either out of clay or suitable material. I used foam because its faster and I have plenty of it. You may ask where to get this kind of foam, might be able to get at craft stores but it probably be thin stuff. I used the orthopedic type (from my real job), it comes in various thicknesses and densities, this stuff is pretty firm.

positive mold is 5in square and 4in tall including the feet




Make or purchase coddles, I used 1x8 clear pine and clamps to hold it in place. Next seal corners with clay to keep plaster from pouring out on to the floor.


Positive placed in the center. The plaster will not stick to the foam


Get plaster, this is about 7 large scoops (around 7 heaping quarts)


There's many ways to mix plaster, you can be all scientific and measure volume, calculate how much water but I have an easy way, the orthopedic way. Add enough lukewarm (warmer it is the faster is sets up) water to cover, go do something else for 15min or so and check. If the water level has dropped add more to slightly cover, repeat this process till there are no more air bubble emerging from the plaster.


When there are no more bubble start mixing, be gentle and mix thoroughly by hand so you can smash lumps with your fingers. Those electric mixers will aerate the plaster too much, it's fast but more work later when you have to get the air pockets out of the dry plaster.


Should look like pancake batter


Pour plaster in slowly so the positive stays in the center


Then dump the rest in and smooth the surface


Also made an foam oval positive if this one is successful.

Tomorrow we'll see how it turned out
 

Boondock

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wow, this is awesome, and an odd coincidence. I went to the library and found a book on making plaster molds just like this, they even used the same board set up (I think there called casting boards, or cuddle boards, I can't remember). I was thinking about modeling a fancy foot for a bonsai pot, and then making a plaster mold for it.

The model you made is excellent. I think it would appeal to many people.

I think I may make a cardboard template of the shape, the feet and the sides on one piece, and then one for the bottom.

I'm looking forward to seeing how this turns out.
 

irene_b

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Won't the plaster (wet) become a part of the mold? I am thinking wet plaster seeping into foam..
Irene
 

ketoi

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Irene,
Depends on the foam, open cell (course texture) will have a slight bond but it can separated with some effort. Closed cell (smooth surface) there is no bond at all and pulls away with no effort.

Thanks Boon,
Working with plaster takes some practice but it's no harder that working with clay. Cardboard will not hold up in the plaster, it will bond to it and all the moisture will collapse it. You should use the silicone molding compound from Tap plastics, once it's dry it will pull away from the cardboard. This will also work better for intricate designs.
 

Boondock

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when I try to make a plaster mold I think I will use leather hard clay walls. Since I was only thinking about making a small mold for a bonsai pot foot, I was going to make a small clay box about 4 X 4 inches (just the bottom and 4 sides), then placing the model in the clay box, and pouring the plaster. After 30 minutes, when the plaster set-up, just remove the leather hard clay walls.

When I mentioned cardboard template, I was referring to just making a pattern to use to cut out a shape in a slab. I make templates for every oval,rounded corner rectangle other other odd shapes. That way I can cut a shape in seconds, and reproduce it accurately. Or I could roll out a big slab, and using the template for your pot, I could cut out the pieces for 20 of them, in less than 2 minutes. If I were mass-producing a single pot, I would cut the templates out of plexiglass for durability.

I make the shapes in photoshop, print them, cut out, and transfer to heavy card board.
 

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ketoi

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Cool! will be interesting to see how it come out.

Removed coddles


Positive encased in plaster


Couple of air bubbles need to be filled and cleaned up
 
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Ok,, Ketoi,
Where did you go from here? I'd like to see the remaining direction of this post.
You got my attention (it wasn't hard):)
Was wondering how to finish the mold process.
Thanks, and great work so far.
KJ
 

ketoi

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Got side tracked on crack pots...but that's another topic.

Mold is finished. What I did was let the plaster dry completley then submerge in water. Once the water is reabsorbed I mixed up a small batch of plaster to fix any imperfections, air holes rough spots or whatever. Corners of the outside are rounded and cleaned , now I just need to pour some slip(liquid clauy) in it and see how it turns out.
 

ketoi

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Poured slip in the mold yesterday


After the clay firmed up the excess was trimmed away


Now I just wait till the clay dries and shrinks so it can be removed easily
 

ketoi

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Press mold for feet

Make feet and walls to hold plaster


Place feet within the walls


Pour in plaster
 

Asus101

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Poured slip in the mold yesterday


After the clay firmed up the excess was trimmed away


Now I just wait till the clay dries and shrinks so it can be removed easily

Im confused at this part. How did you get the empty space here when slip was poured?
 

ketoi

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Im confused at this part. How did you get the empty space here when slip was poured?

Your right asus, I skipped the most important part.

The slip is poured into the mold, you then let it sit. The longer the sit time the thicker the walls of the pot.
 

ketoi

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re: feet press mold

After the plaster has hardened the clay walls and clay feet are removed. Side and edges are cleaned. The plaster needs to dry completely before using, if used wet the clay will not release easily from the mold.
 

ketoi

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After a 30min sit time you can see the thickness of the wall, roughly 1/4"


The wet slip is then poured out leaving what has dried
 

Asus101

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Your right asus, I skipped the most important part.

The slip is poured into the mold, you then let it sit. The longer the sit time the thicker the walls of the pot.
SO you simply fill the mold up? how does it form walls?
I gather that maybe the plaster draws out the moisture, pulling the clay particles together against the wall of the mold?
 

ketoi

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SO you simply fill the mold up? how does it form walls?
I gather that maybe the plaster draws out the moisture, pulling the clay particles together against the wall of the mold?

Correct! The plaster sucks the water out of the slip thus creating shape.
 

meushi

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Thanks for the mini-tutorial ketoi! I will try this technique in the next few weeks I think!
 

ketoi

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Hey Boon,
Sorry if I stepped on yer toes :0)

My memory only goes back so far....
 
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