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Making a mold


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#21 Mark C.

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Posted 27 June 2013 - 12:31 AM

I use a piece of formica but any smooth wood like smooth plywood will work.
The mold will be nicer with clay coils smoothed into all corners-I did a photo post on this last two springs ago.
Mark
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#22 Marcia Selsor

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Posted 27 June 2013 - 07:20 AM

I use a thin sheet of plexiglass. It gives a mirror smooth surface for the bottom and plaster won't stick to plexiglass.
I use the cottle board frame. It is so easy once you have them made. And they are flexible for various sizes.

Marcia

#23 Strelnikov

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Posted 27 June 2013 - 10:53 AM

I use a thin sheet of plexiglass. It gives a mirror smooth surface for the bottom and plaster won't stick to plexiglass.
I use the cottle board frame. It is so easy once you have them made. And they are flexible for various sizes.

Marcia


Great idea Marcia I think I'll go with the plexiglass.

Evan

#24 Strelnikov

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Posted 27 June 2013 - 10:55 AM


One thing to consider is that I made my box out of cheap pine that wasn't very straight. The video I watched on making cottle boards (from Ceramic Arts Daily) said to use oak or birch plywood. That stuff has to be pretty expensive comparatively if I even knew where to get it. Home Depot has oak boards but they are very expensive. I don't remember ever seeing oak or birch plywood there. I guess the good thing is that cottle boards are adjustable and once you bear the expense of making them, they can be used many times.

Another problem is that I don't have a suitable base to put the cottle boards on. In one video the instructor suggested using something like a formica counter base I believe. What are you folks using?


No need to use oak or birch, just use plain old pine or any other inexpensive wood. Mine are pine and are nice and straight and have been used for years and only get better with age. Just use any wood that is straight and smooth. You can set them on any work table. I spray a little WD40 on the table first then use the cottle boards. If I want an especially smooth bottom to the mold I set my cottle boards on my grand piano.

Jim


Jim,

Do you really pour molds on your grand piano???

Evan

#25 OffCenter

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Posted 27 June 2013 - 12:47 PM



One thing to consider is that I made my box out of cheap pine that wasn't very straight. The video I watched on making cottle boards (from Ceramic Arts Daily) said to use oak or birch plywood. That stuff has to be pretty expensive comparatively if I even knew where to get it. Home Depot has oak boards but they are very expensive. I don't remember ever seeing oak or birch plywood there. I guess the good thing is that cottle boards are adjustable and once you bear the expense of making them, they can be used many times.

Another problem is that I don't have a suitable base to put the cottle boards on. In one video the instructor suggested using something like a formica counter base I believe. What are you folks using?


No need to use oak or birch, just use plain old pine or any other inexpensive wood. Mine are pine and are nice and straight and have been used for years and only get better with age. Just use any wood that is straight and smooth. You can set them on any work table. I spray a little WD40 on the table first then use the cottle boards. If I want an especially smooth bottom to the mold I set my cottle boards on my grand piano.

Jim


Jim,

Do you really pour molds on your grand piano???

Evan


No, just joking because I'm always warning people to never depend on the glaze to prevent leaks and I use a vase someone thinks they've sealed with glaze on a grand piano as an example to the point that I'm sorta known here for that and my hatred of splash pans. The clay should not leak with no glaze on it. Obviously, that needs to be pointed out from time to time since just a few days ago someone who has been potting 40-something years mentioned depending on the glaze to seal his pots. I wish I had a grand piano--so I could sell it.

Jim
E pur si muove.

"But it does move," said Galileo under his breath.

#26 Strelnikov

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Posted 27 June 2013 - 01:51 PM

Okay guess I need to read between the lines.

Evan

#27 Benzine

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Posted 27 June 2013 - 02:02 PM

I wish I had a grand piano--so I could sell it.

Jim


Jim, have you thought of selling/ trading your Peter Pugger, for a grand piano? The grand might do a better job of of working your clay, than your Pugger does.
"Anything worth believing, is worth questioning"

#28 OffCenter

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Posted 27 June 2013 - 05:18 PM


I wish I had a grand piano--so I could sell it.

Jim


Jim, have you thought of selling/ trading your Peter Pugger, for a grand piano? The grand might do a better job of of working your clay, than your Pugger does.


That's an idea but I don't know where I'd put it in my studio, besides, people would think I'm weird and snakes would get in it.

Jim
E pur si muove.

"But it does move," said Galileo under his breath.

#29 Diane Puckett

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Posted 27 June 2013 - 06:28 PM



I wish I had a grand piano--so I could sell it.

Jim


Jim, have you thought of selling/ trading your Peter Pugger, for a grand piano? The grand might do a better job of of working your clay, than your Pugger does.


That's an idea but I don't know where I'd put it in my studio, besides, people would think I'm weird and snakes would get in it.

Jim

Or you could swap it for a Giffin Grip.
Diane Puckett
Dry Ridge Pottery

#30 Strelnikov

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Posted 27 June 2013 - 06:44 PM


I wish I had a grand piano--so I could sell it.

Jim


Jim, have you thought of selling/ trading your Peter Pugger, for a grand piano? The grand might do a better job of of working your clay, than your Pugger does.


... and as an added bonus, the grand would make music while working the clay.

Evan

#31 Idaho Potter

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Posted 28 June 2013 - 02:06 AM

Evan, all joking aside, you need cottle boards. Plaster expands as it cures, and if you confine it to a box where the sides are screwed together, you are likely to have the mold split/crack when you are unable to release a corner as the plaster cures. Cottle boards and ratchet clamps are staple items when making a mold. As to what type of boards to use, look for the smoothest surface (at least one side) and lumber or plywood work well once they are sealed and a release agent is applied. You really need to be able to release the form quickly and easily.

Also, if this is your first mold, make your application in layers. Cover the object you are taking a mold from with splashed/flicked on (by hand) layer that gets all the detail you need. Then apply another layer and reinforce it with strips of burlap or some other open weave cloth (only necessary if the mold is large and will be used multiple times). When the plaster has set then pour the next batch of plaster to cover everything and fill up your "box". Jiggle the table a lot to get the bubbles to rise to the top (bubbles weaken the mold) or lightly tap the boards with a rubber mallet.

I never measure plaster and water. I pick my container fill it with water equal to the amount of plaster I figure I can mix and apply before it starts to set up. I then slowly sift the plaster over & into the water--without stirring--until it forms an island. I then (wearing gloves) reach into the bottom of the container and gently lift and mix with squeezing motions. Don't stir with a stick or mixer of any sort--way too many air bubbles.

Good luck!

Shirley

#32 OffCenter

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Posted 28 June 2013 - 07:30 AM

Good advice, Shirley. I've never used burlap. Will have to try that. I still get bubbles even doing most of what you suggest above, but then I usually rush it a little when I start thinking the plaster is setting up.

Jim
E pur si muove.

"But it does move," said Galileo under his breath.

#33 Strelnikov

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Posted 28 June 2013 - 08:44 AM

Thanks for the advice Shirley, I never thought about the mold expanding as it cures. That would be a problem with the box.

Evan

#34 Mark C.

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Posted 28 June 2013 - 10:11 AM

I use cold water (slows the set times) and use a power mixer in a 5 gallon bucket. I weigh the plaster and water so mix is always the same.Always add plaster to water not the other way. I leaned mold work from a master mold maker who is now passed away.I'm not a master mold guy lets get that clear. Making molds was his business in life.Making pots is mine.
I mix it at least2-4 minutes-then drop the bucket several times on a hard surface (cement works well)
This gets all the air bubbles to the top-then pour slowly to cover form and when mold is full you will need to thump the mold -often its easier to slightly drop the table the work is on
so it jiggles the air bubbles to top.
Once set up unscrew or unclamp boards trim edges with fettling knife to round them and let dry.I usually strap clamp them as they can not warp while drying.
I have a recipe #s of plater to #s of water stuck to the studio wall if needed.
Mark
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#35 TJR

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Posted 28 June 2013 - 11:08 AM

Thanks for the advice Shirley, I never thought about the mold expanding as it cures. That would be a problem with the box.

Evan


Evan;
The reference to the grande piano is an on-going joke about pots leaking and ruining your grande piano. It has nothing to do with plaster casting.
ceramics art daily has a good visual tutorial on mold making. Don't have the link, sorry.
Just ignore Jim. Most of us do. But I find him funny, so I don't. I do know how to cast plaster though.Mark Cortnoy has good advice.
TJR.

#36 Idaho Potter

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Posted 28 June 2013 - 01:56 PM

Jim, on smaller molds I've used gauze, lace or anything with an open weave. Burlap is reserved for the big honking molds that get treated roughly 'cause I'm not as young as I used to be. I think we all have developed methods for making molds based on who we learned from, and what we will be using the molds for.

I've made many molds for objects to be cast in bronze. That means a detailed model, then the mold, then cast wax, then plaster & sand investment so the wax can be melted out and replaced with molten bronze (called "lost wax method" of casting). That's why I use layers of plaster to pick up the detail in the original model. It also prevents me from trying to pack on plaster that has started to set up. I don't work as fast as I once did. (is this an age thing? Yep!)

So what is the most important thing to do when making a two-part (or more parts) mold? Keys! It's the only way to make sure mold parts are properly aligned. Any more must dos?


I like slump or hump molds (one from the other) cause it's only one piece.

Shirley

#37 Strelnikov

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Posted 28 June 2013 - 02:48 PM

I decided to go ahead and pour my mold as a learning experience, if only to try mixing and pouring the plaster. If I have to throw away the mold then so be it.

Before I started I tried coating everything the plaster would touch with Murphy's Oil Soap. It made a lot of bubbles so I stopped and washed it off. I then decided to try using vaseline applied with a foam brush. That worked pretty good and covered up some small cracks and undercut areas in the wood that the soap didn't.

Next I weighed out 24 lbs 13 oz of lukewarm water in one bucket (next time I'll use cold water) and 35 lbs of plaster in another bucket. I then gradually poured the plaster into the bucket with the water.

I let the mix soak for 1 minute, then power stirred it with my electric drill for 4 minutes.

Then I poured the plaster into the mold so it was almost full. Deciding that I couldn't leave the mold where it was, outside on my patio, I moved the mold into my back screened porch.

Big mistake, the mold was too full and heavy and plaster spilled all over the patio bricks, my back porch, the sides of the mold, and worst of all my pants, shoes, shirt, and arms (yeah dumb move on my part). Next time I'll pour the mold in its final resting place.

I then topped off the mold, hosed down the patio, cleaned up the porch, changed my clothes, and washed the plaster out of the dirty clothes next to the curb using a garden hose.

The good part is that I had enough plaster to fill up the mold with just a little bit left over, even after spilling some (thanks to you folks). I was afraid I wouldn't have enough or would have way too much.

My mold is hard on top. When should I try to remove the wood from the mold?

Evan

#38 BeckyH

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Posted 28 June 2013 - 03:56 PM

Once the plaster is just warm to the touch. It shuld get fairly hot as it sets. You can let it cool in the mold, but it's easier to move before it sets rock hard.

#39 Strelnikov

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Posted 28 June 2013 - 03:59 PM

Looks like I missed my window of opportunity. It got really warm but has now cooled off. I will see if I can get it apart.

Thanks,
Evan

#40 Strelnikov

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Posted 28 June 2013 - 06:40 PM

Interestingly enough the top of the mold was cool but once I opened it up the inside was still hot. There is some damage on the top of the mold and some air bubbles at the bottom. The sides are okay but are a little greasy from the vaseline. I'm not too concerned about the damage at the top but the bubbles at the bottom bother me. Is there any way to fill them in and smooth off the bottom?

Once again thanks so much to everyone for your help! I can't believe my first mold might actually be usable!

Evan

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