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Making a mold

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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.

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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

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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.

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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

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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

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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

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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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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.

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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

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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

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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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Vaseline at least for me is not something I would have used.

As far as the bubbles NEXT time vibrate the mold more right after pouring and they should go away. The broken pieces are from sticking so better release measures next time.

Go to a drug store and buy some green soap for release agent and use the vasiline for something else.

Looks like a good 1st try.

Mark

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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

 

 

The bubbles on bottom would bother me also. I don't know how technically smooth you need it. But maybe you can fill holes with more plaster . Smooth as much as possible with straight spatula, dry , Sand it smooth. ( it works on dry wall). Guess I've gotten lucky with few I've made.

 

I'm also wondering If you could place master in mold and re pour rim?

 

How are you going to use mold?

 

I use Murphy 's soap for release agent, no problems ....yet.

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Thanks Mark the credit goes to you and everyone else who helped me out.

 

Will do the steps to get rid of the bubbles next time, I read your post about this after I had already poured the mold and the top had hardened.

 

What brand of green soap should I use?

 

Evan

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The bubbles on bottom would bother me also. I don't know how technically smooth you need it. But maybe you can fill holes with more plaster . Smooth as much as possible with straight spatula, dry , Sand it smooth. ( it works on dry wall). Guess I've gotten lucky with few I've made.

 

I'm also wondering If you could place master in mold and re pour rim?

 

How are you going to use mold?

 

I use Murphy 's soap for release agent, no problems ....yet.

 

 

I could try sanding the bottom to get rid of the bubble holes. Either that or next time I pour a mold I could pour a bit in the bottom to cover the holes (if that would work).

 

I don't think the damage at the rim is going to be a problem. I just won't be able to pour slip all the way up to the rim.

 

This mold was mostly a practice mold although I will at least try it to see how it works. My wife (the potter in the family) wants me to make a beer stein mold that will have to be a two-part mold. Before I tackle something that complicated I wanted to make sure I could confidently make simple one piece molds.

 

I would have tried the Murphy's soap but it didn't cover up the mistakes in my woodworking skills (not my day job) and the Murphy's soap really bubbled a lot. Because of that I went with something thicker like vaseline.

 

Evan

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With Murphy's soap I don't use a brush, I use a small piece old t shirt.

 

I started with brush to many bubbles, then wiped bubbles with t shirt rag.... Now I just use small square of t shirt.

 

If any small bubbles I wipe off with moist oils soap rag.

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With Murphy's soap I don't use a brush, I use a small piece old t shirt.

 

I started with brush to many bubbles, then wiped bubbles with t shirt rag.... Now I just use small square of t shirt.

 

If any small bubbles I wipe off with moist oils soap rag.

 

 

Okay thanks for the tip.

 

Evan

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Avoiding air bubbles will probably be a bigger problem than any of your present concerns.

 

Jim

 

 

Damn! I was right again.

 

Jim

 

 

Jim,

 

I need to get a grand piano. An old Steinway would do just fine. And the time to learn Rachmaninov's Piano Concerto No 3.

 

Evan

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After you get that Grand piano cut the top into some coddle boards and save a large square for the bottom of mold.This top may have stain rings from some of Jims vitracation experiments that weeped thru.The rings will not bother the mold process-stick to major brands like Stienway or Baldwin as they have a smoother wood finish.

As far as brands of green soap any green soap will do-better still is mold release sold at ceramic suppliers.Green soap will leave a small foam but it will not show once plaster is poured.

Mark

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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.

 

 

I definitely don't ignore Jim. He's got some of the best stories on these forums, not to mentioned, I'm a fan of his work.

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