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Hump molds


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I got a couple of oval mold patterns from Pure and Simple for making plaster slump/hump molds.  Neither was exactly what I wanted, good size, but not deep enough.  One I added a big slab of set plaster and just surformed it to shape and the other I only wanted a little, so I just set it on a formed to shape piece of plywood.  So far so good.

Not being content, I want a max sized oval for my kiln, so 19x12 or so.  I looking at how to create the master.  The only ideas I have come up with so far are multi stack plywood, cutting each oval smaller from the top down and then hogging off material to get the final shape.  I can do this as I have a bandsaw and other wood working tools.  The trick will getting it laid out correctly on paper. 

The other idea I'm kicking around is thick styrofoam and a heat knife.  I've even seen some advertised with bendable blades.  Maybe I could get a 6" thick piece of styrofoam cut the way I want it in one pass.  No experience with this so totally speculation.

Other than finding the perfect bowl and just pouring the plaster, any other ideas on how to make a symmetrical, oval hump mold to my specs?

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I've done that before myself.  You make a rib the size and shape of the outside curve and use it to work the clay.  You could build a form around the clay and pour plaster or using hydrocal, you can just apply it by hand.  The hydrocal has an extended kick time where by it's almost a putty like consistency.

Forgot about that one.

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2 hours ago, neilestrick said:

Glue up sheets of house foam- the pink or blue stuff they put on the outside of houses under the siding- then carve them down to the shape you want using a rasp or sander.

Isn't that foam the stuff like packing, hobby craft stuff?  I think the problem with it is that the little beads come out any way they want to, making a mess and a not smooth surface.  My revelation was finding that there are hot knives specifically made for cutting the stuff.  Harbor Freight has one for $20.  I'm just sure there are problems lurking under the placid surface of that idea.  I also have tried this approach once and didn't find a glue that I liked for gluing up the styrofoam.  The stuff they make specifically for that didn't work well.  Most glues will dissolve the styrofoam. 

Thanks for helping me think about this project.  I might go back to the clay and hydrocal approach.  I have that on hand.

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15 hours ago, Babs said:

Go for it. Make video and post it

 I am sure folk would appreciate that.

From memory she showed mathematically ..aka string , pencil and pin how to get the oval template. On the ground so to speak

Luckily,   I have the oval I want.  One side of the SlumpHump mold is perfect.

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50 minutes ago, CactusPots said:

Isn't that foam the stuff like packing, hobby craft stuff?  I think the problem with it is that the little beads come out any way they want to, making a mess and a not smooth surface.

Nope, not that stuff but the higher density extruded rigid insulation boards from a building supply place. It can be sanded down after roughing out and it comes out pretty smooth.

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The building insulation foam is the best for this use (not sold at hobby stores) look for sheets at build supply/lumber stores. Its very sandable.

Its dense -there are many types so look for the dense stuff.

I think a clay master is also a good way to go. 

On the slump vs hump deal the best molds are slump molds where the form is inside the plaster form so as it dries it shrinks away and does not bind up and crack. Much easier to leave  and come back to.You never loose one from cracking on the form. Its easy to reverse the thing and make it a slump mold just by one more more plaster pour.I really dislike hump molds and love slump molds.

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6 minutes ago, Mark C. said:

The building insulation foam is the best for this use (not sold at hobby stores) look for sheets at build supply/lumber stores. Its very sandable.

Its dense -there are many types so look for the dense stuff.

I think a clay master is also a good way to go. 

On the slump vs hump deal the best molds are slump molds where the form is inside the plaster form so as it dries it shrinks away and does not bind up and crack. Much easier to leave  and come back to.You never loose one from cracking on the form. Its easy to reverse the thing and make it a slump mold just by one more more plaster pour.I really dislike hump molds and love slump molds.

I guess I haven't seen the right foam.  The building foam Home Depot has is backed on one side with a silver coating.  If I run across it I may give that I try.  Being the tool fool that I am, I'm looking for an excuse to buy the hot knife made for shaping foam.  Got any leads on where to look for the building foam you're talking about?

I need a hump mold because I add features to the outside such as feet and sprig type textures.  It's best to join clay as wet as possible,  so I almost finish the pot before it comes off the mold.  So far with the 2 modified hump molds from Pure and Simple, they have been releasing without any mold release applied.  Either way, unless I build a slump type master, I'll have 2 pours to get what I want.

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9 minutes ago, mnnaj said:

Once you get the form made out of the foam, you could coat it with plaster to make a mold that will dry the clay.   I saw Margaret Bohls make a form that way.  It was lightweight and easy to use.

Nancy J

So the foam form was an inch or so less than the finished mold?  I could do that with hydrocal and the hand applied technique, it might be tricky to get it uniform.  Lots of work with the surform and eyeball, I think.

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2 hours ago, CactusPots said:

I guess I haven't seen the right foam.  The building foam Home Depot has is backed on one side with a silver coating.  If I run across it I may give that I try.  Being the tool fool that I am, I'm looking for an excuse to buy the hot knife made for shaping foam.  Got any leads on where to look for the building foam you're talking about?

I need a hump mold because I add features to the outside such as feet and sprig type textures.  It's best to join clay as wet as possible,  so I almost finish the pot before it comes off the mold.  So far with the 2 modified hump molds from Pure and Simple, they have been releasing without any mold release applied.  Either way, unless I build a slump type master, I'll have 2 pours to get what I want.

https://www.homedepot.com/p/Owens-Corning-FOAMULAR-150-1-in-x-4-ft-x-8-ft-R-5-Scored-Square-Edge-Rigid-Foam-Board-Insulation-Sheathing-20WE/207179253

Comes in several thicknesses.

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The second rule of extreme DIY (according to me) is priority must be given to material on hand.  Turns out I didn't have sufficient plywood for my first idea.  The foam will get a look another time, but here's my strengths.  First I have a pugmill, so I can get really soft clay.  Much easier for this kind of thing.  Second is the material for the rib.  Back in the 90s, I was installing telephone systems.  The main controllers where the size of large refrigerators and the control cards where a full 12"x12".  Every slot that was vacant had a filler card in the slot.  The material was like a circuit board with nothing on it.  It was 2 layers of something like fiberglass with a woven metal core.  Cut on a band saw, there are sparks.  Anyway it's super thin, light as stronger than anything I've seen.  It's no thicker than really thin cardboard and unbreakable and unbendable.  I can use it for extruder dies.  First rule of extreme DIY, never throw anything away.

I'll give it a shell of hydrocal tomorrow and then pour plaster.

 

image.png.c45702cc70b86902ad31daa03861b9f2.png

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the problem with using just the word "foam" is that there are a lot of things with that name.   the stuff neil is talking about is in the insulation department of your home center.  it does not have holes that you can see easily.

then there is upholstery foam that we press clay into with a shape  so it can take that shape instantly.

and the  thin sheet foam that cuts with a scissors and is great for making a shape you can roll into clay.

and the white stuff they make cheap coolers from, styrofoam.   then the stuff in the can that sprays into openings, then the.....................

Edited by oldlady
correction
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I did a lot of work past summer with foam insulation -bought all the specify knifes for it as well even a super saw for it. with the right tools you can do wonders with it-they make a cualk for sticking it togther as well -says for foam board on tube.

Yes clay is the easy material for many things especially molds.

 

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7 hours ago, CactusPots said:

So the foam form was an inch or so less than the finished mold?  I could do that with hydrocal and the hand applied technique, it might be tricky to get it uniform.  Lots of work with the surform and eyeball, I think.

The form that Margaret made out of foam looked similar to the one you just posted out of clay.  She glued multiple thickness together to get the height she wanted.  After carving out the foam, she mixed up some plaster, when it got to the point of being thick - not hard, she spread it on like frosting over the entire top of the hump, she also set the form on some plaster to completely surround it.  I believe she tapped and jiggled  the form a bit to get it to flatten the high spots .  There may have been use of a surform or green scrubby to smooth things out when it was hardened.

Nancy

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You can pour a load of plaster onto a board, and then use a pre-made template (plywood or similar) to drag the plaster into shape.  It's called sledging.

https://ceramicartsnetwork.org/daily/pottery-making-techniques/making-ceramic-molds/how-to-get-the-perfect-profile-by-sledging-plaster/

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  • 2 months later...

Couldn’t find much on slump/hump molds in the forum so thought I’d ask my questions here. Im looking to get a couple molds for plates and bowls for slab building but I’m not sure if the sizes of the molds I’m seeing online is taking into account shrinkage. For example, if a hump mold for a bowl  says its 7.5” diameter it doesn’t mean my bowl will be 7.5” right because of shrinkage. So would buying a slightly larger mold, say  8 or 9”,  be the way to go if i wanted a 7.5” final product?  I cant find a clear answer on this online,  any help would be appreciated thank you!

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21 minutes ago, Cynthia44 said:

Couldn’t find much on slump/hump molds in the forum so thought I’d ask my questions here. Im looking to get a couple molds for plates and bowls for slab building but I’m not sure if the sizes of the molds I’m seeing online is taking into account shrinkage. For example, if a hump mold for a bowl  says its 7.5” diameter it doesn’t mean my bowl will be 7.5” right because of shrinkage. So would buying a slightly larger mold, say  8 or 9”,  be the way to go if i wanted a 7.5” final product?  I cant find a clear answer on this online,  any help would be appreciated thank you!

It's the size of the mold, not the fired product.  

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