moulds for pots

[paddles]

I recently had an idea for a pot... mumbled to a local potter, thought bit.. decided to make a mould for the pot (out of paper mache) potter then told me, that really he needs a mould for the outside of the pot, not the inside... mumbled a bit more, so I have finally begun, my idea is to make an interior mould, a mould of the actual pot, then the outer mould? sound good? maybe, sound feasible, depends on how much you've had to drink today... so has anyone else ever tried this? I also wondered if I could use the mould to make a fiberglass copy of the pot... just cause... you know.... I made the mould and I could???? suggestions? thoughts??? recommendations for an asylum?

[GavinG]

The way potters do it, you get a lump of clay on a board, and make it into the shape of the pot, upside down. Then you make a dam around it, and pour plaster of paris over it - when it sets, you have a negative mould of the outside of the pot. When it's all dried, you pour in a sludge of clay and water, the plaster absorbs some water, and a skin of clay deposits on the surface of the mould. Pour the sludge out when the skin is thick enough, let it dry, and you have one moulded pot. Best to put the feet on afterwards, as they can catch in the mould unless you've designed them very carefully.

Best of luck,

Gavin

[joannm]

I can see you are having some fun there
I make single piece molds for my mame pots. if you are going to use plaster, make sure it is casting plaster. it's not the easiest thing to get right if you don't understand undercuts of the pot you are molding. you also need to keep the piece supported on the board.then it never ends. please don't assume it is easy.
also in this weather the mold can take 10 days to dry before you can pour slip into it. (thats the sludge as one of you called it ) lol

[kcpoole]

I fyou want a rough irregular shape to the outside of the pot, then you can have a mould on the inside.
just make sure it will come out easily as the clay will shrink as drying and may grip too hard

Ken

[paddles]

I"ll post a photo tomorrow of the bottom/inside half of the mould... it's going to be large...

[mudlarkpottery]

I wish you good luck and hope your experiments come out well. As Joann says, it's not easy - so many things to take into account just to get a worthwhile result. Are you press moulding or slip-casting? They are two quite different approaches to making the moulds.
Penny.

[paddles]

I have absolutely no idea what the potter is going to do... I have tried to explain what I'm after... I"ve tried making small models, I've tried showing photos and saying... but I want mine to.... now I'm trying the make a full size copy of what I want (probably either in paper mache, or plaster) and give him the moulds, the copy etc.. and say... go forth and create....

[shibui]

The other thing to remember with moulded pots is that clay shrinks as it dries. Maybe this only applies to slip casting, not press moulding? You need to make your mould bigger than the desired pot to allow for the shrinkage. Not sure how much this is - maybe one of the potters has some idea of shrink rates for various clays.

[vgarth]

Yes, shrinkage applies to press moulded clay as well as to poured slip, thrown clay, hand built, etc etc. Usually about 12% from the plastic state of the clay, though amount can vary. The plastic state should be how the clay is in the bag when it's bought. Slip will shrink more, of course, more water in it. And if you use a lot of water when you throw clay on the wheel, you can expect more than 12% shrinkage from how it looks when you've finished the throwing, cos you've added water.
So if a drape mould, ie clay draped over the outside of a shape, have to remove it when it has firmed up enough to handle, but not dried too much and started to crack as it shrinks. But a mould on the inside of a shape should be able to shrink in from the walls without cracking.... maybe.

Anyway, the potter will know all about that... but as Shibui says, allow a bit bigger in the mould you prepare.
Best of luck
Val Garth

[paddles]

so if I want the finished pot to be 2ft, by 1 ft, how big does it need to be? to allow for shrinkage of the clay? or will that vary according to the clay??

[shibui]

Use Val's 12% as a guide. Your finished pot is 2' or 60cm long + 12% = 67cm long. 1' wide is half that so 33 or 34 cm wide should give you a finished dry pot about 2' x 1' but as Val says this does vary a bit with the clay and whether it is slip cast (with very wet clay) or press moulded with a drier clay slab.

[mudlarkpottery]

Just to make things even more difficult, the clay shrinks more vertically than horizontally. By the time you've got it all sorted out, you could have made a couple of dozen pots by hand but if it does work out, you'll hopefully have a mould you can use for as many pots as you want to make.
Penny.

[Lynette]

If you are talking about just a plain rectangle pot, you should not need a mould. It can just be handmade or thrown and altered. Also make sure the potter has a kiln big enough to fit the size of the pot. If you are slip casting you can either make a one piece mould (by far the easiest) or a two or more piece mould depending on the shape. If using a mould the pot you have to make sure there are no undercuts so the clay will come out of the mould.
Can you do a quick drawing of what you are looking for. and show us, so we can give better feedback.
Lynette

[MattA]

Paddles, I am wondering why you persist when a freeform pot (what you are after) could be sourced from one of several aussie bonsai potters. Its all great to make a mould but in the long run you are doing all the work for the potter... Is there something about this artists work you admire? If so I would be giving them carte blanche, otherwise find someone who understands & can do what you want

[paddles]

Oh, this will be fun... I'll put a photo of the first "model" I made, followed by the beginnings of the interior part of the mould...