Ceramics group?

Where in the world do you live?

Near Tampa Bay, Florida, USA

This is probably as good a place as any. It sounds like you are interested in "slip casting". That will allow you to make hollow vessels. If you don't need the decorations to be hollow, you might be able to make them by "press molding". Tell us more about exactly what you want to make.

The molds in either case are typically made from plaster. You can use plain old plaster of Paris, or pottery plaster (which is better, and cheaper in big bags). You need to make the molds such that there is no undercut area; you should be able to move the clay straight out from the mold without it hanging up on anything. That often requires multi-part molds, but when you are just starting out you would be better off to find something simpler.

If you have a simple piece with a flat back, yo may be able to stick it down in a flat-bottomed plastic container using a bit of plasticine modelling clay, then paint it with liquid soap (as a parting agent) and cover with wet plaster. After it is set up, pull out the original. Let the plaster mold dry for a few days or more, then press a wad of pottery clay into the cavity. It should release fairly easily, since the plaster will suck moisture away from the contact surface and cause the clay to shrink slightly. Then dry and proceed as for normal pottery, (bisque fire, paint with glaze, glaze fire).

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Thank you, just learning the phrase "slip casting" was enough to make my searches more fruitfull.

What I want to make are things like the Christmas houses you see in stores. So they would be partly hollow and partly solid (the base). I think slip casting is the answer.

The part I really want to learn is the making of the molds, but of course I will need to be good at working with them when they are done.

Might I also suggest a good book: "Clay, A Studio Handbook" by Vince Pitelka Chapter 4 deals exclusively with plaster working, mold making, and slip casting, and is comprehensive enough to be of good use to you. Hope that helps, Wayne Seidl

Just a side question, can I depend on the amount of shrinkage? I want my peices to be exactly 6 inches across so I need to know the shrinkage when I (re) make my mold.

You might also check into local businesses. In SE Michigan, at least, there are (were) a lot of hobby ceramics shops that appear to have fallen on hard times. These shops typically buy the molds and just cast their own wares, which they sell as greenware or bisque. There are also shops where people can walk in off the street, select a piece of bisque, paint it with supplied glazes, and come back and get it after it is fired.

These businesses might be your competitors if that is your goal, but otherwise they might have some practical tips. And if you approach one of those that is having hard times, you might be able to buy all your supplies in one go, at cheap prices. I've seen a number of Going-out-of-business sales around here.

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

.....want my

In which case you need to ask your slip supplier about the shrinkage of a particular product, and know to which temperature you will fire it. As a reference, I use porcelain slip, fire it to ^6 or higher (1200 deg. cel.) and it shrinks almost 14% If you make your model for mold out of clay, it will also start shrinking as soon as it starts drying Also, molds for houses will need to be made from several parts so you can get the cast out without braking it. You will need to measure ratio of plaster to water for each part so that they are the same(otherwise the absorption of water from slip will be uneven and cause problems) hope it helps, Andrea

shrinkage will depend on the clay/slip you are using as they differ in rate.allow anything from a couple of percent up to about 14%. it really is trial and error when you want a specific result size.

I'd like to elaborate on the good advice others have given. There are different types and causes of shrinkage. The first is going from slip to bone-dry, and the second is going from bone-dry to fully-fired. As noted, the amounts depend a lot on your clay. I haven't gotten into slip casting, but I suspect the initial drying shrinkage may also depend on how heavily deflocculated the slip is. (A deflocculant is an additive like sodium silicate that makes the slip more fluid with less water.)

It's been my experience that many clays don't shrink at all in going from bone-dry to bisque. The shrinkage only comes when they start to vitrify. I've used this to advantage on pieces that needed to be a certain dimension after firing, since you can sand, file, or grind the greenware before firing. (Possible after bisque, but *much* harder.) Then glaze at low-fire temperatures to keep the same size. Note that this is not a good method for dinnerware or vases, since the unvitrified clay will never really be waterproof, your best glazing efforts not withstanding. And you will need a really low-expansion glaze to keep it from crazing.

Hope this helps!

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Grandpa always said, "...give the hardest job to the laziest man...he'll find the easiest way to do it..." and Grandpa was right, gods rest his soul :>)

Take a kid's plastic ruler, one with raised numbers and lines on it. Cut a slab of clay at least as long and as wide as the ruler, then turn the ruler upside down and press to imprint the numbers into it. Fire that piece as you normally would. When it's fired and cool, lay the ruler next to it, and with a pen or marker, mark the "real" measurements next to the edge.

There's your shrinkage guide for YOUR clay. Make one for each clay you use.

To find 6 inches, look until you see the "real" six inch mark, and then read what the clay size has to be to shrink to 6"

Thanks to someone on Clayart for that suggestion, not me. Best, Wayne Seidl

Hello

There will be alot of trial and error in what you are attempting for a first - timer.

May I suggest going to one of the "hobby" ceramics shops and asking them how much they would charge to do the mold making for you? Many of them have mold making facilities to add fresh designs to their collection for their surface decorating clientele. Having the molds made for you may speed-up the results you want from this endeavor. Of course there is the process which may be intriguing to you that's different than a results oriented intent. At any rate those hobby serving mold-makers have alot of knowledge and experience to tap into, maybe working for one part-time or just inquiring will get you the knowledge and experience you need to go on with your small house making project. Have fun and good luck. Nancy Jane

v

i have a batch of cheap old wood rulers and i glue a fired clay ruler on the back of the 'original" ruler to record shrinkage of all the clays i use. you can readily see the start & finished size of the clay.

steve

steve graber

If shrinkage were only that easy. The shrinkage your measuring is only linear. Have you considered the shrinkage across the width of say a bowl?

When a bowl is thrown on the wheel you create compression in the clay at the base and you stretch the clay at the rim. So when the bowl is drying it contracts in different ways.

The easiest way I can explain this is if you think of a bag with a drawstring neck. As you tighten the drawstring the bag closes in towards the middle.

This is the way the clay behaves in a thrown bowl. If you want a specific diameter and height you have to make allowances for the rim to contract making it smaller in diameter and it rises up as it draws in, therefore making it higher.

Tricky old business if you need to make a specific finished size.

Practice practice is the only way.

Good luck

...and error and error and error... apparently!

That's a great idea, and I will expand my search for such a place. So far, the ceramic places I've visited simply purchase pre-cast peices for "Paint your own" ceramics. One lady pours molds, but only premade molds.

I suspect I will have to go towards Orlando to find anyone who does their own work.

"Nancy Albin" wrote ;

Oh!... and, BTW, I poured slip into my first one-part mold yesterday and it worked! Today I am making part two of a three-part mold.

Of course, there is still firing and ect to do, but at least now I have something to fire besides funny looking clay scraps.

Trial and error indeed...

sounds like you are moving right along ! NJ

v

May I suggest you check your local library for a magazine called Popular Ceramics. It bills itself as "the original ceramic hobby magazine". It has some excellent information on slip casting, pouring, making molds, decorating, etc. I make molds from my own original work and have found this source most helpful. In the magazine, you will find many sources for the type of ware you seek. You can buy bisqued ware for decorating, molds to pour you own, or supplies and information on how to make your own molds. They have also had excellent articles on firing, kiln maintenance, and studio safety.

Good luck,

Jan C.

Have you tried

? They have some interesting links which may take you to folks who will make the molds for you. Perhaps, they may even have links to the exact product you are looking for. Steve in Tampa, FL.