Making a mold out of Clay?

[CDK]

Originally Posted by DCockey

Chavant is a major (only?) supplier of clay to the auto industry in the US, and the clay used is oil base and contains sulfer. http://www.chavant.com It is firm at room temperature though a finger nail easily makes an impression and pressure with a finge type can leave a mark. The clay can be milled and scraped at room temperature. Warm it slightly and it softens so that it much easier to move with hand pressure, and it can also be extruded when warm. My former employer used restaurant "bun warmers" to keep the clay at a working temperature. The clay is re-used though it eventually needs to have more oil added.

The major advantages of clay are that it can easily be worked by hand, and it is almost as easy to add material as to remove it. So it's great for revising a shape.

Several decades ago I visited Peter Monteverdi in Switzerland, where he built exotic cars in very small series (http://en.wikipedia.org/wiki/Monteverdi_(car)). His people were working with the same water based clay my wife uses for pottery (and made the same mess).
Of course she also buys her clay at ridiculous prices because the stuff you dig yourself has lots of entrapped stones and other impurities that cut your skin and spoil the object in the furnace.

But for a simple shape like a canoe, natural clay on a wire frame should present no such problems: there are no fine details and there is no baking involved.

[Steve W]

This is an interesting thread which just shows that there are many ways to do things. It seems to me that if you are going to build over a wire frame plaster of paris would be a fair substitute for clay and maybe easier to buy and cheaper,this has been done before and is quite a common way of building male plugs.
Steve.

[CDK]

That's right Steve, except that the time to shape plaster of paris is limited. There are retarding agents to buy you some more time, but you cannot quit what you're doing and continue the next day. With clay you can.

[Steve W]

I would see using plaster of paris, sheetrock mud and similar products as being very similar to building in Ferro cement in that you would get the frames, lath, mesh as fair as possible and then try to plaster it as fair as possible then, unlike Ferro, when its dried it is very easy to fair further with sandpaper, much easier than polyester or epoxy fillers for example, obviously you need to seal it.
Steve.