What type of clay and any other materials do I need to buy to make a pourable or castable clay ?
I have a rubber mold that will produce a tile about 8" X 8" X 3/4". The finished tile is a relief rather than being flat on the face. The tile will be glazed with different colors. What do I need to make a clay casting of this mold ? The finished pieces will be used as wall hangings.
Tampa Bob
Bob,
You will need to make a plaster mould of the tile form, as clay slip won't dry out successfully in a rubber mould.
Make a plaster tile from your rubber mould and don't let it dry out!
Get some Soft Soap (also known as Mould Maker's Size), and using a soft brush and a little water, work up a thick lather and coat the surface of the tile with it and let it set. Keep coating and re-coating until the tile surface is very slick indeed. Place the sized tile face up on a board (a piece of Melamine faced board is ideal), and seal where the tile and the board meet with clay so that plaster won't run in underneath the tile.
Build a clay wall around the tile at least 1.5 inches away from it and the same height above it.
Mix up enough plaster to fill it up. Try to mix it up as gently as possible to avoid trapping air in it, and pour it gently down a ruler or similar into one corner of the walled area so that it floods gently over the tile. When it is full, gently bang the board for a few minutes to jog any air bubble in the mix up to the surface, and then leave it to go hard.
The Melamine surface of the board won't allow you to lift off the cast, but you should be able to slide it off sideways. Stick something with a strong point into the back of the plaster tile and see if you can prise it free. Do this gently as well because fresh set plaster, although apparently hard is quite soft until it has fully dried out.
When the tile is out, set the mould to dry thoroughly before attempting to use it.
To cast from it use Casting Slip which contains relatively little water compared to clay and water slip and so won't over-wet the mould.
I apologise if these instructions are overly simply stated, but I find it is best to assume no knowledge, unless you know fully who you are trying to help.
Steve Bath UK
In article , Bob writes
I think that was a great description! Short and to the point.
I have saved it in my pottery reference folder.
Thanks!
Marianne
Steve gave you excellent advice about preparing a plaster mold. Note that if you use casting slip with any mold, the normal approach is to fill the mold, wait a while for the mold to suck the water out of the clay in contact with it to produce a certain thickness of clay against the mold, then pour out the excess slip. This produces hollow objects. I don't think you particularly need that for your tile. Instead, consider a press mold. You'd still use a plaster mold, but just fill it with a slab of ordinary clay, like you'd use for hand-building (or a little stiffer). Then you apply pressure to the back of the slab, to force the clay into all the little crevices of the mold. Scrape the back of the mold to make it flat, then invert the whole works and support it by the edges a little bit above a drying bat (piece of drywall, plaster slab, etc) until the finished tile drops out. This is pretty much how they do it commercially, except that they have a press that applies the pressure and then applies air to special passages in the mold to release the tile immediately.
There is a good book on tilemaking you may want to check out called Handmade Tiles by Frank Giorgini (Lark Books).
Best regards,
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
Excellent information Steve, thanks.
Why do I not want the plaster to dry out ? Also, how do I accomplish this ? How much moisture in the plaster is "not dried out" ?
Your instructions were dead on for this newbie to pottery, kilns and firing. I do understand mold making quite well though. My work place has a large mold making shop and a separate fiberglass shop for making molds. The main mold making shop uses liquid rubber, wood, plaster, concrete and a few other things to make molds for architectural precast stone.
Tampa Bob
I had not considered pressing the clay. Thanks for the information. The chief mold maker for my work says the rubber mold I intend to use may have negative draft in some places. He is going to check it over for me. Either way, plaster or pressing will require the finished model to be free of negative draft, if I am understanding things correctly.
Tampa Bob
In article , Bob writes
You need the plaster tile to remain damp, as this prevents the Soft Soap from soaking into the plaster. The function of this material is to act as a release agent, so if it soaks in it won't release the tile from the mould!
Bob is right on the money with that suggestion; much better than slip.
I would also add about 20 percent very fine grog (powdered fired clay) or very fine sand to the clay you use for pressing to help combat warping during the drying process. Dampen it before you add it else the clay becomes over-dried and difficult to handle.
Steve Bath UK
In article , Bob Masta writes
Another tip for drying is to dry the tiles between 2 slabs of drywall (sheetrock, plasterboard, or whatever they call it near you). Of course, this only works if the "pretty" side does not have fine raised areas that would get flattened by the weight of the drywall. I use about 1-foot squares of drywall with up to 4 tiles between them.
Best regards,
Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis
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