Tuesday, January 27, 2009

Plaster Mold for Ceramic or Porcelain

In this tut, we are making a two piece mold. First we need a form to make the mold in. I had the above form made for me by a person that makes plexi cases. You can use wood-but it will swell from the moisture, so the wood has to be protected. You also need C-clamps to attach the L forms together. Next, you need kids play clay-the old fashion kind, a single edge razor blade, and plaster that is specifically made for mold making. I have forgotten what the plaster is called, but the place that sells plaster should know what you are talking about. Regular plaster of paris is too soft, and will not hold up-but if that is all you have, try it. Keep plaster powder in a sealed container.

Note: If you have a piece that is too big, make a mold, pour it in porcelain and when it is fired it will shrink 33%. Then make another mold, pour, fire-shrunk another 33%. And so on.

Now for the piece we are making the mold of. We are doing a 2 piece mold. For each plain of the object, you need to make a mold piece. Example: Say you are making tiles-you would only need a one piece mold-a tile has only one plain. Lets take a vase (it can have one or two handles)-you have two planes: front and back.

Look at the mold on the left. Say you were holding the real object in your hand. Take a soft leaded pencil and draw a line around the object that will cut it in half or plains. (I just notice an optical illusion about the picture on the left-it looks like the piece is rising up-it is NOT. The mold is hollowed out)

Now, put some of the kids clay where the "pour hole" will be for the slip. Usually it is where an opening already is. Or on the bottom of the object. You can always close a hole before firing with wet slip.

NOTE: There always has to be a hole in the piece for the air to escape while firing-except tiny objects.

Now put two of your L forms together( making sure when the mold is finished there will be enough plaster around the object to absorb the moisture of your slip when you pour the mold). When you put your L forms together, you will need to put a small snake of kids clay at the joining point to completely seal the joint. We DO NOT want the liquid plaster to escape. Put these L forms on a appropriate size sheet of plexi.

The next step is like you are making half the mold out of kids clay. Put the top of the pour hole w/the piece you are molding against the L form (inside) making sure there is equal distance around the piece you are molding. (check out the above picture)

NOTE: The smaller the piece you are molding, the shorter the pour hole. You have to get the excess slip out somehow.

Now, I either hold the piece I am making the mold of and do this, or leave it pushed against the two L forms. The object is to make the kids clay seal up the bottom of the piece you are molding, so you can pour the plaster and make the top half of the mold. The kids clay does not have to be solid(thick)-I make a 1/4th inch sheet and work it around the piece I am molding-AT the pencil line. If the piece you are molding is rounded put little stilts under the 1/4th kids clay to make it flat and not slanted. The VERY important thing is to seal the kids clay against the two L forms and around the piece you are molding. WE DO NOT WANT THE LIQUID PLASTER TO ESCAPE!!!! Also, take the end of your pencil and make a 1/8" indention in the kids clay-these are to make sure the mold goes together correctly when finished.

Next we take the next L form (which will make a U out of forms), put a kids clay snake at the joining point-make sure it is right next to the 1/4th in sheet you have made. Push the plexiglass together and clamp it with the C clamp.

NOTE: I have been doing this a very long time-so I am giving you LOTS of hints I have learned. Here is another one. The first two clamps you put together must be put together solid as a rock. The next L form only has to be joined till the (1/8th in dia) kids clay rope is squished just enough to seal. These forms do not fit together perfectly in the end. So when you put the last L form on to make a square you have enough give to push the L forms together without breaking them. The C clamps will hold them till you get things pushed together, then when it is just right tighten the last two Clamps. MAKE SURE YOU PUT YOUR 1/8TH IN. KIDS CLAY ROPE ON ALL MEETING SURFACES-KEEP THAT LIQUID PLASTER IN!!!

If there are tiny gap between the clay and the plexiglass L form, make a tiny rope of kids clay and lay it over the gap-then smooth it to seal. Now, put a fat rope piece (1/2") along the out side bottom edges of the square plexiglass mold form-we have got to keep that plaster IN.

Now we have our form ready to pour the plaster. If you have allergies, I would suggest a dust mask or dish towel over your nose and mouth-the plaster dust will make you sick. This is what I do and it always works. Take a sealed(non pure paper) or plastic cup and fill it with half of the amount of water you think will make the half mold. (I make small molds, so if you are going to make a BIG MOLD you will have to adjust). Take a small cup and shake the powder plaster in the water a layer at a time (make sure you have a big enough cup-the plaster power doubles the water). Now, when the plaster powder no longer sinks into the water-stop putting in plaster. You want an very thin layer of dry plaster powder on the top of your water. DO NOT TOUCH OR MOVE CUP OF PLASTER/WATER. Put the timer on 10 mins. If it is hot in your house, stick to 10 mins-if it is cold in your house wait 15 min. I try to get the plaster a tiny bit thick-it does not escape the unfilled spots as easily.

When the dinger goes off, take a craft stick and stir. This is IMPORTANT-bang the cup on a flat surface to bring the bubbles out of the plaster-bubbles are our enemy. Then take some cooking spray (plain Pam is best) and lightly spray the top of your mold form. Where it puddles, take an old brush (I have had mine for 20 years) and paint it around. Cover everything WELL.

Now, pour your plaster s--l--o--w--l--y. Take a tooth pick and pop the bubbles as you are pouring. Darn bubbles. You can always make too much plaster and not use it-but you can't make more plaster if you do not have enough. Now, we hope and pray that you have sealed every place well. Lift the plexi sheet you have your L form mold thing on and bang it on the table top to loose those pesky bubbles. Don't lift the side too high-about an inch will do. Bang it about 5 time-hope you don't leak.

Now you have to wait. For the small mold I make, I wait 6 hours or until dry to the touch. The plaster, as it is setting up will release water-DO NOT REMOVE THE WATER. It really is best to leave the plaster mold 12 hours-you want it really set but not rigid.

Take off two adjacent L forms and gently lift out the half plaster mold. Clean all the kids clay off. Leave the piece you are molding in the plaster mold. Our old problem with leaking plaster has popped up again. Set the plaster mold in the two L forms that are together. Keep moving it around until you get the best fit. If there is a gap, take a little kids clay and put it on the plexi L forms (kids clay will NOT stick to plaster) where the top of the mold is-then push the mold against the kids clay. Put the other two L forms together as you did before.

NOW-we must plaster proof our mold. Any place there is a tiny gap on the edge of the mold and the L form, make a rope of kids clay and lay it over the gap. Smush and smooth into place. Where the kids clay pour hole meet the L form-make sure that is sealed against the L form. Make sure the 1/2" ropes of kids clay you put on the bottom of the out side of the L forms is secure.

Make the plaster as before. Do not spray too much Pam-make sure you spray enough.

Now this HAS to cure 12 hours.

Gently take the mold out of the L form square(take the L forms apart). With gentle pressure pull the mold apart. Gently take the piece you molded out of the mold. There may be some bubble hole marks. I have discovered how to fix that. Make up a tiny bit of plaster-while the plaster is setting up put the mold back together. When the plaster is ready, open the mold and put a generous amount of plaster in the bubble hole. Immediately, smooth it with you finger. When you are done, put the mold back together (it will warp if you don't and the halves will not go together). Set the damp mold on top of a glass or ceramic cup-rotate often. It takes about 4 days to dry. While wet, you can scrap the outside sharp edges with a dull knife. When dry, put a rubber band around the mold, then pour you slip.

If you have any questions or want pictures(I will add pictures the next time I make a mold) email me at katescats@msn.com.