I use a brush-on mold making technique with a silicon inner an a hard plastic outer shell (mother mold). The silicon gives the texture and the hard outer shell gives the structure.
Step 1: Get an Idea what to make
In this project I wished to make a girl sitting on the grass at a picnic looking up at a friend. She wears a large summer hat that she clasps on her head to hold it from falling off.
Step 2: Research the project
All my source material come from Google and Pintrest image searches. I make a collage of these images, that I print out and pin on the wall where I work.
Step 3: Make an armature
An armature is the inner structure of the clay sculpture. You can think of it as the skeleton. For this sculpture I wanted to make the scale 1:4. I scaled the basic proportions of a person and welded up the base structure from a 6 mm rod. This I bend into the position required and welded it on a angle iron base that I bolted to the wooden base board. Getting the scale right saves a HUGE amount of trouble later on! I cover the armature in tin foil. This allows the the clay to shrink without cracking.
Step 4: Sculpt the figure in clay
I use pottery clay to sculpt. I used clay without grog or paper as I want the smooth areas to be really smooth. In this piece, I got the proportions of the armature legs wrong. I only realized this later on and had to cut them off (I did not take a picture). Luckily the legs are part of the base, so I lost no structural strength from the cut.
The clay sculpture is done, now to make the mold
Step 5: Cover the sculpture with the first layer of silicon
The silicon I use is called Mold-Max 20 (or 25, depending on what is in stock). It is a 2 part mix that you mix together to form a sticky pink goo that behaves like no other material I have worked with. It has the consistency of yogurt and the stringiness of melted cheese. It sticks to everything, except the wet clay. I use 2 spoons to apply it, scraping the silicon off onto the sculpture. A trick to get the silicon in overhanging and detailed spaces, is to attach the armature to the wooden base board. You can then tip it to pour the silicon over the details an let it run into the difficult areas. The first layer will be very thin. Make sure to cover the whole piece.
Step 6: Cover it some more
The next batch of silicon is mixed with a thixotropic agent that thickens the silicon to a paste. This is a lot easier to apply than the first layer and forms a 5 mm thick layer on top of the first. The silicon dries in about an hour (depending on the weather). I add 2 thicker layers on top of the first.
Step 7: Add the ShimsMark the line where the outer shell will split with a pen. Cut into the silicon and stick the shims into the slit. I use ex-ray plates to form my shims. I paste the shims to each other by running packaging tape along both sides. The aim is to form a tight structure that won't sift or come loose when applying the outer mold.
Step 8: Make the outer mold
I use a two part plastic resin called Material 1 for the outer mold, mixed with a "whisk" attachment on my drill. Once it is mixed, it has the consistency of water and have a very faint, pleasant smell. It sets in 15 minutes so you have to work fast. Don't mix too much to handle in that time. To apply the plastic, dip pieces of pre-cut, roof sealing cloth into the mix and smooth it onto the silicon. Wear surgeon's gloves and old clothes because this stuff does not come off easily (especially off your nails). Start at the edge of the Shims and have the cloth overlap each other. Cover the whole piece in 3 layers.
Step 9: Trim the edges and drill holes through the shim flanges
I use a pair of tin shears to trim off the edges of the flanges. Drill 6.5 mm holes through the center of the flanges at about 15 cm intervals for the 6 mm bolts and wing nuts. At the end you must be able to remove the outer mold so you have to plan your split lines carefully.
Step 10: Open the mold and clean out the clay
Pry open the outer mold carefully. You do not want to harm the flanges to much. To remove the silicon mold you have to cut through the shim line into the clay. It should peal right off. Don't be afraid to break the clay sculpture. Wash the silicon mold clean with water.
Step 11: Cast the sculpture
I use reinforced cement to cast my sculptures, but you can use this mold to cast resin or wax(used in bronze casting). For the wet part of the mix I use 1 part water to 1 part bonding or keying liquid.
I cover the inside of the mold with a layer of 100% cement and liquid. It is mixed to a paste consistency and applied with an old cheap artist brush (The brush will be useless after this)
While is is still wet, I pack the remaining cavity with a mix of 1 part cement and 3 parts sand mixed with the water and keying liquid solution. The cement must have a stiff consistency. To test it, form ball of cement in your hand. If it runs through you fingers it is to wet. If you open your hand and it crumbles, it is to dry. Push the mix into all the cavities. When both sides is full, place steel rods in the small areas to reinforce them. For this sculpture I placed a rod along the arms and up the back, through the neck into the head.
Place the two halves together and bolt them up. Store the sculpture upside down with a piece of cling foil over the base to stop it from drying to fast.
Leave the sculpture for at least 7 days to dry.
Step 12: Open the mold
Opening the mold is the most exciting of the process. It's like Christmas! Only a Christmas where you could get a piece of coal. The first cast is rarely a good cast. Unbolt the outer mold and pry it open. Don't do it all at once from one side, but gingerly work your way around the flange so that it opens evenly. Once the outer is removed, peal of the silicon. At this point the cement is very brittle and can easily break of so be very careful and take your time.
This cast came out fairly ok for a first one. She lost a nose and a hand, but that is gets fixed in the last step.
Step 13: Finishing the sculpture
Once the sculpture is out of the mold it is very brittle. Now is the time that you can easily break off the parts that leaked out of the mold with your hands. Don't do to much though as you can easily break the sculpture. Let it dry and harden for a day before you start the serious work.
Chip and grind the unwanted pieces off the sculpture. To fill the holes and cracks in the cement (and to build up the hand and nose) I use a mix of dry cement and quick set cement (mix 1:1) mixed with the same solution of water and bonding liquid used before. This dries in about 15 Minutes. Sand the sculpture with a 100 grid sanding paper.
For the final touch I add a layer of cobra floor polish and brush it off. For the dress I dipped the cloth with polish in green cement oxide. It then collect in the corner and give it green hue. Glue a piece of felt to the bottom and the sculpture is done.
For videos about sculpture view this playlist: