By now you have your clay work finished and have it sitting on a large enough piece of melamine coated cabinet board so that it allows several inches for the mold material. A dam or wall either created with thickness of clay, paper cups or even tin will work, just make sure it can support a little pressure from the fluid pushing against it.
You will do one section at a time keep it simple and do not rush.
Registration marks or keys will be worked into your walls to lock your mold pieces together. These can be made by using a little (key) mold bought at art supply store (or make your own). One side of wall will have a positive and the other a negative. If you use clay for the wall you can poke indention's or tracks where they lock.
Determining where your wall will fall is crucial.
Determining where your wall will fall is crucial.....no I am not crazy just repeating important issue.
This is where you have to look at your piece very carefully for undercuts. For instance if you have a two piece mold and you pull them apart will you be pulling a nose or ear off , etc.
In some cases you can split tiny areas after silicon has set up to loosen area around something but over time sometimes that weakens the integrity of the area. Some unnecessary undercuts can be avoided by just filling in with a little clay knowing you can clean it out later in your wax.
Polytek Platsil 71.20 RTV Silicon Rubber
Be careful of items used (no latex gloves) and be prepared to throw away whatever you mix your Part A and Part B together . Plastic buckets are great and then into a third container to blend the two.The largest problem is deciding how much volume you need for the job you have created. This can be broken down by math (there is only room for a 5% error or so) a scale that measures in grams accurately is great. Most important in the beginning is spraying everything you will be touching with the silicon with release.The wall, board, clay, just don't get it near your silicon itself.
Mix Part A and Part B equally but do not pour together until you are completely ready and set up to continue.They are two separate colors so it will blend to make a third color so stir it until you are sure that it is completely blended...you are allowed a little time so you can make sure. If you are making a one piece mold you have the piece already dammed all around the object and just evenly and smoothly pour the mixture all around the piece until you have it covered sufficiently (this is where you do not want it so thin that it tears or pokes through the mold). This will set up in four hours and you can remove the wall and extend it out farther where it can accept the mother mold that will support it through the months of storage.
This is a plaster mold (instructions on the sack) and you just add a little shredded fiberglass to the plaster to add strength. One thing you want to make sure is that you don't add too much so that you get a pocket of fiberglass like a bubble next to the silicon...this weakens your mother mold. This type of mold is perfect for a relief type piece or a simple piece that can be pulled apart easily..in other words it gets more complicated the number of molds you have in a single piece also if you have a vertical piece that requires a thickener (which they sell for silicon) then you are laying it on vertical surface. For that type of mold you would need a specialist in mold making and that is not me.