Preparing the project
The realization of a mold in two parts has to begin with the analysis of the sculpture to mold to determine how both parts will be cut, as well as the location of the hole of casting...
Defining the separation axis of the two parts
The first stage consists in identifying how the sculpture can be realized in two parts, which mean to visualize the location of the cuts which will correspond to the place where the mold divides and will form the separating joint or wall. Both parts have to be separable, which means not to have any undercuts which would prevent the silicone from getting loose from the sculpture. It is thus advisable to define the separation of the mold along edges and make sure that there are no element preventing the release. Fortunately the silicone is a little bit flexible which allows small bumps to be released (for example the beak, the ears or the claws of the owl). If undercuts are present and it is not thus possible to realize a mold in two parts, it will be necessary either to make 3 parts, or if it is possible to proceed by pieces.
The separation (defined by the separating joint) will rather have to cross on the summits of the sculpture in a not too textured place and avoiding as possible the front of the sculpture so that it is not too visible. It's better to avoid placing the separation in hidden recesses, and place them on plans which will be easier to re-polish on the edition.
If the sculpture was drilled to be then put on a base, you will have to consider the question of the integration in the molding of the stalk. This answer depends on the material of the edition: if the edition is in plaster, there is no reason to integrate the stalk, unlike if it's a bronze or resin edition. The integration of the stalk also depends on the needs of the sculptor: if the edition will be placed on it's base exactly in the same way as the original (base of the same form and same angle for the sculpture) then the stalk can be integrated into the molding. Otherwise it is better not to integrate it and to drill the edition afterward. In this last case, don't forget to block the drill hole with Plastiline to prevent the silicone to enter into this hole.
Identifying the hole of casting
The hole of casting is the hole you'll use, during the realization of an edition, to pour the material. It has to have a diameter of one centimeter minimum to allow an easy casting, be situated on the highest part of the sculpture (which can however be returned) so that the material can fill the whole mold. Also the hole of casting has to be situated if possible on a part not too visible of the sculpture (avoid the front, if possible put it rather down) and not textured so that it's easy to re-polish the edition.
When the sculpture has a flat bottom (as the owl), you can use the base of the sculpture as hole of casting, simply apply a board against the foot of the sculpture and put Plastiline all around to make sure that neither the silicone nor the plaster will recover the flat part.
When there is not this flat part on the sculpture, it is then necessary to create a cylinder in Plastiline to create the hole of casting. This operation will have to be made on the second part of the mold, but it is necessary to make sure at the beginning of the molding project that there is a space not textured in height to place it.
Building the separating joint or wall
The separaing joint or the wall is created to protect the face that will be casted later and thus has to follow the separation defined for the project. It can be realized with Plastiline or with clay. The wall must be at least 4 cms wide to be large enough for the thicknesses of silicone and plaster. The more the molded sculpture is big, the more the wall has to be wide because it is what will assure the solidity of the final mold. The wall has to stick to the sculpture to avoid that the silicone flows between both, it also has to be very solid because it will afterward have to support the weight of the plaster screed. Finally it is important that the wall is very smooth so that the silicone and the plaster also be smooth on both parts and also for the mold to be tight, what will limit the marks of separation on the edition and thus limit the work to be made on it.
To create the separating joint of the sculpture of the owl (about 20cms height), you'll need about 2kgs of Plastiline.
Creating the keys
Keys are volumes, dug or added in the material, placed on the junction between both parts of the mold which will allow that this one emboite and does not move during the casting. You will have to place keys on the silicone and the plaster what will assure the good behavior of the whole mold.
For the silicone part, you can create keys by digging serpentines in Plastiline around the sculpture (as in the image above). You can also dig small returns or holes whith a trimming tool or stick small balls on the separating joint. Keys will have to be situated at least at 1 to 2 cms of the sculpture.
The impression coat
The impression coat is a fine and fluid coat which will be applied with a brush on the whole sculpture to make sure that the silicone recovers well all the parts, even in chinks and to make sure that there will be no air bubbles which then would appear on the edition.
Prepare a small quantity of silicone and mix it with the catalyst (5g of catalyst for 100g of silicone) with a spatula, be careful to mix well to avoid disparities in the taking of the silicone. Apply then the silicone on the edges of the sculpture and let it pour. Pass again then with a small brush on all the places where air bubbles would have been able to form (hidden recesses and harshness). All the sculpture must be covered as well as keys if needed.
Let the silicone spread out and flow and then take. To verify if the silicone is taken, you can touch it with the finger (Protected by a glove!) to see if it sticks or if it is firm. You can also, to avoid touching the sculpture whith the risk of damaging him, test the rest of silicone in the bowl which was used to mix him. With the spatula, check if the silicone sticks or if it gets loose easily. To be able to do the same during the next coat, it is necessary to remove all the silicone of the bowl, what is very easy because when it is taken, it withdraws very easily. To clean the brush which served to apply the undercoat, use some white spirit.
For the impression coat of the sculpture of the owl (about 20cms height), 100grs of silicone and 5grs of catalyst have been used.
The thickness coat
Once the impression coat is taken, you can create the thickness coat. Mix a more important quantity of silicone with 5 % of catalyst. Once mixed, add a little bit of thixotropic agent, a thickening which will allow the silicone to stay in position on the sculpture without flowing too much. Thixotropic agent's quantity to be added has to be controlled: mix, then let rest one minute because mixing prevent the thixotropic agent from acting. Verify then the consistency of the silicone by letting it flow of the spatula. It has to almost not flow any more.
When the silicone is ready, apply it with a spatula in thick coat on the whole sculpture and separating joint by making a movement to raise the silicone on the edges, because in spite of the thixo it will tend to flow slightly. It is necessary to try to have a regular coat on the whole sulpture, what can be difficult when it has a complicated shape (where from the interest to know well or to have studied well the sculpture before beginning, certain moulders photograph even the sculpture in the molding position to keep track of its shape).
Then you just have to wait until the silicone takes. Then cut the edges of the silicone but be careful not to cut the keys. To cut the silicone precisely, make a bait and then cut with a safe gesture in once by holding the cutter vertically.
For the thickness coat of the sculpture of the owl (about 20cms height), 500grs of silicone and 25grs of thixotropic agent have been used.
The plaster screed
Creating the keys on the plaster screed
You can then create the keys that will maintain the plaster screed by making small bricks of Plastiline that you will position on the edge of the separating joint. These bricks have to have 3 bevelled edges and 1 straight edge which is the one that we shall position along the edge of the separating joint. This shape is important because it will facilitate the opening of the mold and will limit the risks of break-in of the keys which are the most fragile parts of the mold.
When the mold is not very big, it is also possible to realize round keys by digging the Plastiline with a trimming tool. Pay special attention that the realized holes are rounded off well and smooth inside to obtain beautiful plaster keys.
It is also possible to realize the keys only later, once the plaster thickness coat of the first side is done. It will be easy then to dig keys (rounds) in the screed.
Preparing the work
The plaster screed will be realized with a mix of molding plaster (preferently Molda 3 because it can be used either for the screed and the editions), water and oakum.
The taking of the plaster being rather fast, it is necessary to organize its work before beginning.
The first stage thus consists in preparing the oak so that it can be quickly used when it will be necessary. The oak allows to create a lighter and more solid mold. Pull a small handful of oak and aerate it by pulling at fibers and by forming a "nest" into which try to put the ends of fiber which exceed (so that they do not risk to exceed of the plaster screed).
Don't forget to cover the wood you're working on with vaseline or a plastic film so that it will be easy to remove.
The preparation of the plaster: put some water in a basin where you put vaseline before or in a silicone bowl and sprinkle with some plaster until saturation of the water (more there is of plaster, faster this one takes).
The plaster can be accelerated with some warm, dirty or salted water or with lemon.
Separate the plaster in two bowls, one for immediate use, the other one for the second coat, then you'll be able to manage two speeds of taking. Indeed add in the bowl intended for the second coat more plaster to accelerate the taking (the second coat must be more thick than the first one but does not have to pour.
Take then a big brush and apply the most liquid plaster on the whole sculpture and reach up to the edge (including keys): it forms the impression coat. Do not forget during all the work of the plaster to clean your hands and your tools regularly by scratching them.
When the texture of the second bowl of plaster is creamy, take the oak, dip each of the nests into the liquid plaster and apply it on the sculpture until it covers it uniformly. Use a ribbing tool on the edge of the separating joint to avoid the oak to exceed.
Then realize thickness coat of 3cm maximum by applying the harder plaster with a spatula and by making a circular gesture to raise the plaster on the top of the sculpture when it flows on sides. It is necessary as far as possible to try to make so that the screed is more or less regular on the sculpture, what implies to know well this one, as for the coat of silicone. It is also necessary to make so that the edges of the separating joint have enough thickness, use the ribbing tool again to form them correctly.
Once the plaster is taken (it's been getting hot and then cold again), you can improve the plaster screed by grating the harshness with a surform or a grater, then use a 180 sand paper with water and clean with a wet sponge (to avoid any mark, use the side of the sponge which is generally smooth).
The second part of the mold
When the screed of plaster is dry, you can begin to work on the second part of the mold by turning delicately the sculpture, taking care that the silicone stays firmly in place on the sculpture.
Take off the separating joint in Plastiline to release the not yet molded part and clean it well of all the rests of Plastiline and all the silicone which exceeds (use a cutter to cut the silicone which exceeds).
If there are small spaces between the sculpture, the silicone and the plaster, fill them with a little bit of Plastiline. In the same way, if the sculpture has a hole for the stalk of the base (and if you don't mold the stalk with the sculpture), fill it with a seal of Plastiline.
Impression and thickness coats
Don't forget before beginning to apply a release agent on the silicone (but not on the sculpture), otherwise it will be impossible to open the mold! You can use for example some vaseline or a mixture of linseed oil and liquid soap to apply generously with a brush but pay attention not to create packets which would risk to cause problems with the waterproofness and the accuracy of the mold.
Pour the silicone impression coat (or fluid coat) and let it dry, put some humidity if needed to accelerate the taking.
Then pour the thickness coat by covering entirely the sculpture and the keys with enough thickness on the edges to ensure the strength of the silicone mold. If you have a doubt on the thickness of silicone, you can use a pin to check it, if it is too fine, mark where with a cross and add a little silicone on these places, then let it take.
Once this coat is taken, cut the silicone in the dimensions of the other side (by following the cut which has been done previously).
If you haven't done the keys of the plaster in the Plastiline at the beginning, you can do them at this moment by digging cones in the plaster screed of the first side well in the middle of the thickness and by arranging them regularly around the sculpture so that the mold closes well.
Don't forget then to apply a release agent on the plaster and on the wood which might be under the bottom of the sculpture. Put also vaseline on the tools and prepare the oak nests the same way as you did for the first part.
The you can do successively the bond coat (fine coat applied with a brush + oak dipped in plaster) and the thickness coat and grat the screed to improve it's appearance.
The end of the molding
The release from the mold
When the plaster screed of the second face is finished, you can open the mold carefully. It is a rather delicate operation because you should not break the fragile parts of the screed, in particular the keys.
First of all you have to identify the place where the both parts meet (if they are covered with plaster the separation might be invisible, then scratch the plaster to find them) and use a fine blade to separate both parts, being very careful not to cut the keys. Then separate slowly both parts of plaster and then both parts in silicone.
Realizing a cleaning epreuve
Close the mold and maintain it well closed with two parallel wires very well tightened with pliers. Pour then the plaster inside by tilting the mold to fill well the undercuts and chase away any air bubble. Tap along the process to take off bubbles.
Fill the mold to the brim (but don't create a dome) with the mold straight and let the plaster set at least two hours. Release the epreuve, paying attention again not to break the keys. Clean the mold.
Retouch the edition by using small rifflers to take off the mark of separation between both parts of the mold. If it is necessary to make some additions of material on the edition, wet the sculpture first then realize a plaster filled until the surface of water.
You can also use the mold to realize an hollow edition, which is an edition of which the center is empty (here a plaster shell armed with oak). It is in particular useful for a big sculpture to obtain a light edition. It is also the technique used when you can't or don't want to pour the material of the edition (edition in clay for instance to limit the retreat when drying).
To make a plaster hollow edition, put the mold opened in front of you and we realize the various stages for each of both parts. Make a normal plaster and keep a part of itthat you will thicken and use the first part to make a bond coat on the mold putting it everywhere to avoid bubbles. Attention not to exceed the limits of the inside of the mold. Arase carefully with a ribbing tool (otherwise you risk to have an edition where both pieces will not be joined).
When the bond coat is taken, dip the oak nests in the rest of the plaster and put them on the plaster coat, pay attention to have no air bubbles and that the oak does not exceed the plaster. Let the plaster take.
Make a new rather creamy plaster and make a coil for the joint between the plaster of the edition and the silicone of the mold on one of the sides of the mold, then return the other side and put it on the first one, press and close the mold and wait until the plaster takes.