HEMP: I purchased a bale of hemp, which will probably last me a lifetime. I might suggest you talk to your foundry or another artist and see if you can purchase some of their hemp from them. Be careful, if you buy a bale and clip those metal straps around the bale, the thing will explode, and you will be trying to figure out where to store all that hemp.
DO NOT MAKE A MOTHER MOLD WITHOUT HEMP. If you break your mother mold you have lost your piece. Hemp increases the strength of your mold, and if you are in a hurry, like I usually am, you are going to want to get this mold pulled and into production as quickly as possible. Molds, just after their completion, can be quite fragile.
Fill your plastic bucket with water to the amount needed for the amount of Hydrastone mixture you will want to use. I am not sure how to tell you how much to use; this knowledge will come with practice. Always mix the Hydrastone into the water. I start by making mounds of Hydrastone and letting them soak. You can get ratios from your Hydrastone supplier, but I really do this part by sight. Usually I add a bit more Hydrastone as I impatiently wait for the bucket to begin to “kick”. I mix all my plaster by hand and don’t use any mixers. This really helps me know the different stages of the plaster. It is something I do by feel. While the Hydrastone is soaking, I’ll take some of the hemp that has been soaking in water and arrange it over the rubber section that I am about to cover in Hydrastone, being sure to put some on the seam area. Then I will take this hemp off and dip it into the Hydrastone mixture and lay it back on the rubber. As the mixture begins to "kick", I must work quickly, putting the plaster on the sculpture, forcing it into the hemp and building the areas that I need, including the seams. I strive to make the entire mold an inch thick or less.
When you are finishing the Hydrastone mother mold section and the Hydrastone is curing—you will know this is happening because it becomes increasingly warm, go over the section one last time with a wet hand to “polish” the outside surface of the mold, making it smooth.
HINT: When creating a section of mother mold next to another section of mother mold, I also rub the Crisco or cooking shortening on the edge of the completed Hydrastone piece. This will help in separating the pieces and keep splashes of Hydrastone from being stuck to the already hardened plaster.
There are a couple of tools that I think are necessary to create the plaster. For example, I love having this large loop tool around when creating my Hydrastone mother mold. It is great when you want to clean up your Hydrastone seams and not make them jagged. This tool also cuts through the Hydrastone, even after it begins to cure, which is helpful, if I feel I have too much Hydrastone in one place.
Once all parts of the plaster are made you will want to separate the mold. Waiting a day or two for the Hydrastone to cure is helpful. It will take longer in high humidity. As the water evaporates from the Hydrastone, it will strengthen and will become a bit lighter.
HINT: Small molds are often held together with rubber bands, larger molds may require bolts. Making a seam that comes up at the edge provides room for bolts. These bolt holes can be drilled before pulling apart the mold. Be careful not to jeopardize the rubber underneath. Often there is so much rubber and plaster, it is sometimes hard for me to make out where the sculpture actually lies underneath.
Separating the shell can sometimes feel like an impossible endeavor. I have several screwdrivers that I can shimmy into the seam. Once one is in a seam, I’ll try to move it down a bit and put another in, doing the same thing all of the way down the seam, being careful not to puncture the rubber underneath. More than once I have to struggle with a seam, only to flip the mold over and try another one. As a l ast resort, I may try to scoop out the clay from the open section where the wax will be poured, giving me something to hold on to, and I use this to pry the pieces apart. It takes practice to know when and where to apply pressure and when not to. This part of the process can take a good deal of strength.
NOTE: Be very careful if you are prying one piece of Hydrastone off another. It is easy to shatter a corner or an edge. Go slow and steady, and if you feel frustrated walk away, get lunch, and come back later.
Once the mother mold is separated, you can pull the rubber away from the original art. You will lose the integrity of your original art, but I keep the sections, especially the face. Should the wax come back to me and not look right, it gives me something to compare the wax against. Before sending these molds to the foundry, the rubber will need to be washed both inside an out. This can be done with kitchen soap and a good brush. Sometimes a toothbrush helps to clean small textured areas. When cleaning the inside, be sure that all the clay debris is out of the crevices and details, as this will be picked up by the wax, rendering the wax unusable.
The Crisco should also be washed off the outside of the rubber. Then the rubber is dried and placed back into the mother mold.
HINT: Do not leave the rubber in a heap. Instead let the mother mold do what she is supposed to do, cradle and hold the rubber. Rubber can have a “memory”, and leaving the rubber tossed about outside the mother mold may distort its shape and cause it not to fit back into the mother mold securely.
I would suggest that anyone who is wanting to learn to make molds for his or her own bronze casting, watch the process being done under a master mold maker. Contact a local foundry or artist and ask them if you can intern or apprentice under them to learn this process. They will most likely appreciate your eagerness and the help, especially if you can do it in their time frame. Remember they are on a deadline.
I might also suggest you take your education a bit further and ask the foundry if they will teach you to pour and pull your rubber molds. There is no better education in knowing if you created the mold properly than experiencing how the mold pulls from a wax.
Though the process of creating a mold for bronze casting can feel daunting, with a little practice it is possible to begin to create your own molds, and in turn, save yourself a great deal of money in the process of bronze casting.