I made this bas relief stone carving when I was 19, at the time I tried to saw it off a larger chunk of stone for display and cracked it on accident. Not wanting to throw it out, I glued it back together and decided recently that I could cast it and have copies that wouldn't show the cracks as much. This is how I did it using mold builder thickness liquid latex and gauze. If you have the funds I would suggest using a two part silicon mold. I had a gallon of the mold grade latex so I used it, this is the more traditional method.
I will be making a mold of only one surface, so I will need something to set the original copy on top of. I will be using a piece of masonite, which is a slick MDF. For a release agent I will use Ky jelly. Plaster of paris will be my copy material. Liquid latex for the mold material (I threw away the container recently, in the pic is thin latex, don't use that kind) and gauze to strengthen the mold.
You will need to put release on anything that the latex touches, otherwise it will be impossible to pull off later - without destroying the mold. I start with the board, and then the original sculpture. Be careful to not leave globs of Ky jelly on your original, otherwise it will cast that, you want it to be as smooth as possible. If the surface is porous you may need to wait and then reapply. My original was carved in Indiana white limestone, so I had to apply several coatings, the board only required one coat.
I always add two or three layers of latex before adding the gauze to make sure the rubber (latex) is evenly covering the surface of what i'm molding. You will need to wait for each coat to dry, a fan will help speed the drying process.Be careful not to apply more coats until each layer is completely dry, otherwise the bottom layer will stay wet and ruin your mold. You can tell latex has dried by its color. It goes from white and milky to a translucent tan color. The corners or recesses will take longer to dry so be patient. When you have built up enough layers then you can reapply latex and while its wet add a layer of gauze. The gauze gives the mold its real strength so make sure you work it into the wet latex and over lap the layers. Once that layer dries you can build up another couple layers on top of the gauze. Once that dries, you are ready to remove your mold. Be careful while removing to not tear or stretch your mold. The last shot shows the finished and dried mold.
When the mold is pulled off, you should have a near perfect inverse copy of the original. Inspect it for any defects, you may want to touch it up by rubbing latex into any problem areas. I trimmed the corners to make sure they wouldn't unravel later. If you chose, you can place the original back inside and then cast plaster onto the topside to make a hard mold. I chose to keep mine flexible since I want to make a bunch of copies and a hard mold can get damaged when you pull your copies out. As you can see the mold is floppy and will need something to support it while the castings are being poured. Now add some release to the inside of the mold, being careful to not leave any globs.
I use sand to support my mold and it works pretty well. Wet sand helps as well, since the water makes the sand form better. It helps to scoop out the center where you form will go, just be sure its completely supported. Place the original in the mold and press the piece into the sand shaking it slightly to aid in the water and sand leveling. When you are sure its pressed in place, carefully remove the original. In case you forgot, make sure you have release on your mold before pouring or else you might ruin your mold.
Figuring the ammount of plaster can be a pain, but if you are shy a bit, mix up more and pour it in before the first layer drys and you'll be fine. Obviously fill to the top of your mold. Then while its wet place your hanger into the wet plaster. I didn't want mine to touch the front of the casting, so i devised this method to hold the hook in the plaster above the molds surface (see last pic) The plaster will have directions on mixing and dry time. Mine was two parts plaster to one part water. I used three cups plaster to one and a half cup water.
After the plaster is dry pull it out. I managed to get a bubble of air caught in the snakes eye, it gives him a googly eye look, I will be careful to shake the wet plaster better in the trough. The last pic shows a better casting that I made next, I poured the plaster and then gently shook the trough back and forth on the table. You will see bubbles surface everywhere when you do this, its the same principle as vibrating concrete when you pour.
Now that I have shown you the traditional hard way, here's the modern easy way. You can get two part silicoln molding materials from companies like Smooth On. The one I used was the brushable type. The original piece I molded is cast iron from a scratch block I made at a local iron pour. I have glued it to my staircase at the shop so I had to cast it in place. It literally speeds the process up from a day of mold making to about ten minutes! My tip is to use a heat gun to make the silicoln harden faster. I used a smaller sand trough for this one and added a hanger like I explained in previous steps. the last pic is my original piece, you can get faux finish kits to make plaster look like iron, bronze, or whatever you like. I hope this helps you make something! If I forgot anything, feel free to leave questions in the comments below. Thanks!