At Halifax Makerspace, we teach free monthly workshops at the public libraries. Recently, we decided to teach seed bombs. Seed bombs are little balls of soil, clay and flower seeds that can be thrown into empty lots, neglected flower beds, and otherwise inaccessible and unsightly areas. When it rains, the seeds sprout, bringing color and life to drab cityscapes. We decided to make ours a little more interesting by making them in custom-made molds. These are a two part mold that uses latex for the details with a plaster "mother mold" that keeps the latex from warping as the seed/soil mixture is pressed into it.
Step 1: Mold Positives
To make the molds, we designed two different objects to 3D print: a half grenade and a flower. These were done in Fusion 360 and 3D printed on the Makerspace's printer. It's beyond the scope of this tutorial to teach Fusion 360, but there are excellent tutorials on YouTube. If you don't have a 3D printer, check for makerspaces in your local area. Or you could find and print something from Thingiverse.
Alternatively, you could find objects like toys or chocolates or decorative soaps that you want to recreate. They should have a volume similar to a golf ball.
Step 2: Latex Mold
A local company donated 60 litres (three 5 gallon buckets) of liquid latex to the Halifax Makerspace for our members to use in various projects. You can get this at various industrial suppliers, hobby and makeup shops, or order online.
Lightly hot glue the positives to a non-porous surface, such as acrylic or finished piece of wood or masonite. Brush with 8-10 light coats of latex using a disposable brush. Brush the latex an inch or two around your positive as well. Make sure the latex dries fully between coats. You can hit it with a heat gun to speed up the process. Without a heat gun, it might take 20 minutes to 2 hours to dry between coats.
Step 3: Mother Mold
Mix plaster of paris (you can get this at hardware stores) and cold water together according to the directions. Leave it until it just starts to thicken up. Then put a large blob on each positive/latex pair. When it's almost set, use a board or something else flat to flatten the top, so that later, when you're using it inverted, the bottom will sit flat and not rock.
Step 4: Finishing the Molds
Peel the plaster mother molds off, then peel off the latex, taking care that it doesn't stick to itself. Dust the latex all over with chalk dust or talcum powder, etc. This will make sure it doesn't stick to itself in the future. Your molds are now ready to use.
Step 5: Seed Bomb Mixture
Combine 1 cup bentonite clay powder (available in pottery and health stores, but you could also blend clay kitty litter) with 4 cups topsoil or potting soil. Sprinkle in a handful of flower seed mixture or a very small handful of flower seeds. The mixture we used came from the dollar store and has coconut husk in it. Add just enough water so that it clumps when you squeeze a small handful.
Step 6: Making Seed Bombs
To make seed bombs, put the latex mold in the mother mold and press a golf ball-sized clump into the latex. Peel the latex out, then carefully demold the seed bomb onto some paper. Let the seed bombs dry in a warm, dry place. Once dry, they should be viable for a year or more. To use them, just toss them where you feel there should be a little more color! Keep them in a bag in your pocket, backpack, purse or car to use when the mood strikes.