For this project, craft foam is used to create a multi-part mold which is then held together with modeling clay. The foam creates a nice "matte finish" on the surface of the plaster. Then paint is mixed with non-sanded tile grout to give it a chalkboard finish in any color of the rainbow.
Step 1: Supplies
Plaster of Paris
Container in which to mix plaster (about 3-4 cups volume)
Plastic Silverware to mix plaster
Laytex Paint (Matte or Eggshell finish) OR Craft Acrylic Paint
Non-Sanded white grout
Fine grit sandpaper
Step 2: Creating and Preparing the Mold
Cut a heart of the desired shape and size for your plaque from a piece of paper. Trace it onto the foam and cut it out. (For reference, my heart was about 7.5" wide by 6" tall.)
Measure the length of the sides of the heart and cut the foam strips to this length. Either use a fabric tape measure and transfer the measurement or simply lay the foam along the edge, mark and cut.
Lay your heart onto a piece of paperboard, cardboard or other stable work surface. This surface will get wet and covered in plaster so make sure it's scrap.
Place the foam strips along the edges of the heart, uncut side down (notched side up). Use modeling clay to secure the edges in place at a 90 degree angle to the base. Press the clay firmly to the work surface around the OUTSIDE of the mold to seal the edges well. (I had a a very small amount of plaster leakage on one small section of mine but it it was easily neatened-up with sandpaper.) Use more clay to seal the seams between the foam strips at the top and bottom of the heart.
Prepare your hanging assembly using wire and modeling clay as done here: Recessed Hangers for Poured Plaster Pieces. There are other ways to make a hanger but I find this works well on this project because it allows the heart to be easily leveled and hang flat against a wall.
Step 3: Pour the Plaster
Pour the plaster into the foam mold. Tap the surface the mold sits on very well or else pick up the work surface and gently tap it against the table top. Make sure you've tapped out all of the air bubbles. Once you are done, place your hanger assembly toward the top of the heart, making sure the wire is embedded in the plaster.
Allow the plaster to harden, about an hour, before removing the mold. Clean up the edges of the heart with tools and/or sandpaper, if necessary. Allow to cure for a few days to drive out any residual moisture before finishing.
Step 4: Chalkboard Finish
Paint the surface of your cured plaster heart using a brush or roller. Once it is try, apply a second coat of paint. Apply a third coat of paint or more if you'd like, making sure to allow the paint to dry between coats. Lightly sand the surface of the paint to even out any brush marks.
The back side of the heart should be sealed or have a piece of foam or felt glued to it to protect your walls, as unsealed plaster can leave a chalky residue on walls.
Once all of the paint is dry, prime the chalkboard surface by rubbing the side of a piece of chalk against it and covering the surface with a chalky layer. This makes it easier to remove subsequent layers of chalk over time. Wipe the chalk off, write your message and hang.