Select a bowl or similar object for your vessel. If you prefer, choose one of the many vessel sink forms available for this type of project. Remember that you must create an opening in the bottom of a bowl mold for the sink’s drain opening.
Measure the height and diameter of the bowl. Cut two square pieces of non-stick melamine board at least 2 inches larger than the diameter of the bowl, using a circular saw. Cut four pieces of melamine the length of the square you cut to use as sides for the mold. Cut the four pieces the height of the bowl plus 2 inches.
Turn the bowl upside down on top of one of the pieces of melamine. Trace around the perimeter of the bowl. Set the bowl aside. Drill a hole inside the line. Insert the blade of a jigsaw into the hole and cut around the line.
Center the bowl upside down on top of the square with the solid surface. Seal around the edges of the bowl and the melamine with modeling clay rolled into a snake-like tubular shape. Place the melamine board with the hole cut in the center around the bowl as well. Align the two boards and mark one of the sides so you know where they align. Seal the edges of the bowl onto the second board with modeling clay as well. Create a circular opening for the sink’s drain by making a snake-like piece of clay and joining its ends.
Heat a hot-melt glue gun until the glue is ready to dispense. Glue two acorn nuts diagonally in three corners of the melamine board. Glue three acorn nuts onto the fourth corner in a triangular pattern. These nuts create alignment devices for the sink mold once it sets.
Attach the four side pieces to the base of the mold and to each other, using drywall screws. Seal all the seams with a bead of hot-melt glue. Spray the inside of the form as well as the bowl with mold release product.
Mix equal parts mold rubber with mold catalyst for three minutes, using a paddle attachment for a drill. Take care to mix any of the product that sticks to the side and bottom of the container. Pour the rubber mixture into the melamine box to within 1 inch of the bottom of the bowl. Let the rubber mixture cure for at least 16 hours at 72 degrees Fahrenheit.
Measure and cut a piece of melamine board to fit inside the top of the form. Secure the board to the box with drywall screws.
Turn the box containing the bowl over. Take off the bottom of the box by unscrewing the screws holding it in place. Remove the board with the hole in the center as well. Wipe the mold with a soft cloth to remove all traces of the clay. Pop off the acorn nuts.
Place modeling clay into the screw holes in the form. Reheat the glue gun. Seal the seams with a bead of hot glue. Spray mold release into this portion of the mold.
Mix mold rubber and catalyst as you did in Step 7. Pour this batch of rubber into the remaining open portion of the form box. Let the rubber mixture cure 16 hours.
Remove all the screws holding the box together. Carefully take the sides and bottom piece away from the mold. Separate the top section of the mold from the bottom and pull out the bowl you used for the model.
Cut a melamine board the same dimensions as the board you cut in Step 2. Cut a 4-inch hole in the center of this board using a hole saw.
Spray mold release product onto the surfaces of the rubber mold. Assemble the mold, using the indentations made by the acorn nuts to align it and keep the pieces together. Place the full-size melamine square you used as a form on the vibrating table. Set the rubber mold on top of the board with the larger portion of the mold facing down. Finish the stack by placing the melamine square with the 4-inch hole on top. Wrap two or three packing straps around this bundle to hold it all together.
Add water to ready-mix concrete following the manufacturer’s directions. Pour the concrete into the 4-inch hole while the vibrating table is in operation. Without the use of a vibrating table, the concrete vessel sink may have bubbles in it. Let the concrete cure in the mold the length of time recommended on the package.
Unstrap the mold. Pull the mold away from the concrete, taking care not to damage the mold so you can reuse it. Your new concrete vessel sink is ready to install.