Although the Egyptians, Babylonians, Greeks and Assyrians all used arches for the construction of underground vaults and drainage systems, the Romans revolutionized arch architecture with the invention of the keystone. The keystone is a triangle-shaped stone at the pinnacle of the arch which distributes the weight of the structure down the columnar stones or voussoirs. A model Roman arch can easily be constructed with Plaster of Paris. Put weights on the top of your finished arch to test its strength.
Cover a 12-by-14-inch piece of cardboard with aluminum foil to serve as the base for your arch. Smooth the foil tightly and secure the edges to the cardboard with tape.
Cover your work surface with newspaper. Mix 2 pounds of Plaster of Paris in a disposable container according to the manufacturer's instructions. Add a few drops of brown poster paint to the plaster to color the arch.
Carefully spoon the plaster into ice cube trays so that each section is full but not overflowing. Allow the cubes to dry completely, then pop them out of the trays.
Glue two cubes together to form the keystone of the arch. The narrow ends of the cubes should be pressed together. Wrap an elastic band around the keystone to keep the cubes together while they dry.
Glue three cubes, which will serve as voussoirs, to either side of the keystone. Lay the arch on its side until the glue hardens completely. You can expedite the process by using a hot-glue gun.
Connect several arches to make an arcade.
Wear a dust mask and goggles to protect your eyes and respiratory tract from fine particles of Plaster of Paris.
Use a spoon to mix Plaster of Paris. Do not touch it with your bare skin.