Mixing the Concrete
Scoop two shovels of refractory cement, two shovels of sand, three shovels of gravel, a half-shovel of lime onto the center of the tarps. Slowly add 1 1/2 to 2 gallons of water to the mixture.
Stir the concrete mixture with the shovel, adding water until the mixture can stand up when stacked and no water pools out from the base.
Mix the concrete until a sample can be packed into a ball and keep its shape when you let it go.
Use the concrete as quickly as you can before it starts to dry out, making it harder to form.
Making the Oven
Set firebricks tightly side by side on open, flat ground. Add bricks until the platform measures 25 inches square. Tamp the bricks down so they are even and flat.
Spread refractory concrete over the surface of the bricks. Smooth it out with the long end of a board. Redistribute any excess concrete from one side to where it is needed to create a smooth, flat surface.
Allow the base of the oven to dry and cure for 24 to 48 hours under a plastic sheeting, sprayed with water. Remove the sheet and let the refractory concrete dry completely.
Pile damp sand up into a dome shape that is 17 inches tall at the peak. Shape the dome so it is symmetrical on all sides with a 27-inch-wide base.
Bend a thick piece of cardboard so it makes an arch that is 15 to 17 inches high. Trim as necessary. Set the arch into the sand to frame the oven's doorway. Push the arch into the sand by 1 to 2 inches.
Cover the sand dome with two or three layers of wet newspaper. Overlap a little over the cardboard arch. Do not cover the bottom 2 inches of the sand dome.
Spread refractory concrete over the dome so it makes a 3- to 4-inch-thick layer. Smooth it out with a flat board, your hands or other tools to get an even covering.
Cover the dome with a plastic sheet spritzed with water for 24 to 48 hours. Pull the cardboard arch away when the concrete is set up and will not collapse when the support is removed. Uncover the dome and let the refractory concrete dry completely.
Scoop the sand out through the opening. Pull the newspaper from the inside of the oven dome.
Light a small fire inside the outdoor oven and let it burn out to get any remaining newspaper out and further set the refractory concrete.