Clay bricks of various kinds have been basic building materials for millennia. Do-it-yourselfers still like homemade bricks, whether for the satisfaction of doing it from scratch or to make uniquely designed bricks. Clay bricks require adding nonclay soils to prevent cracking, and fire brick requires a stabilizer to withstand radical temperature changes. Modern brick manufacturers generally use Portland cement as a stabilizer, but one effective stabilizer that has been around throughout history is wood ash. A kiln-dried brick with 50 percent clay, 10 percent wood ash and 40 percent soil and aerators is very stable in high heat.
Cut three squared lengths of your board 2 feet long. Cut two squared pieces of wood 5 ½ inches by 8 ½ inches. Cut one squared piece 5 ½ inches by 4 inches. The 2-by-6 board is actually 5 ½ inches by 1 ½ inches, so these are all single cuts. Nail two of the 2-foot lengths along the third 2-foot length to form a simple trough. Nail the 5 ½ by 8 ½ pieces to the ends of the trough, forming a rectangular box. Fit the 4 by 5 ½ piece in the center of the trough, dividing the trough-box into two equal compartments. Nail the divider into place from the outside-in. This is your completed two-brick form.
Shred the straw in the leaf shredder. Mix five parts clay, one part wood ash, one part straw and three parts topsoil in the tub. Knead the mixture until it is homogenous, adding only enough water to make the mud mixable. Mix until the mud is approximately the consistency of bread dough. The ground straw will burn off in the kiln, leaving pores in the brick that lighten it and ensuring that the kiln heat reaches the middle of the bricks.
Place the tarp on a firm, flat surface. Wet the brick form. Pack the mixture into the two-brick form. Scrape your straightedge over the top of the form to square off the wet bricks. Lift the filled form to the tarp and flip it over onto the tarp. Tap the sides and bottom of the form with the hammer until the wet bricks release onto the tarp. They will hold their shape. Make enough wet bricks to fill the kiln.
Heat the kiln according to its instructions and specifications. Higher heat makes stronger bricks. Carefully place the wet bricks on the wheelbarrow, and move them to the kiln. Place the bricks equidistant from one another inside the kiln. Fire the bricks according to the specifications of the kiln.