Fill your 5-gallon bucket about one-third to one-half full with silica sand. Make sure you have enough sand to fill one of your wooden boxes to the rim.
Add sodium silicate. The amount is roughly 3 percent to 4 percent of the weight of the sand in the bucket. You can either use a scale to measure the weights, or you can estimate it. It does not need to be exact.
Mix the sand and sodium silicate together thoroughly.
Pour enough of the sand mixture into your first box, or drag, to cover the bottom with about an inch or two.
Coat your casting model with mold release agent according to the provided directions. Place the model into your first box, which will be the drag. Place it to one end of the drag, leaving about 5 to 6 inches of space on the other end for your runner. If the middle of your model does not reach the top edge of your drag, remove it and add more of the sand and sodium silicate mixture in a flat layer until it does.
Pack the rest of the drag to within 1/2-inch of the top edge. Place your 1-inch square dowel piece so that it touches the end of your model and runs toward the side of the drag where you left space. This will be your runner.
Pack the drag with the sand mixture to the rim of the drag, making sure the sand is flat.
Turn on your carbon dioxide canister. Insert the canister tip into the surface of the sand mixture and hold it there for about five seconds. You will notice the sand mixture begin to harden around where the tip is inserted.
Remove the carbon dioxide tip and move it to another spot in the sand mixture. Continue pushing carbon dioxide into the sand mixture until it solidifies completely. Wipe away any loose sand that did not harden.
Coat the surface of the rigid sand mixture with mold release agent according to the product directions.
Place your other box, or cope, on top of the first one. They should match up exactly. Set the 20 oz. soda bottle top upside down on the end of the runner that is not touching the casting model. It should form a funnel and tube shape. Make sure that the top end goes above the top edge of your cope, and get a taller bottle if necessary. This will be your pouring cup and sprue.
Hold your round dowel piece against the exposed top of your casting model. Make sure it touches the topmost portion of the model. Pack your cope with the sand mixture all the way to the top. Make more of the mixture if necessary, and discard any that has started to harden from naturally occurring carbon dioxide.
Insert the canister tip into the surface of the sand mixture in the cope and hold it there for about 5 seconds. Remove the tip and move it to another spot in the sand mixture. Continue pushing carbon dioxide into the sand mixture until it solidifies completely.
Remove the round dowel piece and the soda bottle top. The hole left by the dowel piece will serve as your gas vent. The pieces should pull out fairly easily.
Separate the cope and drag. They should separate easily. Remove your casting model and square dowel piece. Be careful when removing the model so that you don't damage the interior of the cavity it leaves.
Place the cope back on top of the drag. Secure the two halves of your mold together using your scrap 1-by-4 boards, nails and hammer to prevent separation while pouring.