To start, I made concentrated solutions of copper sulfate (root killer) and sodium carbonate (washing soda). I used about a 2:1 mass ratio of these solids and completely dissolved each. Baking soda (sodium hydrogen carbonate) would work just as well, but the ratio might be different. With my solutions prepared, I mixed them thoroughly together. This made a lot of bubbles, so I had to quickly transfer my experiment to a larger container. When the solutions mix, they make copper carbonate, which can be decomposed into copper (II) oxide. I knew that I had added enough sodium carbonate because the blue color of the solution became clear (once the blue copper carbonate had settled out).The byproduct of the reaction is sodium sulfate, so I filtered this off, along with the excess water, using simple coffee filters. When the copper carbonate dried, I was left with a robin's-egg-blue powder. To transform the copper carbonate into copper (II) oxide by releasing CO2 gas, I heated the powder in a soup can. My hot plate wasn't hot enough, so I put the can in a bonfire for 15 minutes. When everything had cooled, I was left with a dark black powder--the copper (II) oxide. I also noticed some pink inside the can, so I wonder if the fire was somehow reducing some of the copper oxide back into copper metal.
Untouched, the copper oxide would probably work. I have heard that copper thermite is very energetic, so it probably doesn't require extra fine ingredients. However, I want the best performance from this reaction, so I milled the powder by hand using some steel ball bearings in a plastic jar. Shaken enough, the bearings break up any clumps. I also ran a magnet in a plastic bag over the powder to remove magnetic particles that came from the soup can. I was left with 55 grams of fine copper (II) oxide powder for an exciting thermite reaction.