MOD EDIT - WE DO NOT RECOMMEND THAT ANYBODY TRIES TO DO THIS AT HOME. THIS IS VERY DANGEROUS STUFF TO PLAY WITH, AND SHOULD BE LEFT TO THE PROFESSIONALS. YOU COULD EASILY BE KILLED, OR KILL SOMEONE ELSE. WE REALLY MEAN IT - DO NOT DO THIS AT HOME!
Extracted from (http://www.rotteneggs.com/r3/show/se/146343.html
If youíve watched action movies you know what C4 is. Itís a dense, putty-like explosive compound used widely by the military and demolitions crews. You canít find Composition 4 at your local hardware store, but itís easy to make your own. Hereís how.
Mixing these chemicals can be VERY DANGEROUS. Storing the finished product long-term can also be hazardous, as the mixture can decompose explosively while in storage.
12 oz. spackling putty or 20 packs of Silly Putty
1 container Petroleum Jelly (Vasaline)
12 Oz. Isopropol (rubbing) alcohol
A heat source (hot plate, stove, etc.)
A hydrometer or battery hydrometer
A large Pyrex, or enameled steel container
6 inch length of polyester yarn
Flour or Corn Starch
24 Oz. cooking oil (Canola Oil works best)
1 package wire pipe cleaners
2 packages (2.8 Oz) clear gelatin
Place four cups of tap water in your Pyrex container, and begin heating it until it boils. Add the gelatin, and mix thoroughly. Remove from heat, and allow it to cool completely, then add 4 Oz. of cooking oil, and stir. You should notice a thin layer of solids beginning to form near the top. Scrape off the solids, and place them in a small container. Place them in the freezer.
Next, light your candle, allowing the melted wax to accumulate. In your bowl, mix the petroleum jelly, cooking oil, putty, and corn starch until well blended. Test the mixture with the hydrometer. You should get a reading of 17. If you do not get this reading, keep mixing.
Now remove the chilled mixture from the freezer. BE CAREFUL. The mixture is highly unstable at this point. Add it to the compound in the bowl - the blend should have the consistency of ice cream. Stir in the mixture VERY CAREFULLY.
Now, take approximately 1/3 of your compound, and form it into a ball shape. Often you will see C4 molded into a square, but a sphere-type charge guarantees the highest detonation velocity. You should mold the shape to a density of 1.3 grams in a cube. Dip the ball into the candle wax to seal the compound from moisture. Form a second ball, slightly smaller than the first, and also dip it into the wax. Stack this ball on top of the first. Finally, make a third ball, smaller than the first two, and stack it on top. This three-tiered formation will cause a cascading detonation effect that is absolutely devastating.
You will need your pipe cleaners now. You will insert them into each side of the middle ball - this is where your detonator will attach to the device. There will be three hookups for each side, so you must twist together three pipe cleaners so that they branch out (see Illustration).
Insert the pipe cleaners on opposite sides of the 3-tiered shape. Next you will need your yarn. Tie the yarn around the compound, at the joint between the top second and third sections. Tie it loosely, and let the remaining yarn hang down in front of the charge. This will form a secondary fuse, should it become necessary.
Now, to mark your bomb. This is extremely important. You will need to identify your device according to the internatinal munitions protocol. The first marking will be two black dots near the top of the charge, on the top sphere (use a fine-point black magic marker for this). These dots should be horizontal, about one inch apart. This signifies that it is a class 2 explosive (meaning itís homemade). Below these dots, place a single third dot. This means it is a class 1 hazardous material (meaning the most dangerous). Then, below this formation line up five dots close together - that will indicate that this is Composition 4. REMEMBER: IT IS ILLEGAL TO MAKE YOUR OWN EXPLOSIVES IF YOU DO NOT MARK THEM PROPERLY.
The finished product is below. As I stated before, it is dangerous to store this material in a closed area. The ideal storage medium is out in the open, preferably on a coffee table, where you can keep a close eye on it. You will probably want to leave it there during the winter and holiday months, but it is wise to dispose of it during spring and summer, when the chemicals become the most volatile.