Home Made CapacitorsOver the years, copper and aluminum have been the predominant metals used, but the dielectric can include just about any form of resistive material. Since aluminum is easy to work with and obtain (aluminum foil, disposable aluminum pie pans, foil air-duct tape etc) I tend to use it, although you can use strips of copper or even copper tape if you can find it. Capacitors have been effectively carved out of the copper on a copper clad circuit board. I have made variable air core capacitors out of bolts and washers, with the washers cut in half. As far as the dielectric, I've worked with plastic (everything from Saran Wrap to garbage bag material), paper, masking tape, electrical tape, glass, cotton cloth, and even a water & wood pulp paste. An Interesting Twist I once had a large screen television I had to repair. In the vertical circuit, there was a very small value capacitor that died. Of course, I didn't have the capacitor in my home shop, and the parts store, from where I was, over a 2 hour drive - one way. So I improvised. Taking a piece of insulated wire about 3 inches long, I bent the wire in half, and twisted it. After soldering the two ends of the wire into the circuit board, I cut off the "top" of the wire. The capacitance was close, but not quite right. After 2 more cuts, it was a perfect match for the circuit, and my TV worked fine for years afterward.
Experiments:Using a Multimeter, measure the voltage of a battery. Now measure the resistance of a resistor. Once you have a known voltage and resistance, you can use them to measure the charge time, from which you can calculate the capacitance of a capacitor.  Cut paper into three 1 inch wide strips. Cut aluminum (pie pan or tape) into two 3/4 inch strips. Solder a separate piece of thin copper wire to the END of each of the two aluminum strips. Measure the aluminum strips, and find their area. Lay the paper and aluminum out as in the picture on the left.  Make very certain that the two layers of aluminum never touch. Now beginning at the wire ends, begin wrapping the strips in a very tight roll. When you reach the end of the roll, tape the whole roll up with electrical tape. You have just made a capacitor.
Using the multimeter, battery, and resistor, measure the capacitor you just made, and calculate its capacitance. (T=RC so C=T/R). Try the same experiment using different widths of aluminum strips, or different dielectric materials (plastic bags, cotton cloth, etc). Have FUN!The Borax / Mason Jar CapacitorInterestingly, the dielectric can also be some form of electrolytic (chemical salt). This is the theory behind the electrolytic capacitor. Factory made electrolytics were made with a paste of borax - yes, the same stuff that they call Mule Team laundry detergent. An interesting salt I played with along the way was Sodium Bicarbonate (Household Baking Soda). ==============ADVANCED CONCEPT===================== 
Chemically speaking, Sodium Bicarbonate (NaHCO3) can easily be broken down to form pure table salt!
By mixing baking soda with muratic acid (Sodium Bicarbonate with HydroChloric acid), the outcome is salt water and carbon dioxide. The carbon dioxide simply floats out of the water, forming pure salt water. If you then evaporate the water, you have pure sodium choride, or table salt (NaCl).
H2CO3 ? H2O + CO2(g) ============END of ADVANCED CONCEPT=============== Using aluminum foil separated by a paste of water mixed with sodium bicarbonate as the electrolyte, you can make some decent electrolytic capacitors. Alternately - I found that simply suspending strips of aluminum in mason jars full of a baking soda and water (2 cups of water/1 tbsp baking soda) you can make a fairly healthy electrolytic capacitor. I have performed this with water right out of the tap, so some other impurities may exist, and not cause a problem, but my tap water is not full of iron oxide/rust).
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