They are wirewound resistors in a light brown rectangular molded case. The "bottom" is a white, grainy substence that is cast in place, and looks like sand. For the wattage, they are quite inexpensive.
To figure the resistor to use, calculate the RC time - that's the product of the capacitor (220 mikes) time the resistance (your choice). The time to charge a cap through a resistor is about 5RC, so the discharge time ought to be about the same. The size of the resistor will give you the current - I = E/R, so you can use the current figure to get to the resistor wattage - P = I squared R. You may very well not need 10 watt resistors.