Kung fu iron palm can be trained on various surfaces, but the most common is to use a specifically-made training bag (usually constructed of canvas) that are filled with progressively harder materials over time. Traditionally, the first bag a kung fu student will use is filled with rice or mung beans. These materials are solid, but aren’t too hard, and provide a good base upon which to develop a base level of conditioning. Over time, mung beans may be broken and crushed by months of continual training and need to be replaced.
After a long time with mung beans, the student’s hand will have developed a base level of conditioning and he may progress to a second bag which is filled with small rocks or pebbles. The pebbles are harder than the mung beans and provide a step up in difficulty. The student will often continue to warm up with the mung bean bag, but then over time do the majority of his training on the bag filled with rocks.
After a certain amount of time (often months or years) with the gravel filled bag, the kung fu student may choose to progress to a third bag which is filled with something very hard like steel shot, or sometimes iron pellets. This should only take place when the student has made significant progress with the rock-filled bag, and only if the student wants to take his training to the ultimate level. Anyone wishing to go this far should always do so with the consent of their instructor.
Progressing from one bag to the next should always be done very slowly, over a long period of time. It is better to progress too slowly than to move up to the next bag too quickly. Each kung fu school and style will have its own criteria for how long to stay at each level.