Hi guys,
For what its worth i tell my students to use dried peas or rice initially, then on to soft sand after 3 months when they understand the basics of the punching structure.
You should be able to hit the bag fairly hard without any long term damage, assuming you form the correct fist shape (surprising how many people still struggle with this) and use your stance efficiently.
What i tell my students never to do is:
a) elbow the bag - the force will be redirected straight into your shoulder
b) kick the bag with power - force will go straight into the lower spine - not good
I was once advised by a former teacher that you could using ball bearings after a while . IMHO, this is not a great idea unless you want to permanently damage your bones and increase your chances of arthritis.
Ken Chung may have no damage, but he presumably was tauight in a structured manner when he was first learning under LS, and is a pretty big bloke by the looks of it with strong fists. The same rule does not apply to all. If your students have any weaknesses in their hands, they can always wear light bag mitts, or tape their hands the boxing way, focusing the bandages around the weaker areas such as the wrist.
All the best, Stu
Ip Ching Ving Tsun in South Wales - www.swanseavingtsun.com