A lot of Muay Thai fighters look to condition their shins by kicking hard wood or something similar. This is not a good way to go about doing this and may result in injury. If you want to properly condition your shins, then you’ll need to kick something a bit softer than a hard wood. Certain soft woods can be kicked with moderate intensity, but you still run the risk of damaging your shin.
Kicking hard objects damages your shin because, frankly, they are usually harder [or at least less brittle] than the bone in your shin. This will lead to the bone getting a bunch of small fractures that can culminate in a serious injury or break. This is something you want to avoid when conditioning your shins because it can lead to long term damage that may be irreparable.
The correct way to condition your shins is to hit soft wood at a moderate intensity or to hit bags. A firm heavy bag is a great way to condition your kicks and you’ll be able to go at it all day. After a while, though, you will feel your shins begin to bruise and you’ll probably want to stop then. I’m not saying you should stop because it hurts, but just because it’ll slow down your training and make it more difficult to condition your shins properly in the long run.
So why do your shins get conditioned? Well, the main reason is that calcium deposits form in the bone and essentially build it up. This will increase the strength of the bone and allow it to take more damage before becoming damaged. The other reason that your shins get conditioned is because the nerves become less sensitive – this means you can actually get hurt more and not realize it. This isn’t particularly safe, but it’s useful in a fight because it allows you to continue fighting for longer periods of time.