Pounding herbs and spices with a pestle and mortar releases the natural oils they contain, which give curry pastes and nam prik sauces a more intense flavour. This is even more important when preparing fibrous spices and herbs, such as lemon grass, galanga and kaffir lime peel; When processed in a blender, the resulting paste is less moist than when crushed in a pestle and mortar. Also, when these herbs are pounded together, their flavours meld into one, giving an extremely fragrant paste.

Here In Thailand, there are 3 different kinds of Pestle and mortar, each designed to create a different resultant crush.
Curry pastes, a heavy stone (Granite) mortar and pestle, is the most effective; with this you can pound fibrous herbs and tough seeds to a fine paste very quickly. The pestle (Pounding baton) and the inside surface of the mortar (Bowl) are polished smooth, and are not porous. Both are and heavy, they do not chip and will last for years, even when subjected to vigorous daily use. But be careful not to drop the pestle as it will easily break in half.

Three types of mortar and pestles:

This Grey Granite, mortar and pestle set is available in small , medium, and large sizes. You should the largest size, as you can use it for big and small chores. It also enables you to work faster,  without worrying about the contents overflowing.

If you only need to prepare nam prik dipping sauces,
a lighter less strong mortar and pestle set will be enough. With a marble set, you will be to crush, to a paste small amounts of the less fibrous, wetter ingredients like garlic and chillies. A Thai-style, baked-clay mortar with hardwood pestle from a our market. is a good alternative to a marble set and both the mortar and the pestle are much larger than a marble set, making pounding easier and faster.

The dark brown mortar comes in two different shapes – one deeper and more bowl-shaped, and the other with a noticeable moulded-in stand and a wider, denser rim around the top. Because both are tall and deep, they keep the juice from the wet ingredients from splashing everywhere and, when you've finished crushing them, the  remaining sauce ingredients can be added straight into the mortar and stirred with the pestle until the sauce is well Mixed.

Making curry paste:
When making a curry paste in Thailand, the ingredients are added step by step, starting with the hard spices and herbs, followed by the softer ingredients, when crushed, they are pounded together in the mortar. Often, the softer and wetter ingredients like garlic and shallots are placed in whole as they mash up relatively easily. Coarse sea salt crystals provide some abrasion to reduce the harder and more fibrous herbs and spices, as well as release their flavours. The pounding goes on until everything in the mortar is mashed into paste and is no longer distinguishable.

Method:
Pound one ingredient at a time, from the firmest and most fibrous to the softest and wettest.
Herbs reduce more quickly when pounded with a sturdy, straight up-and-down motion.
Move the herbs around with the pestle so that a single layer is pounded at a time to maximize the efficiency of the hard pestle beating against the hard surface of the mortar. When they are reduced, push them aside and move uncrushed pieces to the center to be worked, and so on.
Just because your mortar is large doesn't mean that you can pound and reduce a lot of herbs at a time.
For fibrous herbs, a thick layer of them will  take longer breakdown and require more energy from you to crush them, as each piece will cushion another.