The best use is definitely to crush fresh Ligurian basil (together with garlic, oil and pecorino cheese) in order to make the "pesto", a green sauce to be used for pasta's first courses.
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Basically, a mortar is a grinding tool - a food processor - just exactly like a coffee or spice grinder or electric processor, but because of its design it is far more versatile.
The mortar is the 'bowl' and the pestle is the 'stick' used to grind, crush, mix and stir. In some parts of the world the average kitchen has several mortar-and-pestle sets for different uses; for example, a wooden set used only for grinding dry spices; a stone (granite or marble) set used as an all-purpose heavy-duty grinder and mixer; and a metal - frequently brass - set used exclusively for mashing and mixing wet ingredients. Some kitchens also have a set used only for grinding coffee in small quantities and also for blending different roasted coffee beans.
Pesto, as excellently described by the previous contributor, would probably be crushed and mixed in a stone mortar, which is heavy and solid enough to take a lot of pounding. Similar sauces, such as guacamole, would be treated in the same manner. The large stone mortar has the advantage of being able to handle all stages of the sauce, starting dry with spices to be crushed and gradually moving to moister foods such as chilli and garlic, anchovies, and so on. A little coarse salt might go in now, to help crush these to a paste. Tomatoes, avocados, fresh herbs and other ingredients can now be mashed in and wine, lemon or lime juice, oil or other liquids such as fish sauce for Asian pastes can follow.
The great advantage of a mortar over an electric processor or blender is that the cook retains total control of the process at all times and is able to smell and taste the sauce as it develops, keeping watch for the desired finished texture. This adds to the enjoyment of cooking for even the most experienced cook, and because sharing food with others involves sight, taste, aroma and conversation, serving a pesto, guacamole, or other sauce at table in the mortar in which it was mixed looks fantastic and adds a special feel to the dining experience. I've a medium-sized granite mortar used for this purpose and it is frequently the star of the show.
Working food in this way also means no messy blender to wash up, since the mortar and pestle does the lot, with just the assistance of a decent knife.
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Crucible, pestle and mortar
The mortar and pestle isn't made by players in the game. You can purchase on from herblore shops such as the one in Taverley.
Crucible, pestle and mortar
A mortar and pestle made of glass is typically used for grinding and crushing substances into fine powders or pastes. It is commonly used in laboratories for grinding chemicals or in kitchens for grinding spices and herbs. Glass mortar and pestle are preferred for their non-reactive nature and ease of cleaning.
Try Pestle as in Pestle and Mortar
The mortar is a bowl-shaped container used to hold the ingredients, while the pestle is a heavy tool used to crush and grind the ingredients in the mortar. The pestle is typically a heavy and blunt object, while the mortar is a bowl with a smooth interior.
http://www.phisick.com/images/apoth/mortar-and-pestle-brass-101.jpg
it has traditionally been made of marble yet I am sure they can be made from many sources
Chemicals are ground into powder using a mortar and pestle.
The word pestle is generally used in connection with the word mortar. It is from Latin pistillum, an instrument used for crushing; the mortar is a kind of bowl, and the pestle is used to crush a substance to powder in the mortar.
A pestle is a bowl, usually made from a very heavy and dense stone used to crush seeds, herbs and spices in by using a mortar, which is made usually from the same stone. The two are usually used together, "Now grind the rock salt with your mortar and pestle until you have fine granules."
for powdering...