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A burr mill or burr grinder is a device to grind hard, small food products between two revolving abrasive surfaces separated by a distance usually set by the user. Usually the device includes a revolving screw that pushes the food through. It may be powered electrically or manually.
Devices with rapidly rotating blades which chop repeatedly (see food processor) are often described as grinders, but are not burr grinders. Burr mills do not heat the ground product by friction as much as blade grinders, and produce particles of a uniform size determined by the separation between the grinding surfaces.
Food burr mills are usually manufactured for a single purpose: coffee, pepper, coarse salt, spices, or poppy seeds, for example. Coffee mills are usually powered by electric motors; domestic pepper, salt, and spice mills, used to sprinkle a little seasoning on food, are usually operated manually, sometimes by a battery powered motor.
Coffee grinders 
The uniform particle size is desirable for the specific case of coffee preparation by any of the various methods. Some methods may be more tolerant of a range of sizes produced by a blade grinder; this may be the case for drip (percolated) coffee.
Burr coffee grinders are especially significant for keeping the flavor and aroma of the coffee beans intact. Burr mills differ from blade grinders in not producing as much heat from friction—this is especially important for coffee aficionados looking to get the most flavor from the freshly ground beans. Friction heat would lessen or taint the natural flavor. Burr grinders create less friction and require fewer motor RPMs which reduces potential flavor loss due to heat.
Manual coffee grinders have been largely supplanted by electrically powered ones where the object is simply to get the job done; manual grinders are used more for their "traditional" appearance: they are made to look good, and are often more costly than the cheaper electric models. An exception is the manual Turkish coffee grinder; these are inexpensive, and can grind coffee to fine powder for Turkish coffee, unlike the cheaper electric models.
Many grinders are free-standing; some larger coffee grinders are fixed to a wall.
Manual burr grinders 
Manual burr grinders are turned by hand, rotating one grinding surface against the other. Coffee mills usually have a handle, providing leverage for the many turns required to grind enough coffee for a cup. The ground coffee is collected in a container which is part of the mill.
Salt, pepper, and spice mills, essentially the same as coffee mills, usually do not have a handle, the entire top rotating instead. While this is less convenient, only a few turns are required to grind enough. The ground product falls directly onto the food being seasoned; the mill has no container. A few mills have abrasive surfaces which do not rotate; each squeeze of the handles moves one flat plate past another, then the plates are restored to their original position by a spring. Many hard spices are available in containers incorporating a simple cone burr grinder, intended to be discarded when empty.
Most grinders can be adjusted to set the fineness of grind.
Manual mills can be used for grinding other food products than they are intended for. Be aware that mills designed for pepper grinding are inappropriate for grinding flour. However, coffee mills are a suitable substitute. Laura Ingalls Wilder's novel The Long Winter describes a family grinding wheat in a coffee mill to make flour during months of hardship.
Peugeot of France created the pepper grinder in 1842. The mechanism of case hardened steel cracked the peppercorns before the actual grinding process. The grooves on the Peugeot mechanism were individually cut into the metal and then case hardened, making them virtually indestructible.
Types of materials 
There are several types of materials used in pepper mills. Each has its own particular advantages.
Stainless Steel: One of the most suitable and durable materials for grinding peppercorns. The male and female sections of the grinding mechanism are usually made from powdered, pressed metal. This material is preferred by professional chefs. The teeth of the grinder are machined to cut the spice. Stainless steel is not suggested for grinding salt.
Zinc alloy: Perhaps the most common mechanism found in pepper grinders, zinc alloy is composed of a mixture of metals, primarily zinc, with chrome plating to resist corrosion. It is a good choice for grinding pepper, but is not suitable for grinding salt.
Carbon steel: An extremely hard metal, carbon steel provides the sharpest edges and most efficient grinding capability, and is preferred by professional chefs. Carbon steel is not suitable for grinding salt.
Ceramic: Ceramic is extremely hard and provides the best performance for multi-use grinding. It is non-corrosive and suitable for grinding pepper, salt and spices.
Acrylic: Durable and low cost, acrylic is a non-corrosive material suitable for grinding salt and spices.
Electric burr grinders 
Electric burr grinders are powered by electricity from a battery or mains supply. An electric motor drives the grinding elements against each other. Electric grinders grind faster than manual grinders with no effort, but heat the ground product a little by friction.