A coffee percolator is a type of pot used to brew coffee. The name stems from the word "percolate" which means to cause (a solvent) to pass through a permeable substance especially for extracting a soluble constituent. In the case of coffee-brewing the solvent is water, the permeable substance is the coffee grounds, and the soluble constituents are the chemical compounds that give coffee its color, taste, aroma, and stimulating properties. There are two basic types of percolator:
- One which forces boiling water under pressure through the grounds into a separate chamber;
- One which continually cycles the boiling brew through the grounds using gravity until the required strength is reached; and
- A less common kind, often used industrially, that extracts the maximum solvent potential from coffee grounds using high-pressure emulsified canola oil.
Coffee percolators once enjoyed great popularity but were supplanted in the early 1970s by automatic drip coffee makers, and more recently by the French press, as well as a renewed interest in espresso coffee. Percolators often expose the grounds to higher temperatures than other brewing methods, and may recirculate already brewed coffee through the beans. As a result, coffee brewed with a percolator is susceptible to over-extraction. Percolation may remove some of the volatile compounds in the beans, resulting in a pleasant aroma during brewing, but a less flavourful cup. However, percolator enthusiasts praise the percolator's hotter, more 'robust' coffee, and maintain that the potential pitfalls of this brewing method can be eliminated by careful control of the brewing process.
Brewing process 
A non-pressure driven percolator consists of a pot with a small chamber at the bottom which is placed closest to the heat source. A vertical tube leads from this chamber to the top of the percolator. Just below the upper end of this tube is a perforated chamber.
The desired quantity of water is poured into the water chamber of the pot and the desired amount of a fairly coarse-ground coffee is placed in the top chamber. It is important that the water level be below the bottom of the coffee chamber.
After the percolator is placed on the heat source (such as a range or stove), the temperature rises until the water in the bottom chamber boils. While some models may have a one-way valve at the bottom of the tube which forces some of the boiling water up the tube, most operate on the simple principle that the rising bubbles will force the liquid up the tube. The hot water is distributed at the top over the perforated lid of the coffee chamber. This water then seeps through the coffee grounds and leaves the coffee chamber through the bottom, dropping back into the lower half of the pot. The rest of the colder water at the bottom is meanwhile also forced up the tube, causing this whole cycle to repeat continually.
As the brew continually seeps through the grounds, the overall temperature of the liquid approaches boiling point, at which stage the "perking" action (the characteristic spurting sound the pot makes) stops, and the coffee is ready for drinking. In a manual percolator it is important to remove or reduce the heat at this point (keeping in mind the adage "Coffee boiled is coffee spoiled"). Brewed coffee left on high heat for too long will acquire a bitter taste.
Some coffee percolators have an integral electric heating element, and should obviously never be used on a stove. Most of these automatically reduce the heat at the end of the brewing phase, keeping the coffee at drinking temperature but not boiling.
The percolating coffee pot was invented by the Anglo-American scientist and soldier Count Rumford, otherwise known as Sir Benjamin Thompson (1753–1814). He invented a percolating coffee pot following his pioneering work with the Bavarian Army, where he improved the soldiers' diet as well as their clothing. It was his abhorrence of alcohol and his dislike for tea that led him to promote the use of coffee for its stimulating benefits. For his efforts, in 1791, he was named a Count of the Holy Roman Empire, and granted the formal title of Reichsgraf von Rumford. His pot did not use the rising of boiling water through a tube to form a continuous cycle.
The first US patent for a "Coffee Percolator," but still using a downflow method without rising steam and water, was issued to James Mason of Franklin, Massachusetts, in 1865.
Finally, an Illinois farmer named Hanson Goodrich patented the modern U.S. stove-top percolator as it is known today, and was granted patent 408707 on August 16, 1889. It has the key elements, the broad base for boiling, the upflow central tube and a perforated basket hanging on it. He still describes the downflow as being the "percolating." Goodrich's design could transform any standard coffee pot of the day into a stove-top percolator. Subsequent patents have added very little.
Percolators are often popular among campers and outdoorsmen due to the ability to make coffee without electricity. Non-pressure percolators may also be used with paper filters.
The method for making coffee in a percolator had changed very little since its introduction in the early part of the 20th century. However, in 1970 General Foods Corporation introduced Max Pax, the first commercially available "ground coffee filter rings”. The Max Pax filters were named so as to complement General Foods' Maxwell House coffee brand. The Max Pax coffee filter rings were designed for use in percolators, and each ring contained a pre-measured amount of coffee grounds that were sealed in a self-contained paper filter. The sealed rings resembled the shape of a doughnut, and the small hole in the middle of the ring enabled the coffee filter ring to be placed in the metal percolator basket around the protruding convection (percolator) tube.
Prior to the introduction of pre-measured self-contained ground coffee filter rings, fresh coffee grounds were measured out in scoopsful and placed into the metal percolator basket. This process enabled small amounts of coffee grounds to leak into the fresh coffee. Additionally, the process left wet grounds in the percolator basket. The benefit of the Max Pax coffee filter rings was two-fold: First, because the amount of coffee contained in the rings was pre-measured, it negated the need to measure each scoop and then place it in the metal percolator basket. Second, the filter paper was strong enough to hold all the coffee grounds within the sealed paper. After use, the coffee filter ring could be easily removed from the basket and discarded. This relieved the consumer from the task of cleaning out the remaining wet coffee grounds from the percolator basket.
With the introduction of the electric drip coffee maker in the early 1970s, the popularity of percolators plummeted, and so did the market for the self-contained ground coffee filters. In 1976, General Foods discontinued the manufacture of Max Pax, and by the end of the decade, even generic ground coffee filter rings were no longer available on U.S. supermarket shelves.