The heating source may be an open fire, stove-top or electric power outlet. As water in the bottom chamber heats, it is forced up the tube, where it spurts out over the grounds, making the distinctive blub-blub sound.
Hot water percolates or seeps through coffee grounds to again eventually trickle back down into the bottom chamber. When coffee is percolated well, the water never quite reaches the boiling point making good strong coffee. Percolated coffee gives off great color, taste and aroma.
There are two basic types of percolators. One which forces boiling water under pressure through the grounds into a separate chamber. The other type continually cycles the boiling brew through the grounds using gravity until the required strength is reached.
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