"And you shall make a laver of copper, and its pedestal of copper... " (Ex. 30:18)
The laver, a large vessel which appears like a kettle, is actually the first vessel which the priests have contact with every day, for they must sanctify their hands and feet from its waters before commencing any sacred task in the Holy Temple.
The original laver which was constructed for the desert tabernacle in Moses' time included 2 spigots for releasing the water. In the era of the Second Temple, the High Priest Ben Katin, who fashioned the muchni, also fashioned 12 faucets for the laver, so that the entire shift who participate in the offering of the daily sacrifice may sanctify themselves at once.
The Laver Must Be Constructed of Copper
According to the verse above wherein God commanded Moses to make the laver, it must be constructed of copper. Although many other Temple vessels were constructed of costlier metals such as gold or silver, thus giving greater honor to the will of God - the laver was to be made exclusively from copper. Even later, throughout all the subsequent generations of the Holy Temple, and even when the nation knew times of great prosperity and decorated the entire Second Temple in gold, the laver still remained of copper.
The Midrash (Bamidbar Rabbah 9:14) relates that the original laver was made from the contributions of the righteous women of Israel, who donated their shiny mirrors towards this cause. These mirrors, made of highly polished copper, were melted down and it was from these that the laver was created. This act of sacrifice was precious in the eyes of the Holy One - the fact that the women cared more about fulfilling the word of God than about their own appearance. He declared that the laver must be of copper throughout the ages, to invoke the merit of these righteous women, so the memory of their action will always be before Him.
The Reason for the "Muchni"
There is a specific concern of the laws of Biblical purity with regard to the copper laver, although the same problem can effect certain other vessels as well: anything left overnight in a sacred vessel becomes unfit for use in the morning. If water was left in the laver overnight, the priests would not be able to sanctify themselves with this water. Thus, the laver had to be emptied of its contents at the end of the day. But how could it be refilled quickly with enough water? In addition to this priest who officiates at the removal of the ashes, all 12 priests who will soon be offering the daily morning sacrifice must also wash their hands and feet at the laver!
Ben Katin, one of the High Priests who served during the era of the Second Temple, devised a system whereby this problem of ritual impurity can be circumvented: the mechanism of the muchni, meaning "machinery," or possibly derived from the word for "prepared." By emptying the laver of its contents from the previous day and then submerging in into a specially-made pool under the court, fresh water was obtained for the new day when the laver was hoisted up by the first priest in the morning - he who goes to remove the altar ashes. It was this sound of the laver being raised up with the muchni that could be heard by the other priests in the court, on the other side of the altar.