The tide mill is a medieval invention that uses the non polluting energy of the tides, that is, of the cyclical difference of the water level between high tide and low tide.
The first mill of this type in Portugal was constructed in the year 1290 in Castro Marim but, afterwards, the tide mill became very common in rivers and estuaries in the Algarve.

In the Ria Formosa alone there existed about 30 tide mills. The new mill of Marim, constructed in 1885, includes a stable and living quarters for the miller and his family. It functioned up to 1970, being the last tide mill of Ria Formosa to stop working.
In the 1980's the Quinta de Marim was integrated in the National Service of Parks and the mill has been restored to its original state.
It is a nice family trip to go and see the Ria Formosa Park , visit the working mill and learn something interesting about the traditional system of milling.

The incoming tide opens lock-type gates and lets the water pass into a pond (water reservoir of the mill) until the water level is identical on the two sides. As the tide falls, the first out-flowing water closes the gates. The restrained water cannot retrocede through the floodgates.
When the tide had fallen sufficiently the miller opened the sluice gates at the mill race and the released water, rushing out, turns the wheel and therefore the machinery. According to the quality of the grain and the desired flour a mill stone was chosen.
Mills like the one in Marim used to have 6 mill stones. The stones had to be recut at regular intervals, or the efficiency of the mill would be lost. They were carefully lifted and the miller sharpened the cutting edges with the miller ' s bill.
Normally, a mill had a capacity of 50 kg of grain and it took one hour to mill it. When the force of the tide allowed the functioning of all mill stones simultaneously, the miller was quite busy indeed.
The miller ' s day depended upon the movement of the tide and his working hours were quite irregular. No two consecutive days were the same for him, the variation of tides at different seasons making his live even more difficult.

The miller kept for himself a percentage of the flour he milled for the customer. This percentage was the payment for his work.