Premise:
Before dealing with the Mills of the Valley, it is appropriate to go back to the time when this territory was the northern border of the Maritime Republic of Amalfi. Which historians date its birth to the mid- 9th century and its decline to the mid-12th century.
About three hundred years of history in which a small territory managed to compete with the other much larger and more powerful Italian Maritime Republics (Genoa, Pisa and Venice). The borders of Amalfi extended to Cetara, Positano, including the island of Capri and the Li Galli archipelago and inland, beyond the Lattari Mountains, up to Gragnano, in the province of Naples.
In these three centuries a defensive system was created on the northern side: the Castle of Lettere, the fortress on Mount Pendolo, the Castle of Gragnano (which gives its name to the village of the same name) and the Castle of Pino, kept under control the Sarno’s plains ( which go from Vesuvius to the foot of the Lattari Mountains ) and a good part of the Gulf of Naples. The Valle dei Mulini, being in the immediate vicinity of the Castle of Gragnano, was the village where the millers returned after working at the mills.
CLARIFICATION
This document is the result of work done by the volunteers of the Lattari Mountains’ Historic and Cultural Center "Alfonso Maria Di Nola" who, with their assiduous presence in the ‘Valle dei Mulini’ in Gragnano, aims to give value and spread awareness primarily among our fellow citizens, but also to let know this industrial district "ante litteram" to the whole world. Because we believe that a place where History, Ecology, Technology and Industrial Activities have coexisted for eight centuries should not be kept hidden any longer. It would not only mean wasting an unparalleled tourist opportunity (and consequently prejudicing the employment future of the new generations), but also denying the merit and a fair recognition to those who, many centuries ago, with their cleverness and work, with approximate tools, created a marvel of this kind . A work that we cannot maintain despite our modern technologies.
For this reason we will continue until we find other people who demonstrate that they can replace us by having the same spirit and passion that we have shown despite our limitations.
Birth of the Mills
The territory of the Lattari mountains has always been an area with lots of
Water sources, from which various streams derive. This climatic and morphological characteristics allowed an energy supply in the past but which limited the location of activities near watercourses.
Intercepting, channeling and using this waters to transform it into a driving force was the brilliant intuition of the engineers of the time; in other words, they hypothesized to exploit the natural slope of the territory and leave the force of gravity to make the water pass through sequentially to the other mills, which were built further downstream but at lower altitudes. Even though water mills were already used in ancient times, they only reached their maximum diffusion during the Middle Ages. Their development occurred in parallel with the end of slavery starting from the 9th century. The use of hydraulic energy instead of animal or human energy allowed an unprecedented increase in productivity. In fact, the energy produced by the water wheel of a medium-sized mill could grind approximately 50 kg of grain in one hour. The movement generated by water wheels was used for many uses until the advent of the industrial era:
- for grinding cereals (the oldest use);
- for the operation of sawmills (in the forestry sector);
- to drive looms (in the textile industry);
- metalworking (to operate millstones, forges and forging hammers);
- the production of paper (from the 13th to the 18th century when the energy of the mill was used to weaken rags by operating mallets and hammers equipped with tips).
Mills for grinding cereals were certainly the first practical applications of rotary movement, but the energy thus obtained also made it possible to automate other activities that required continuous and repetitive operations: weaving, rag maceration and paper manufacturing, an activity of considerable importance if we consider that until the nineteenth century there were fourteen factories working in the ‘Valle delle Ferriere’ on the Amalfi side. Even today the Amalfi’s Paper Mill is in activity and produces very beautiful and refined paper. It is possible that the two ancient documents still preserved in the Cathedral of Amalfi and which relate to the first concessions for the "Molendinum in flumine Graniani, of 1266 and 1272”, were written on this paper.
Slope & Aqueduct:
The main masterpiece of the entire Valley, which may look like a simple accessory element for the functioning of the mills, is the ‘hydraulic jump available’ and the power of the water jet. If we consider the way in which the mills were positioned and built and the optimization used for the spaces, we can deduce that a single project was created for all the Mills in the Valley.
To clearly understand the hydraulic functioning system of the Valley of the Mills of Gragnano, we must consider that it was made thanks to an optimal slope of the territory, on which a series of mills were built in sequence at different altitudes, connected to each other by a canal/aqueduct and driven by the force of water falling from enormous tanks which were an integral and functional part of the Mills themselves. The slope allowed skilled engineers to construct numerous buildings with internal mechanisms activated by the impact force of the water against the fins of a "retricine" wheel that began
to turn.
The waterfalls, interspersed with hydraulic jumps all the same, would have driven the millstones in sequence, spaced equal in height from each other, so as to have a series of mills and give them equivalent production capacities.
The minimum slope of the canal/aqueduct was designed to allow a lot of water to flow but at a low speed, avoiding spillage in the event of a steeper slope. This need to keep the speed of the water low meant adapting the slope of the canal to the natural slope of the valley, which oscillates around 5.5% on average. This difference in slope on the various sections explains the non-uniform distance between the mills, but depending on the place where they were built.
As previously mentioned, it was necessary to maintain the same hydraulic jump between the mills in order to to obtain equivalent production capacities. Therefore, where the slope was less, the Mills were positioned further apart and where, instead, the slope was greater, the Mills were built closer together.
The canal/aqueduct was built in masonry to ensure the maintenance of the same water flow rate and was adapted to the morphology of the area with different shapes and sizes so as not to reduce its water flow capacity. The practicability and immediate accessibility was ensured along the entire itinerary for maintenance needs and, fundamentally, to keep it free from debris and branches falling from the walls of the Valley, which could obstruct the passage of water, compromising the functioning of the next mill and, obviously, also those located further downstream.
Functioning:
The principle of series functioning of the mills was valid because the water coming out at the base of a Mill, as a new position, could exploit the slope of the canal (coated in cocciopesto mainly with a rectangular section which followed the trend of the rocks, adapting to the orography of the places), to reach the entrance upstream of the next Mill and so on, for all the other places further downstream but always built at a lower altitude than the previous one which took into account the 1% slope of the incoming channel and the need to have an 8 meter high tank upstream of each mill to guarantee the pressure necessary to turn the millstone.
T
he position where to build the Mill was determined by its height, to which was added the difference in height from the previous Mill which would allow the creation of a slightly sloping arrival channel. But also to ensure that the outgoing channel could feed the one further downstream in the same conditions.
The natural slope of the territory was therefore the key element on which the entire water system of the Valle dei Mulini was based. The force of gravity made the water flow from one mill to another after it had given motion to the millstone.
The Mills:
The mill was the shell of a hydraulic machine, built with the technique and materials of the time and its shape was not an aesthetic choice but a technical requirement. It contained technologies including fundamental hydraulic machinery that determined the dimensions strictly necessary for its correct functioning.
The production capacity of the mills was closely related to the available driving force, which in turn was a function of the quantity of water coming out of the source. Therefore the very dimensions of the mills were due to the main element that gave motion to the machines. It would not have made sense to build large millstones if the energy needed to make them move was lacking. In this regard, it is useful to remember that the water was taken directly from the source which obviously had a seasonal pattern.
This led to the construction of small production units compared to those present in other areas. To somehow remedy this situation, more mills were built along the route and that was made possible by the 6 km distance of the source from the sea and with a more or less homogeneous slope of 5%.
The Mill was composed of three main elements:
From the tank, basically an above ground well (Tower) always kept full of water.
From the basement called "prison" and inside there was the horizontal hydraulic turbine or "retricine", whose movement was powered by the impact force of the water coming out of the nozzle located at the base of the tank.
From the milling room, to the ground floor where the "mollaria" stone was positioned.
Technical aspects:
The mills of Gragnano differ from the river ones due to the presence of horizontal and not vertical wheels. This technology was particularly widespread in the area of southern Europe, in the Middle Eastern countries and in northern Africa where the availability of water was limited compared to the vertical wheel mills present on the large rivers of central/northern Europe.
In order to function, the hydraulic system of a mill had to respect some technical parameters, mandatory and interdependent with each other, which served to generate the pressure (kinetic force) necessary to move the upper millstone weighing approximately 1.5 quintals (q.li). . A fundamental part of the Mill was the tank, which was not a storage tank but had a very important function as a volumetric balancer as we see below. To obtain this impact force of the water against the fins of the driving wheel, the column of water contained in the tank (Tower) had to be no less than 8 meters high.
The diameter of the hole (21 cm nozzle) for the entry of the water into the so-called prison room, where the driving wheel was located, was the terminal point of a canalization with progressive reduction of the section that started from the base of the tank and which effectively regulated the quantity and speed of water that drove the "retricine" turbine, approximately 25 km/h at a flow rate of 7 m³ per minute.
Water tank:
At first glance the tank may seem like just a large container. But is not so. It must be considered that the load capacity was distributed vertically and not horizontally. Today it might seem like an insignificant difference but if we think about the material and construction techniques of that period, we can easily understand that it would have been much simpler to build a long and wide but shallow tank instead of an above ground well of a certain height.
This need to build a cylinder in height and not a tank horizontally makes us understand that the tank was not a simple accumulation of water, but the only possible way for the weight of this water to generate a strong pressure at the base, which then was directed against the turbine blades.
Only 3 centuries later, a Dutch mathematician Simon Stevin, in his studies of hydrostatics, stated that the pressure (weight/force) of a liquid on the bottom of the container that contains it depends on its area, on the height of the column of the liquid and on its specific gravity, while it is independent from its shape and volume.
At this point it must be kept in mind that the sizing of a mill was based on its use and therefore knowing the kinetic force necessary to move the millstones was the most important thing. Everything was built to overcome the friction of the two overlapping stone millstones and the wooden service mechanism to make them turn.
We know that the millstones had an average diameter of 1.25 meters and that the weight of the upper one was around 1.5 quintals, but we cannot know the friction of the wooden mechanism as we do not know enough the construction techniques.
The height, the diameter of the tank and the diameter of the nozzle (water outlet hole) were
fundamental parameters for the functioning of the mills which in the valley have different
shapes but all with similar characteristics. Therefore the height of the tank was of extreme importance because it could not be less than 8 meters (below which the right pressure would not have been created) but nor exceed 8 meters in height, because as demonstrated in the laboratory of the Federico II University of Naples, with an experimental thesis, a greater height would have generated a "turbulent" release of the
water and therefore a jet with less pressure, which would also have generated strong vibrations throughout the tank and on the blades causing damage to the static nature of the Mill. So, while the diameter of the tanks varied from 2 to 4 metres, the measurement of 8 meters remained unchanged which was the piezometric height necessary to generate the pressure necessary to turn the millstone.
The tanks had a capacity of 50 to 100 m³ depending on the diameter. This quantity of water coming out through the nozzle with a diameter of 21 cm would have made the mill rotate for only a few minutes, progressively slowing down, until it stopped completely.
The engineers of the time overcame this problem. They created a connection between the quantity exiting in the ‘carceraria’ with the quantity introduced at the top of the tank. With this continuous topping up, the tank was always kept full to guarantee constant outlet pressure. They had actually invented a compressor using only gravity.
The towers inside are all cylindrical, the external trapezoidal shape is due to an important aspect that makes the Gragnano Mills something technologically advanced. The masonry buttresses were not built along the entire external circumference of the tank, but only on the part opposite the water outlet through the nozzle.These reinforcements were created to counteract the outflow reaction that is generated in a container on the opposite wall from where the exit hole is. A pushing force that would have created problems for the static nature of the Mill.
To make this physical phenomenon understandable, let's think of the engine of a jet plane that pushes in the opposite direction from where the energy used to make it fly comes out. These choices were almost certainly not the result of calculations but these solutions were arrived at empirically as with other choices in the past. After all, much of our current knowledge is the result of past experiences made over the centuries. Morphology of the territory & Altitude: From the first mill immediately under the ‘Forma’ spring to the last remaining mill in the valley (crib area) there is a difference in altitude of 120 metres. The distance is 2.1 km with an average gradient of 5.5% and in this stretch 13 mills were built which are the remaining ones of a system that had at least 20 up to the sea of Castellammare. The distribution across the territory was made by considering the
technical needs and the aim of optimizing the available spaces, which at the time were the basis of efficient production.
Conclusions:
Not the single mill was used by the local community but an INDUSTRIAL ECOLOGY system, active since the 13th century, where grain was ground in large quantities by using the force of gravity and the most natural of elements, to operate the millstones. The WATER that, at the end of the journey, returned to the riverbed without interrupting its natural cycle.
A sign that even in the dark ages of his past man has never stopped using his genius to create works to improve his living conditions. Economy historians agree that the development of technologies made possible the great shift from the
Medieval Age to the Modern Age and created the conditions for the Industrial Revolution, which dates back to the beginning of the 17th century.
This thesis is based on the consideration that technological development exploded only after the scientific revolution with which Copernicus, Kepler, Galileo Galilei, Descartes, Boyle, Newton and many other restless brains demolished the old beliefs and replaced them with rigorous knowledge, based on the observation of reality. The mills of Gragnano constitute very rare proof of the fact that, even before that turning point, human ingenuity had given birth to technical changes of enormous importance for the economy of those times.