A material may decay for a variety of reasons: normally we consider those that derive from environmental interactions, to which we should add events that are limited in time but of high energy that may be far more dangerous for the maintenance of heritage.
Suffice it to think of floods, earthquakes and fires, without forgetting that in today’s world acts of terrorism may also take place, especially if the heritage in question represents a cultural identity. When we speak of safeguarding heritage we must therefore take into account the various risks outlined above and speak, more correctly, of the security of heritage, including in this the concept of conservation.
Leaving aside for the time being catastrophic events characterized both by their sudden nature and by the development of considerable energy, we will now consider the effect of gravity on load-bearing structures. A well-designed stucture can bear a certain weight but if this weight increases, the stability risks to the structure also increase: this is exactly what has happened in the Domus Aurea, where even the load-bearing structures have become more fragile over time and are therefore no longer able to support the maximum load for which they were designed.
The enormous quantity of earth that over the centuries has accumulated on top of the vaults of the Domus endangers the load-bearing structures due to its excessive weight; after heavy rains, this may increase by as much as 30% if the water is not rapidly discharged.
For security reasons it is therefore necessary to remove some of the earth above the structures. However, whilst from a structural point of view this would ensure greater security, the same cannot be said for thermo-hygrometric stability.
For conservation purposes we must try to keep the conditions to which the works have adapted over the centuries stable; if these works have survived until today they must have found a way to compensate for external stresses by adapting to them.
Any variation in the original conditions forces the materials to readapt to new conditions and, if given values are exceeded, depending on environmental stresses and the type of material, there is a strong probability that decay will occur.
Environmental decay is caused by the extent and duration of exchanges of energy and matter between the material and the environment. Energy exchanges are principally the heat flows which may occur in both directions between the environment and the material; flows of matter are prevalently flows of water, both in the liquid and the vapour phase, without ignoring its transformations that lead to condensation and evaporation.
All transformations of matter occur as a result of exchanges of energy and matter. If these exchanges are modest, the transformation may be reversible; in other words, when the stresses end everything goes back to its original state after the requisite time has passed. If the stress is considerable, the process may be irreversible, as in the case of the decay of materials. A parameter that calculates the extent of these transformations is the entropy generation.
Leaving aside the interaction between the various physical magnitudes, masterfully expressed by Onsager, we can say that: the higher the value of the entropy generation over time, the higher the probability of decay.
If we consider the system consisting of the environment and the work of heritage in question, we can see three categories:
- Open systems, where exchanges of energy and matter take place;
- Closed systems where only exchanges of energy take place;
- Isolated systems where neither exchanges of matter nor energy take place.
The reason why some painted tombs appear in all their splendour when they are first opened is linked to what we have said: this was an isolated system that, immediately after opening, becomes an open system resulting in the instant appearance of salts on the surface, thus further confirming the concept.
From the point of view of conservation, the best system is an isolated one but this entails significant restrictions on access. An appropriately devised closed system may allow for access by reducing exchanges, but the size of the Domus makes this solution unsuitable: as a consequence, for conservation purposes, all we can do is try to keep thermo-hygrometric conditions as close as possible to those of the present by stabilizing them.
If we analyse the situation in greater detail, we see that the daily temperature oscillation on the surface, due prevalently to sunlight, does not spread further than a few centimetres from the surface of the earth, whereas annual temperature oscillations penetrate deeper and reach the vaults. This is because heat diffusion is conditioned by the frequency of the thermal wave: the higher its (daily) frequency, the lesser its penetration.
As such, the pattern of temperature oscillations currently manifested on the inner surface of the vault, resulting from the heat flow penetrating from the outside, will depend heavily on the thickness of the overlying earth, as well as on the thickness and composition of the overlying masonry.
The diagrams below show the annual thermal wave curve as a function of the thickness of the earth; we can see that as the thermal wave penetrates into the ground there is an attenuation and a phase shift.
Excavations have show that already at the time of Trajan a brick cavity was created above the Neronian vaults as an underfloor, at least in the area of the Baths used as a garden.
In creating modern roofs we now use new technologies and products.
A first experiment thus involved comparing the two techniques to see if the ancient system is still usable while respecting the aforementioned conservation conditions, so as to replicate the preexisting situation and collect all the information of help for design work.
Moving on to the test phase proper, we used containers in an insulating material, inside which we created the two technological test structures:
- one with a cavity (simulating the original ancient situation)
- the other a modern structure with the necessary equipment.
Within the sample with a cavity condensation occurred.
This problem was overcome by removing the plugs present in the container, thus encouraging the exchange of air with the outside: the cavity must therefore be able to communicate with the outside, but only when the specific humidity outside is lower than that inside the cavity itself.
Next, an imbibition test showed the extremely high velocity of imbibition of the earth; this makes it essential to discharge rainwater rapidly, designing the slope of the ground and collecting water with drainage systems of appropriate location and size.
In a second test we created a technological package as close as possible to the real situation, thus highlighting all the problems it entails.
Above the vault we placed two different types of plastic modules with wedge-shaped protruberances that were filled with expanded clay. One module had holes for the passage of water while the other did not.
On the ground surface we then planted two types of vegetation perpendicular to the arrangement of the two modules so that we could see the effect of both.
We naturally recorded all the parameters, both on the ground and inside the structure, and on the inner surface of the vault; finally, we set up an automatic irrigation system. The results are currently being processed but we can already see that in calculating heat exchanges it is important to consider the convection taking place on the underlying vault in addition to the heat conduction above.
Irrigation, as well as ensuring plant growth, also guarantees a reduction in the heat flow penetrating into the structure as an effect of evaporation.