In issue 50 of News in Conservation (October 2015), we published Part 1 of a paper describing experiments in devising systems aimed at increasing the reversibility of joints used in the conservation of marble sculptures while reducing the use of adhesives within such systems. This is the final chapter of the paper with two more case studies focusing on the use of magnets and the authors’ conclusion.
This model is an improved version of one we presented in our previous publication (News in Conservation, Issue 47, August 2013). By using a magnet between the pin and the sleeve inserts we have transformed the mechanism from a pure and simple safety system into a traction system and a safety system, eliminating the need for an adhesive at the joint interface (Fig. 2a).
Figure 2a. In this image, at the top is shown the pin with threads (A), and with boron-neodymium magnets (B) screwed into each of its ends. The elements marked C and D show the exterior and the interior (right) of the threaded inserts; in particular, note the chamber to hold the ring magnet D (here seen in cross-section) at the end of the threaded inserts. Lower down is the centring rod (E) and finally the extractor bolts with inset hexagonal heads, one with a right-hand thread, the other with a left-hand thread for the two threaded inserts (G, H).
For a number of years magnets ten times as stronger as traditional ferrous magnets and with a near-horizontal curve of demagnetization lasting over decades have been available. This has finally led us to overcome our previous reservations concerning the use of magnets in conservation, due to our doubts about the limited lifespan of (traditional) magnets and meant we have re-evaluated their possible use.
The following sequence illustrates schematically the way the mechanism functions. Firstly, a threaded insert is fixed either with a mortar or an adhesive in a hole in one of the pieces of stone which are to be joined (Fig. 2b). Then a non-ferrous (carbon fibre, Teflon) centring rod is inserted halfway into the insert (Fig. 2c). The main shaft of the rod nee
The centring rod holds at the ends of its central shaft two non-magnetic ferrous metal rings (F) with a threaded hole. These are screwed onto the (threaded) lower part of the projecting bayonet with the help of an adhesive fixed to the central shaft. In this way, during the positioning and centring of the threaded inserts, the centring rod will simulate exactly the volume and the contact between the magnets of the inserts and those of the final pin assembly.
Once the first insert has been fixed in the stone, with the centring rod positioned in it for half its length, the second insert is placed over the centring rod until perfect contact is established between the magnet and the metal ring. The second insert is ready to be fixed inside the second piece of stone which is to be joined. To complete this operation, the joint surfaces are protected as in Case 1 (Figs 1c, 1d) using cyclododecane and tin foil to avoid adhesive accumulating between the two parts, and adhesive is applied to the second hole and the two parts are temporarily joined together until the second insert sets in the correct position (Fig. 2d, cross-section). This guarantees that the inserts are perfectly aligned both along their axes and at their points of contact.
Figure 2e shows a detail (only the right hand side for simplicity) of the end of the main shaft of the centring rod.
Once the inserts have been positioned and set, the pieces of marble are separated again, the centring rod is easily removed (as it is smooth) and the real pin is inserted (Figure 2f). A tool is then inserted at right angles through the hole at the centre of the pin and used to screw the pin simultaneously into both inserts as the two ends of the pin are threaded in opposite directions, in correspondence with the opposite threads cut inside the two inserts (Fig. 2g). Once the two threads on the ends of the pin are screwed far enough to pass through the threaded section of the inserts, the pin passes smoothly towards the ends of the inserts and becomes at a certain point subject to the increasing attractive force of the magnets placed in the ends of the inserts (Fig. 2h).
At this point, the attractive forces pull the magnets on the ends of the pin in contact with the magnets in the inserts, with the consequent movement of the inserts and pieces towards each other until there is perfect contact (Figs. 2i and 2l, details). To summarise, the pieces are held together by the attractive force exercised by the magnets, while the safety of the join is guaranteed by the threaded portion inside the insert which blocks the complete exit of the pin, and reversibility is observed as not only can the pin be unscrewed from the inserts, but the inserts can also unscrewed from the stone (using the hexagonal indents or, if the inserts are made using reinforced fibre, by drilling them out using a core drill bit).
Given we are discussing systems which require magnets or springs it is appropriate to consider the matter of their longevity.
The longevity of a spring of a given diameter d (for example 12 mm) subject to a continuous tensile force by
a load x (for example 2.5kg) is much inferior to the longevity of the magnetic effect produced by the most recent families of rare-earth magnets (such as boron-neodymium) with equal diameters and retaining force. However, a conservator’s approach is to always leave as little as possible to chance and to take into consideration how things degrade. For this reason we should not forget the clear advantages of springs acting as anchors in terms of their being components of a construction – a magnet clearly lack this characteristic: for example, if a magnet lacks sufficient attractive force, the two parts it attaches will suddenly separate; however, if a spring fails to exert sufficient force, even if it elongates it will continue to provide a connexion between the two parts, even if they are no longer held together.
Furthermore, the maximum attractive force presently available between two magnets is in any case rather limited if we want to limit our components to diameters of 8-12 mm and heights of 3-4 mm. On the basis of these considerations, where possible and where the project and its dimensions do not create excessive complications, we are inclined to unite the advantages of magnets with those of springs using a combined, or hybrid retaining system.
The next example is an evolution of the prototype presented in Case 1. Here the only modification necessary is to introduce a pair of disk (or cylinder) magnets mounted using a threaded pin placed with opposite poles facing each other and inserted inside the spring (made of non-magnetic material, for example chrome-nickel austenitic stainless steel) so they are touching or almost touching (see Figs. 3a, 3b, 3c). In this way, the work required by the spring to hold the parts together is reduced to a minimum until the magnets begins to lose their power. Consequently the longevity of the system is increased by decades.
An even simpler version of a pin with combined magnets and springs can be used in some cases when we have smaller diameter holes than the preceding example (between approx. 4 to 8 mm). This would be when at least one of the pieces to be joined (whether stone, plaster or stucco) is quite light, with a horizontal inclination or tilted downwards. Further – and this condition is not always met in these situations – the hole needs to be sufficiently long and regular (as a rule of thumb, the length should be three or four times the diameter) and either pre-existent from earlier pins, or able to be created without risk to the surrounding material. Typically, such conditions may be found when attaching finger joints or entire fingers, noses, folds of drapery and other extremities.
In the simplified version sketched out above, the two external components – to be inserted in the two holes in the two parts to be joined – are made from two sections of carbon fibre and aluminium tubes (which can be found in archery equipment). Each tube has inside itself a divider which separates it into two compartments. The two compartments which touch when the two pieces of stone are joined together house the spring which is joined to the dividers in each of the tubes. The other sections of the tubes are also open, which means they can take in some of the adhesive used to fix the tubes inside the pieces of stone to be joined. Further, at the centre of the spring, when in the closed position, are placed two cylindrical magnets with opposite polarities. Both of these are fixed to a single spiral of the spring at the opposite end to that in contact with the other magnet, and in such a way that the spring is not impeded in its movement when extending or contracting. (Fig. 4)
The magnets must be chosen on the basis of the size of the parts they need to join, but should never be too strong, for if they require excessive force to separate them (to open the join), this might damage the stone (or plaster) being joined.
It should also be remembered that if the magnets have to be inserted in gypsum or other hygroscopic materials, it would be better to use those less subject to oxidation (for example ferrite or – with higher magnetic coercivity – samarium-cobalt magnets) and after they have received a protective or anti-oxidant coating.
If the holes are less than 4mm diameter, we have to abandon the magnetic component of hybrid pin types; thus to hold the pieces together we can use a reversible adhesive, and to save them from falling we can employ either a spring attached to the ends of two aligned tube elements (Fig. 5), or a rod with a plunger set in one of the pieces which slides inside a tube in the other piece, but which is blocked from exiting the tube by a partially closed-off opening (fig. 6a, 6b).
Another possible system might be inspired by the clicking mechanism of a ball-point pen, where for a sculptures, instead of pushing down once or twice to have two different extensions like with a pen, you would have to pull down once or twice for the same result. This would involve a modification to the design of the components, using an extended rather than a compressed spring, and with the inversion (and extension) of the sprocket of the internal harpoon mechanism.
On the other hand, where the holes are not sufficiently deep to house a long joining element, two small disk magnets could be glued into place which have a slightly stronger attractive force than that required to hold the pieces together: clearly an adhesive is necessary in this case, but it will be safer and more efficient with the aid of the magnets. In this case the reversibility of the system depends on the expertise of the specialists studying adhesives: we hope that sooner or later someone develops a structural adhesive which can be reversed using an exterior source, but without employing solvents or heat (for example, using microwaves). In this case we could also try to put a fine spring (in harmonic steel) around the perimeter of the magnetic disks, attaching them to the ends, so that if the magnets detach suddenly, the two (marble or gypsum) parts which they are holding together are held by the spring and do not fall (Fig. 7).
A special category of joins are those involving small parts (fingers, noses, etc.) of sculptures which are very exposed to accidental damage or vandalism, injuries which at times the works themselves seem to attract, repeatedly and in exactly the same places. Special precautions need to be taken for these joins, even when the points being joined are pointing upwards (with less risk of falling).
In these cases, apart from providing a stable join, the new joining system should also have two other specific characteristics: 1. The point of the old fracture must remain weaker than the surrounding material, to avoid another mechanical stress creating a new break near the old one; 2. Another mechanical force or break should not allow the piece to fall even if it detaches. As far as the first point is concerned, we have already mentioned that one of the principal reasons for avoiding the use of structural adhesives at joins is so as to not create a point which is stronger and too rigid in comparison with the rest of the work, apart from obviously making a joint more reversible. Above all when we are dealing with repeated vandalism which a particular work may be subject to over the course of the years, and in particular when there is the risk of lateral forces being applied, it makes sense to introduce “weak” joins. Recently, a sculpture employed a join in wood, which saved the work from new breaks when it was once again subject to vandalism .
When we examine possible solutions to the problem of avoiding fragments falling when they get detached, we find we are dealing with holes to join elements which are an order of magnitude smaller (diameters of less than 3-4 mm) than the prototypes discussed above in Cases 1, 2 and 3 which contained springs and magnets. However, apart from the small sliding tubes with systems to block the ends falling out and the small disk magnets with externally attached springs mentioned a short time ago, we could also use small tubes of semi-rigid plastic (such as drinking straws) which contain either a spring or two small cylindrical magnets which would be in both cases glued by their ends directly to the marble.
Finally, there are several open questions concerning the application of magnets to the interiors of pieces of marble, and it seems an appropriate moment to briefly mention them. One is the possible changes which might be brought about by magnetic fields to traces of metals found in stone; another is the possibility, particularly in large cities, of the surface capture of micro-particles from the atmosphere in the presence of magnetic fields. Another area which requires further study is how much risk there is of oxidation of magnets (or springs) inside these holes in stone, even if the levels of internal humidity are quite low, which could make them act as catalysts for further oxidation processes along their edges and thus hinder the free sliding of the mechanisms they are part of.
All images are copyright of the authors. Translation from the Italian text by M. L. Pellerito, M. Gittins