In the issue 37 of News in Conservation (August 2013), the authors presented a similar-titled 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. The Vatican Museums Studio for the Restoration of Marble is continuing research in this direction and part 1 of the present article aims at describing, through case studies, possible solutions that are today in an advanced phase of development. Part 2 of the paper will follow in the next issue.
In the cases which we examined, the mechanical component remained as the fundamental constraining element in the join, and was designed to prevent a part from falling if there was mechanical failure of the adhesive applied to the joining surfaces. With this in mind, our first objective was to devise methods which ensured that any material filling the gap between the pin and the original stone did not need to have any adhesive properties. There thus remained the adhesive at the interface of the join, and it is this material which prevents the pieces from separating and supplies in most cases the required tractive force. Similar recent work by other colleagues is also in line with these basic principles: research into really – and not just theoretically - reversible joins, reduction in the use of adhesives with consequent research into the engineering aspects of the problem rather than the more usual concentration on the chemical aspects, and diversity and elasticity used in seeking solutions on a case by case basis, even when dealing with different parts of the same work (“zone system approach”).
The next step to take, as indicated in our previous discussion, was to provide the mechanical joining element – whether metal, reinforced fibre or a hybrid of the two - with a tractive force which not only would stop the two parts of the join from separating and falling, but would by itself hold them together (thus eliminating the need to use any type of adhesive) The Vatican Museums Studio for the Restoration of Marble is continuing research in this direction, seeking solutions with a range of mechanical characteristics, structures and designs, with varying levels of complexity, which can be employed or modified as a case requires. Although we are far from having a general scheme which puts together in one place the pros and cons of each system so that the best solution can be extracted for each specific case encountered, we are, however, pleased to be able to illustrate here several ideas which are today in an advanced phase of development.
We will firstly look at a series of projects which employ springs as their basic component to provide a tractive force to the mechanical component: a prototype is illustrated in Case 1.
The next series of projects using magnets, will be discussed in the next instalment of this paper.
In this image, “A” is an active cylindrical component of the pin, “B” is the piston held by a spring. “C” is the inserting cup element while “D” is the receiving cylindrical component “E” represents the centring rod, “F” are spacers (simplified), “G” is an open spanner, “H” is the extracting shank and finally “L” and “M” are internal threads of the cylinders.
The dowel pin is made up of two components which are inserted at two different stages of the operation into holes in the two parts to be joined. The first component is made up of an external cylinder (A in Fig. 1) which holds the element which actively anchors the pieces, made up of a piston (B) fitted into a sliding cup element (C) (which is however prevented from sliding out of A by the flared end of the piston B). This piston is held at one end by a spring inside the closed cylinder head, while the cup element is slid towards the outer rim of the cylinder into the “receiving” component (D), which it is then screwed into. Thus the piston is used to temporarily stretch the spring under tension, while the two parts to be joined are held apart by spacers (for example wedges, threaded sleeves, etc.).
When the spacers are removed the two parts are pulled together by the spring. As this system requires up to three separate concentric components in some parts (external cylinder, cup element and piston), and each one needs to be thick enough to withstand significant forces, the total thickness required is at the limits of acceptability even using the minimum thicknesses. For this reason the system described is generally more suitable where the diameter of the holes is not an issue, preferably thus where there are already pre-existent large holes, or when joining very large fragments.
The sequence of the procedure is demonstrated in the following set of images:
Figure 1a. Inserting the “active” component into the hole in the first piece of marble, using a structural adhesive
Figure 1b. Once the adhesive for the active component has set, the centring rod is inserted. This is a cylinder with a smooth surface used to align the receiving component both with the first (“active”) component and the hole in the second piece of stone. Figure 1c. The second, receiving component is then inserted in the hole in the second piece of stone and held in place with an adhesive. Prior to this, the surface of the join is temporarily protected (for example with cyclododecane and tin leaf) from any oozing of the adhesive, thus preventing the pieces from being accidentally stuck together.
Figure 1d, 1e. The pieces are temporarily joined together to allow the adhesive to set with the cylindrical receiving component in the correct position. The pieces are then separated, the centring rod is removed and the temporary protection of the join surfaces is removed.
At this point, if necessary, the two pieces of marble can be temporarily held in place at a distance from each other by spacers (here by two blocks), so that the following operations are not compromised by accidental movements of the parts. Figure 1f. The inserting cup element is provided with a lip with a hole or some other attachment method which always extends beyond the rim of the cylinder of the active component, even when it is not extended. A suitable tool (such as a pin or shank) is inserted in this part of the cup element and used to draw out the cup element until a part of the element provided with a hexagonal section emerges.
Figure 1G. Use an open spanner or similar tool to hold the hexagonal section and pull on the piston to draw it towards the “receiving” component. The tool inserted in the attachment point on the cup element is then removed.
Figure 1H. The spanner is then used to screw the cup element into the receiving component as far as it can go. The spanner is then removed. Figure 1i. The spacing blocks separating the pieces are removed and the spring in the active element contracts, pulling the two pieces of stone tightly together.
In constructing the prototype described above, a number of technical considerations need to be observed. One of these is the method used to fix the piston to the active cylinder housing. The system employed must block the axial movement of one of the two ends of the piston plus cup while leaving the piston free to follow the rotation of the cup with the least friction possible as it is screwed into the receiving component (Fig. 1i). If this were not the case the spring would be put under rotational pressure and risk permanent deformation.
Further, inside each of the two external cylinders there are two short stretches of threading (Fig. 1, L and M), with an opposite twist to that used on the outside of the cylinders. These threads would be used if it were necessary to remove the active and receiving components: threaded extractors would be inserted into the cylinders and used to draw out the cylinders – in the case of the active component (to the left of the illustration in Fig. 1), prior to this, the spring would have to be cut and the piston and cup assembly removed. Finally it must be mentioned that there may be a practical limit in applying this method, due, in some cases, to the difficulty in inserting and using effectively the spanner and/or tightening tools in what could be a limited or irregular space between the two pieces to be joined. In this case some alternative methods may work: use an octagonal or decagonal rather than hexagonal section for the tightening element, use tools with flexible heads or construct a miniature flexible head ratchet spanner, with gearing up to 6°.
In the next issue of News in Conservation two further case studies will focus on a series of projects using magnets.