Reassembling broken pottery from archaeological excavations is a periodic feature of an object conservator's professional life. In past times pots have been reassembled with adhesives (such as unstabilized cellulose nitrate) which have deteriorated quite rapidly over a few decades to deform and become brittle - it is not safe to handle objects with such deteriorated adhesives since the adhesive joins can fail unpredictably. Other adhesives (such as early poly vinyl acetates) yellowed and caused staining of the pottery fabric. Some early restorers, in their desire to produce a beautiful result, zealously painted over original decoration and added handles to give what they thought was a pleasing appearance to the restored object. Over-restoration leads to a false impression of a pot, so we are careful when reassembling pottery to make restoration clearly distinguishable from the original and to use adhesives which have good aging properties, are reversible, and will show little color change as they age. In general, conservators are guided by the ideal of minimum intervention - intervene as little as possible.
Sometimes "gapfill" materials are required to provide structural stability - without these added materials the reassembled pot cannot be handled safely. In this case we adopt three basic principles:
- A pottery sherd should always be held on at least two edges.
- Fill materials must be reversible and isolated from the original pottery fabric.
- Fills should be appear different from the original pottery fabric both under visible light examination and when the pot is x-rayed.
In general, for low-fired archaeological pottery we use a cellulose-modified gypsum filler such as Polyfilla, often applied over a support mesh.
First, we carefully paint any pottery edges which the filler will touch with a long-lasting acrylic resin such as Paraloid B72 (manufactured by Rohm & Haas, USA) so that the filler will not penetrate into the porous pottery and so that, if necessary, the fill can later be removed cleanly from the original material. Once this precautionary layer is in place, a central profile of the section to be filled may be produced with a thermoplastic mesh called Varaform (manufactured by Runlite SA, Belgium). This mesh, a cotton fabric impregnated with polycaprolactone resin, becomes very flexible and slightly tacky when laid in warm water (ca. 60°C) but hardens again when it is cooled. Polycaprolactone has a fairly low specific heat capacity and so the warmed mesh is quite easy to handle with the water acting as a separating layer. The mesh is lightly molded against the inner or outer surface of the pot and allowed to cool, then the mesh shape is refined with scissors to be be slightly larger than the hole it will fill. The edges of the mesh section are then dipped in warm water to soften them and the fill section is pressed into position on the B72-coated edges of the pot. Polycaprolactone is relatively transparent to x-rays. Thus, fills built around polycaprolactone have a characteristic "waffle" pattern on x-rays.
Close-up of pottery fill materials: two Varaform mesh supports (center front and center back of image) and, at left, a Varaform mesh support covered with Polyfilla.
Once the mesh support is secured it is time to apply the structural fill. Sometimes the inside and outside pottery surfaces around the area to be filled are coated with cyclododecane (CDD) - a wax-like material which slowly sublimes at room temperature - to prevent fill materials penetrating the pottery. Application of this material is not particularly easy, typically we heat it to about 60°C with a little hexane and then paint it onto the surface. Polyfilla is then applied with spatula-like tools. Polyfilla has several nice properties amongst which are reasonable coherence and relatively small dimensional change on drying. Unfortunately, Polyfilla takes quite a while to dry so, for thick areas, it is generally better to use two thinner coats. Finishing the fill is accomplished with abrasives varying from sheetrock rasps down to medium-grit sandpaper. Again, it may be necessary to temporarily mask adjacent cracks and surfaces with CDD to prevent fine Polyfilla dust entering porous pottery fabrics.