ADHESIVE REPLACEMENT: POTENTIAL NEW TREATMENT FOR STABILIZATION OF ARCHAEOLOGICAL CERAMICS
3 RESULTS AND DISCUSSION
The customary disassembly-reassembly method and the adhesive replacement method can both be employed to stabilize the structure of an old ceramic restoration. Aesthetically, the argument can be made that the traditional method produces a more appealing object than the adhesive replacement method. However, because the traditional method is so laborintensive and time-consuming, many objects, especially the multitudes in study collections, often go untreated.
If the goal of retreatment is to stabilize a large collection of stored or study collection objects, then the adhesive replacement method seems to provide a cost-effective alternative in the reconstruction of an unstable vessel by avoiding the time-consuming disassembly process. Also, removing the excess embrittled cellulose nitrate flakes from the surfaces around the failures can both help preserve information by preventing the brittle flakes from breaking off fragile surfaces and markedly improve the object's appearance.
Other than the pearlware chamber pot, the objects treated in this survey were fairly porous. Based on the effectiveness of this treatment on the pearlware vessel and on entirely glazed vessels, it is the author's belief that it would be effective on more impermeable ceramics and possibly even glass. With less porous materials, the solvent evaporation of the Paraloid B-72 would be much slower due to its inability to escape into the clay body. More support for the vessel would likely be necessary during treatment. The less porous the material, the lower the possibility of tidelines and the greater the chance of removing more of the original adhesive. Following this train of thought, less porous glazed vessels can be seen as particularly good candidates for this treatment. Glaze can act as a barrier, forcing the acetone through the break and not as readily through the clay body. Glaze on both the interior and exterior of porous ceramics may inhibit full water soaking of the body before treatment. On porous glazed ceramics, the acetone may still be able to enter the clay body, other than solely at the join, through pits, discontinuities, or crackle patterns. In these cases, the acetone may dissolve the cellulose nitrate and move it into the clay body and under the glaze. Since glaze impedes materials from escaping porous bodies, caution and testing should be practiced before treating these objects.
On the unglazed surface of one of the objects treated, a darkened tideline was noticeable after the treatment was complete. However, this was the first object treated, and water had not been used to pretreat the joins. The author acknowledges that the creation of tidelines is a possibility with this treatment. When considering the options, though, a tideline is less potentially damaging to an object than the collapse of the form or the loss of glaze due to brittle adhesive. In the example treated, the resultant tidelines were far less unsightly than the original excess adhesive.
3.1 TIME SAVINGS
Because no two archaeological ceramic objects fail in exactly the same manner, it is not possible to directly compare the time required to stabilize objects by the traditional disassembly-reassembly method and the adhesive replacement method. In an effort to gain some relative sense of how the two compared, a colleague unfamiliar with adhesive replacement research was asked to prepare estimates for treatment of a group of bowls, urns, and plates by the traditional method. The objects were then treated by the adhesive replacement method, and the elapsed treatment times compared to the estimates. The results suggested that for similarly sized and damaged objects, the adhesive replacement method could be up to three times faster in producing a stable object.
Seventy-two hours after treatment, the six Jamestown vessels were reexamined by sight and sound for stability. Visually, the gaps, fills, losses, and flaking surfaces appeared to be stable. A more reliable auditory indicator of increased stability was the clear tonal ring produced by gentle tapping. It has now been three years since the six vessels were treated. All the objects remain stable by these same indications. Sixteen years have passed since the first vessel was treated with AYAF. As mentioned in section 2, this vessel was recently examined and determined stable enough for travel. Considering that the adhesive in the joins of the re-treated vessels are likely a mixture containing some cellulose nitrate, one could draw the following conclusion: Since the vessel re-treated with AYAF remained strong after 16 years—even though it may contain some residual cellulose nitrate—we can assume that the vessels treated with Paraloid B-72 will remain strong, owing to B-72's proven long-term stability and good aging characteristics.
3.3 FURTHER ANALYSIS
There are many scientific and material property issues that could not be addressed in the scope of this work. The occurrence of adhesive mixtures is an issue in all branches of conservation, yet it is rarely studied or discussed. Research on adhesive mixtures in a systematic manner is ongoing at the National Park Service. At Harpers Ferry Center, Fourier transform infrared spectroscopy (FTIR) was performed on adhesive samples taken from three of the joins of the original treated vessel. Two samples contained only AYAF and one contained some cellulose nitrate and some AYAF (Bischoff 2002). This study indicates that although most of the cellulose nitrate is likely removed, some may remain. The stability of a mixture of Paraloid B-72 and aged cellulose nitrate—that is, the effect of residual plasticizers in the deteriorated cellulose nitrate on Paraloid B-72—is a topic for consideration.
Other potential topics for further study are determining adhesive concentrations in mixtures and how they relate to mechanical strength, and what types of ceramics or glass are best suited to this treatment and which are not. What is the result of solubilized adhesive in the clay body? Could this actually help to consolidate the edge? The author invites colleagues in the conservation community to try out this treatment and report back with their results.