Conservation can be a funny thing. Sometimes it requires microscopy, vacuum tables, lasers and exotic materials. Sometimes it just takes a little warmth.
A major piece by a prime Art Nouveau designer recently came in for conservation. The large piece was sound and stable structurally but had some very obvious problem areas in the finish caused by the introduction of moisture. The problem areas were all at the bottom but were still extremely visible, obvious and a serious distraction from the exquisite beauty of the entire piece.
The main problem was a white/light colored circle (ten inches in diameter by one quarter inch in width) right in the middle of the horizontal display area at the bottom. This outer circle also contained five smaller, various-sized white circles within it. As you moved towards one end of this surface, there were large areas of variegated damage that spilled out over the edge and onto the angled and rounded apron.
These light areas were typical of finish delaminated due to moisture exposure. Moisture exposure can cause different materials to react in very different ways. This can vary greatly due to the amount of moisture, the duration of exposure, the temperature of the moisture, etc. If a substrate and a finish move or react in different ways, separation can occur.
When a finish delaminates, a space is opened and light can no longer pass cleanly through the finish to the substrate and reflect back out. This will cause an opacity that does not allow the true color of the substrate to come through - in this case the dark, rich, red-brown of the mahogany wood and veneer. When a finish delaminates, it becomes more fragile and can crack, fracture, and flake with consequent losses.
The first step was to inspect the piece in a dark space using ultra-violet light. This showed the finish to be almost completely intact with only typical wear areas on the prominent edges of surfaces and scattered small, shallow scratches. The finish appeared to be original with no evidence of finish repair except for some very small, scattered areas of touch up. The areas of delamination did not show under UV light which meant the finish was still complete, just no longer consistently attached.
Due to the period and type of construction , it could be assumed that the finish would be a spirit-based varnish (probably shellac) or an oil-based varnish. The finish under UV light did not appear to be shellac, but this was a very imprecise test due to the variations in age, exposure, application, and make up of varnishes. Solvent testing showed no reaction to alcohol, but a slight softening with turpentine. This led to the conclusion that it was probably an oil-based coating of some kind that appeared almost complete and original. A complete microscopic analysis was not required for this project and this gave us enough information to explore treatment possibilities.
The importance and quality of the piece required that we do nothing to alter or add to the intact original finish, while stabilizing it for the long term and removing the visual marring of the damaged areas. The location and type of damage meant that any added finish or touch up, reversible or not, would not give an acceptable result. Any additions of solvents or glues would not be possible without damage to the delaminating finish or alteration of the surrounding intact finish.
We did not need to add to the finish, we only needed it completely attached to the substrate again. Warmth was all we needed. Most natural older finishes are susceptible to heat but they are also easily damaged by heat. It is also very easy to alter the color and patina of the underlying wood with excessive heat. We needed only very minimal amounts of heat applied in a very controlled and contained manner.
We used a small heating iron with a small triangular head that had been sanded so the edges are rounded and smooth. It had a range of temperature settings, most importantly very low. By the introduction of very small amounts of heat applied only to the areas of delamination, we were able to very slowly soften and reattach the finish. This had to be done very slowly over a number of days. Short amounts of heat were applied a couple of times a day with long cooling times between so we could monitor the color and surfaces of all the elements involved and not go too far too fast. The heating iron needed to be used like a small brush, gradually blending and darkening as the delaminated finish settled down and the true color of the wood was made visible again. The applications had to be varied and altered to maintain the variations in the wood grain and color while recovering the smoothness and integrity of the finish. The damaged areas gradually faded, became ghost images , and then disappeared.
The entire piece was then lightly cleaned using cotton balls and an absolute minimal amount of water with a small amount of non-ionic detergent. A coat of microcrystalline wax was applied and buffed to the proper patina of a well-maintained but historic artifact. The process was incredibly satisfying due to the amount and quality of the result attained through a very minimally intrusive treatment.