The key to successful preservation is retreatability. Many of the finishes that were customary before the second quarter of the 20th century are still easily soluble in alcohol generations later. Although there is an obvious real life potential for a careless spill of alcohol, the long term benefits of resoluble (i.e. thermoplastic) coatings are inestimable. If you are looking for your work to last 200 years use shellac for varnish and hide glue for adhesive. Period.
When these old “spirit varnishes” are crazed and deteriorated after 150 years it is possible through the introduction of alcohol to redissolve and rework the coating. This treatment has the benefit of readhesion to the substrate, amalgamation of the coating (restoring cohesion), and restoring the “optical saturation” of the finish (For when a finish is described as “dry and lifeless”).
There can be other problems with finishes, however. In some cases, varnishes exposed to the sun’s UV rays for an extended period can lose their color. Occasionally, solvent action alone can remedy this but in extreme cases more is needed.
This chest of drawers I worked on recently was severely sun bleached and application of solvent alone wasn't retrieving the original color. Because this finish was soluble in alcohol I was able to restore the color without stripping away the coating.
|Before Treatment - Notice original color preserved under the brasses|
This was my method: TransTint metallized dyes well diluted in alcohol padded into the damaged coating. I prefer this method over spraying a toner because I find hand application much more discriminating with regard to where it lands. (Overspray into secondary areas is common.) As long as the dye is well diluted, I can safely work my pad with a wet edge on one surface at a time. It’s important to avoid stopping mid-pass as this can leave a tell-tale mark. To be honest, this technique does require experience and practice. It takes a controlled touch to keep everything even but I like how fine-tuned the application can be.
|Quick passes are required|
After all the dye was applied over each surface (20-30 quick applications), it was left to dry. In order to lock the color in, I later padded 10% Paraloid B-72 in toluene lightly over the piece. After that was dry I was able to pad a couple coats of shellac.
|Paraloid B-72 in Toluene|
A final rubbing and paste wax completed the treatment. Though the dye is not entirely retractable, the original finish was retained and the color was restored.