Volume 12, Number 2, May 1990, p.31

Letters

Editor:

Thank you so much for publishing the article "Solvents and Sensibility; Parts I, II, & III" by Sharon Blank and yourself in the May 1989 Newsletter. It was the most thorough and well written feature article I have ever read anywhere on the subject of oil paintings conservation. In the articles there was no mention of using a neutralizer to wash the soaps and gels off of the painting surface being cleaned. Is this a concern?

AUTHORS' REPLY

With Richard Wolbers' soaps and gels, clearance is indeed a very important issue. The materials in a soap or solvent gel must be removed from the surface. The active ingredients will not evaporate as solvents do, so it is imperative that as much thought be given to removing the gels as went into selecting and formulating them.

Neutralization is not necessary, with the exception of enzymes which are so fragile that they denature (loose their activity) nearly completely in the clearing process described below. Richard's research has shown that simply allowing lipase to dry out will air denature the protein, rendering it inactive. All trace of enzyme activity is lost in the clearing process necessary to remove the gelling agent, buffers, and detergents present in the gel. [Note any residual organic solvent on the surface of an area about to be treated will inhibit an enzyme for some minutes after application.]

When formulating a cleaning system, one should also be considering the formulation of an appropriate clearing agent. The issue of clearance is complicated. For the soap and enzyme gels, a buffer solution of 1/2 to 1% ammonium acetate of the same pH as the water borne gel works very well. The buffer is made from a 1/2 or 1% solution of ammonia to which acetic acid is added until the pH is correct. This can be followed by "spit cleaning" and/or rolling with distilled water and finally, just to be on the safe side, a quick wipe with an organic solvent like Shell Solv or xylene. Xylene will also aid in removing any detergent residue (i.e. Triton X-100) from the surface as well.

While on the subject of buffers, the stock soap solutions mentioned in the article should be diluted to the final working concentrations (2-4%) with a buffer solution rather than just distilled water. It is not clear if there is an optimum concentration, but between 2% and 6% triethanolamine in distilled water adjusted to the working pH with acetic acid (never above a pH of 8.5 if oil films are present) seems reasonable. Proper buffering will assure that the resin soaps will not precipitate out of solution when used on a highly acidic surface.

[A buffer solution is a mixture of a weak base (or weak acid) with an acid (or base) which will help to hold the pH stable against dilution effects. It will also minimize the effects of any additional acidic or basic materials which are dissolved into the solution as it is being used. Buffers only work when there are both acidic and basic forms of the active agent in solution at the same time, i.e. triethanolamine and the triethanolammonium ion for an alkaline buffer (or acetic acid and the acetate ion for an acidic system). This means the buffer range is generally only +/- 1 pH unit from the pK of the active agent. Some pK's of useful buffering agents are: triethanolamine, 7.8; ammonia, 9.2; acetic acid, 4.8; and carbonated water, 6.4.]

The buffering agents also add counter ions and increase the ionic strength of the cleaning solutions which also make them more effective. To clear solvent gels: first remove the excess gel with a dry swab; and then use a solvent that is just polar enough to dissolve the gel, but not powerful enough to dissolve the paint or varnish surface which you have been working so diligently to preserve. With Carbopol based solvent gels neutralized with Ethomeen C-25, xylene alone is normally not sufficiently polar and causes the gel to whiten (the Carbopol begins to precipitate). Water will not clear the solvent gels very well, unless the solvent itself is water soluble.

Hope that helps.