THE USE OF IMMISCIBLE SOLVENT COMBINATIONS FOR THE CLEANING OF PAINTINGS
ABSTRACT—The working properties on various immiscible solvent combinations used in the cleaning of paintings are described and interpreted.
THERE HAS BEEN considerable debate of the use of various solvents and combinations of solvents for the cleaning of paintings. One common choice in selecting a cleaning solution is to combine an active solvent, such as ethanol, isopropyl alcohol or acetone, with a relatively inactive solvent, such as Stoddard solvent (white spirit), petroleum spirit or turpentine. This paper considers the role of moisture in such a system.
I had noted that some traditionally apprentice-trained conservators from Europe would add some water to the cleaning solution. A typical cleaning solution might be 30% ethanol, 65% turpentine and 5% water by volume. Such a solution separates into two immiscible layers (phases). These were shaken into a cloudy suspension each time a swab was taken.
Similarly, should a solvent mixture like 15% ethanol (or isopropanol) and 85% white spirit be chosen, it is likely that it will separate into two layers if there is sufficient water in the ethanol (as anhydrous ethanol will absorb moisture from the atmosphere). A water content in the order of 5% in the ethanol or 1% of the combined cleaning solution is sufficient for this separation to occur.
THE SEPARATION OF a miscible cleaning solution into two immiscible solvent combinations considerably changes the nature of our cleaning system. In order to determine the composition of these combinations, five cleaning solutions were prepared as shown in Table 1. Each solution separated into two solvent combinations, one mainly ethanol phase and one mainly hydrocarbon phase. The mainly hydrocarbon phase “floats” on top of the mainly ethanol phase. These were analyzed using gas-liquid-chromatography and the component amounts calculated as percent by volume.1 The results are shown in Tables 2 and 3.
Table 2 (mainly hydrocarbon phases)
Table 3 (mainly ethanol phases)
For example, if one prepares cleaning solution system 1 by adding 20 ml ethanol, 78 ml white spirit (Stoddard solvent) and 2 ml water, the water tends to pull the ethanol out of solution with the white spirit: the solvents separate into two phases. They are a mainly ethanol phase (80.2% ethanol, 10.6% hydrocarbons, 9.2% water) and a mainly hydrocarbon phase (96% hydrocarbons, 3.1% ethanol, 0.12% water). The two phases are shaken into a cloudy suspension to produce a cleaning system which is composed of a very active ethanol phase dispersed as tiny droplets in a mainly hydrocarbon phase.
3 INTERPRETATION OF THE RESULTS
- Whilst changing the ethanol component in our original solutions from 20% to 15% to 10%, we see that the ethanol component in the mainly ethanol phases remains much the same, i.e. around 80%. The solvent power of the active component in each cleaning mixture is therefore very similar. The mainly ethanol phase of systems 1, 2 and 3 are plotted on the Teas Chart shown in figure 1 (as the mainly hydrocarbon phases).
Teas Chart showing location of ethanol and hydrocarbon phases of immiscible solvent combinations relative to regions of peak oil swelling and varnish solubility.Whilst the solvent power of systems 1 and 3 are very similar, system 1 will tend to be more active in varnish removal because it contains many more ethanol-rich droplets.The rate of solvent action (and varnish removal) can be varied by changing the proportions of hydrocarbons to ethanol, whilst maintaining the solvent power of both ethanol and hydrocarbon phases around the same level. Similar variations in the proportions of ethanol to hydrocarbons in a miscible combination would result in substantial changes in solvent power. In immiscible solvent combinations, the hydrocarbons can be regarded as a diluent.
- The limitation of using the immiscible solvent combinations described is that the varnish has to be soluble (or swollen) by the ethanol phases plotted on the Teas Chart. Experience has shown that the use of miscible solvent combinations is required for many varnishes.
- If we compare cleaning solutions 1 and 5, we can observe that by adding more water to the initial cleaning solution, the hydrocarbon phase remains relatively unchanged after separation; however, the active ethanol phase has substantially more water and less hydrocarbons. Too much water in the ethanol phase could be a disadvantage in treating water-sensitive paintings, so the water content of this phase is best minimized by adding water drop by drop until separation occurs.
- There was an interesting observation in the analysis of the ethanol phase in Table 3, in that the hydrocarbons in it were largely aromatic. The active component therefore consists largely of ethanol with toluene, xylene and water. Since aromatic hydrocarbons tend to be more active solvents than saturated hydrocarbons, this needs to be taken into account when formulating cleaning solutions and plotting solubility parameters.
4 IN PRACTICE
WE HAVE ESTABLISHED that we are cleaning with tiny ethanol-rich droplets dispersed in a relatively inactive hydrocarbon phase. The method is not dissimilar in approach to the surface cleaning method of working through a Stoddard solvent or petroleum spirit layer with an ammonia-water solution. One reason for employing this technique is to utilize the properties of immiscible solvents to reduce the active solvent's action after the swab is removed. The requisite of the solvents to separate tends to interfere with the wetting and swelling of the varnish. My empirical conclusion is that solvent action is more directly related to swab action: a “feeling” of greater control over, and response from, the swab.
Another advantage of using an immiscible system is that as the varnish is reduced in thickness, we require less solvent action to remove the remainder (or to reduce it to whatever level one wishes to clean). This is accomplished by reducing the number of ethanol droplets available for contact. Remember that in this immiscible combination, the hydrocarbons behave as a diluent.
Using a so-called non-active component such as white spirit offers other advantages in cleaning. As a slower evaporating solvent, it remains on the area, helping optically to saturate the varnish and paint layer, thereby increasing visibility of the varnish removal. It also affords a method of “harnessing” the active solvent; that is, it allows for the application of a manageable amount of active solvent with a suitable solvent strength, evaporation rate and diffusion rate, while controlling the action of same.
The selection of a cleaning method naturally depends on the particular requirements of each painting. The use of immiscible solvent combinations is a method worthy of consideration when making the selection. In any case, the conservator should be aware of the role of moisture in the cleaning solutions.
Analysis carried out by Larry Corklan, Government Chemical Laboratory, William Street, Brisbane, 4000, Australia.
Hedley, G.“Solubility Parameters and Varnish Removal, A Survey,”The Conservator, Number 4, 1979, pp.12–18.