Workshop ‘Materialen en methoden voor oppervlakte reiniging en het verwijderen van film vormende materialen’. Deze driedaagse workshop werd gegeven door Paolo Cremonesi en was georganiseerd door Nora van der Veer. Het vond plaats in het Teylers Museum op 13-14 en 15 november 2019.
Dag 3 Maria Vicente student Escuela Superior de Conservación y Restauración de Bienes Culturales, Madrid
SOLVENTS: SAFETY, PROCESSES AND TYPES.
The last of the three days workshop was dedicate to the solvents used for film-forming materials removal. First we checked some safety indications and reviewed the actual regulation.
Afterwards we focused on the three different processes that happen when we apply a solvent onto a porous surface: evaporation, solubilisation and diffusion.
The evaporation is a physical process that mostly depends on the boiling point and the vapour pressure, in other words, onto its volatility. We have no way to predict the evaporation of a solvent from the porous surface of an artwork. We have to rely on models like the “Evaporation-retention curves” by L. Masschelein-Kleiner or the Radio-labelled solvents by R. Wolbers and the Getty Conservation Institute.
The part of the liquid that doesn’t evaporate is retained and penetrates into the porous surface diffusingand dissolving part of the film-forming materials. Diffusion is influenced directly by physical process and conditions such as the viscosity and surface tension of the solvent. Although the solvent that had already penetrated tends to evaporate, it cannot escape freely without retro diffusing through the porous material.
The solubilisation occurs thanks to the primary and secondary, chemical and physical, molecular bonds. The primary or chemical bonds are the ones that constitute the molecule. These forces bind the atoms. The secondary bonds are the intermolecular associative ones that hold them in solid and liquid states.
Depending on the nature of the chemical compound, it can use one of these forces or both of them at the same time. This last case consists in acidic and alkaline solvents. Although they have been used in conservation, this cleaning method is too toxic and aggressive for the artwork. The only way we can control acids and bases is in an aqueous medium.
All of these processes happen simultaneously. Whether one or the other takes more or less place depends mainly on the solvents’ ionizing power. According to this we can find the following groups of organic solvents: neutral, dipolar non-Protogenic and acidic or alkaline.
Neutral organic solvents use its physical action, the inter-molecular associative forces based on polarity to dissolute solid material molecules. The solvent molecules associate to the solid ones breaking their intermolecular forces and resulting into a solution.
In this group of solvents we can find alcohols, ethers, ketones, esters, hydrocarbons, siloxanes, and poly functional compounds (such as hydroxyethers -metoxipropanol- and hydroxyesters -ethyl lactate).
Although we have called these solvents “neutral” it does not mean there are no poles or charges at all.
Some of them have covalent bonds and are basically compounds of carbon and hydrogen atoms as hydrocarbons. Hydrocarbons have indeed no polarity, and could be divided into aromatic and aliphatic ones.
Aromatic hydrocarbons like xylene or toluene are toxic; but even worse to our health are the hydrocarbons derived from benzene, becoming carcinogenic.
Aliphatic hydrocarbons, depending on the type of covalent bonds present in their structure, are divided into saturated or unsaturated. These saturated are simple bonds. They are present in different structures as cyclic (cyclohexane), open-chain (isooctane), or more complex as petroleum ethers, Ligroin. On the other hand the unsaturated or double bonds are directly related to the reactivity of the molecule and its facility to polymerize. This fact is not suitable for helping us carrying out a harmless cleaning or film-removal.
Instead of using aromatic hydrocarbons we can start with pure aliphatic hydrocarbons and add gradually up to a 20% of benzyl alcohol.
Polar organic molecules contain oxygen or nitrogen as well as carbon and hydrogen. Here the intermolecular forces are more intense due to the polarity of the bonds. This can be polar forces or hydrogen-bonding forces. We can express this forces through numbers and representation in the TEAS or solubility Triangle. Just as we can depict the solubility parameters of a solvent, we can use this tool to determine the solubility areas for solid film-forming materials.
In case of physical action a neutral solvent is not enough for dissolving (or at least swelling) the film forming materials; we need the chemical forces of dipolar non-protogenic solvents.
Dipolar non Protogenic solvents are mainly amides. Cremonesi recommends the only less toxic alternative: dimethylsulfoxide (DMSO). We must be warned about the strong solving power of this solvent, even towards inorganic compounds. This kind of solvents also shows a strong retention in oil paint layers. If we are in the need of using DMSO it is better to mix it with an ester up to 10% (if we increase further the amount of DMSO we will swell the oil painting).
MIXING ORGANIC SOLVENTS AND WATER
Some solvents previously mentioned are miscible with water. “Lower” ethers, esters, ketones (with 1-3 carbon atoms) and alcohols with 4 or less carbon atoms form a homogeneous mix with water, a solution.
Solutions solving power becomes unpredictable. If we want to use an organic solvent mixed with water it should be a not miscible “Larger” one. This way we obtain heterogeneous phases, which we can turn into emulsions with surfactants.
GELLING MATERIALS FOR ORGANIC SOLVENTS
If we want to combine the benefits of using gels for the application of organic solvents properties we can choose between cellulose ethers, Richard Wolbers solvent-surfactant gels or silicone gels.
Cellulose ethers as hydroxypropyl cellulose (HPC) or KlucelTM is a non ionic, useful for gelling polar solvents in temperatures lower than 40ºC. For clearance we prepare a rinsing solution compound of adjusted water to the specific painting parameters of pH and electric conductivity.
- Wolbers solvent-surfactant gels are the result of a combination of a Polyacrylics acid gelling material (CarbopolTM or PemulenTM) and a second component, which is a base and surfactant (EthomeenTM). The rinsing solution used in this case would depend on the polarity of the gel we prepared. For non-polar gels we can wipe off the remains with a simple hydrocarbon. If we used a polar gel we would add to the hydrocarbon a small amount of acetone or ethanol.
Table showing clearance solution for solvent surfactant gels
Silicones are chemical compounds based on an inorganic chain bearing organic groups. This structure provides them with unique properties such as a strong non-polar character, low solving power, low surface tension and high water repelling ability. There are some commercial, ready-made silicone gels available from the cosmetic industry.
We can use the specific properties of this compounds combined with the aqueous medium to temporary hydrophobizate a surface or creating a water-in-oil particle emulsion (silicone-based).