MITRA Forum Question Details

I generally use only ethanol in pigment pastes. This is really just a case of ease of access and expediency. I always have a good supply of the solvent as it is the diluent for most of the conservation paints that I use and teach.

I have found that I do sometimes have mold occur in pigment pastes despite the addition of ethanol. Since these are used by multiple students who may not immediately recap after use, I am not really sure that it is not the case that the ethanol has completely evaporated.

Water and ethanol will form an azeotrope which means that at a certain proportion both the ethanol and water evaporates at the same rate. The problem there is that the azeotropic proportion is 95.63% ethanol and 4.37% water, which would never happen in the circumstance we are discussing.

On the side of phenol is the fact that it evaporates more slowly than water (unlike ethanol) which would mean that it is a more effective preservative since it would still be present even if the paste dried out.

I am not a proponent of clove oil, especially in oil films, but it is a super slow evaporator. I do not think that anyone has performed a true scientific experiment to discover at what point clove oil would cause problems in an egg tempera film. In painting conservation there is anecdotal evidence to show that oil films documented to include clove oil tend to be far more soluble in the solvents used in conservation.

Isopropyl alcohol, particularly in solutions between 60% and 90% alcohol with 10–40% purified water, is a more effective disinfectant against bacteria, fungi, and viruses. Once alcohol concentrations drop below 50%, its usefulness as a disinfectiant drops sharply. Higher concentrations of alcohol are also not as effective as a bactericide, virucide, or fungicide.

The presence of water is a crucial factor in destroying or inhibiting the growth of pathogenic microorganisms with isopropyl alcohol. Water acts plays a key role in denaturing the proteins of cell membranes. The water content slows evaporation, therefore increasing surface contact time and enhancing effectiveness. Isopropyl alcohol concentrations over 91% coagulate proteins instantly. Consequently, a protective layer is created which protects other proteins from further coagulation.

Phenol or carbolic acid is usually dissolved in glycerin or propylene glycol to form a solution, because it has limited solubility in water. This solution in turn is dissolved in water to a safer concentration, usually 4%. Carbolic acid does not readily evaporate at normal room temperature, so it remains effective for longer periods than perhaps isopropyl alcohol which is highly volatile.

Based on this information, however, alcohol may be less effective in this application, since dilutions of less than 50% posses much less effectiveness ,which is how it would be used in the preparation of pigment pastes. Of course, it is more effective to add phenol in higher concentrations than 4%, which would then be further diluted in the preparation of pigment pastes. However, handling it in higher concentratoipns raises more safety concerns that may outweigh any benefit it may provide.

Overall, I think it is uncessary to add a disinfectant or preservative in home made pigment pastes. Any mold that forms on the pigment paste can be quickly eliminated by spraying it with 70% isopropyl alcohol.

The waterborne pigment concentrations we prepare are made with a preservative that is both highly effective and safe in the concentrations used. These preservatives are widely used in waterborne pigment concentrations and paint today, and have largely supplanted the use of phenol in commercial paint products.

​Thanks, that is very helpful.  Koo​