Both mediums use the exact same pigments and are equally very photochemically stable (fade-resistant). Both mediums are considered archival - they should last for many generations (hundreds of years) without breaking down, assuming they were properly applied to an archival-grade canvas or panel. Since acrylics haven't been around for the centuries like oils have, the last sentence is unproven... but as far as I know, accelerated aging processes have shown acrylics to be at least as durable as oils, and very possibly more so.
As far as the differences go, oil paints (which generally use a linseed oil base as a vehicle for the pigment) dry much more slowly than acrylics. This can be an advantage or disadvantage - depending on the type of painting being created. Oils are ideal for a large portrait or landscape with vast amounts of sky or water, for example, where the colors and shades can be reworked and blended for hours, even days (especially if more linseed oil is used to retard the drying time further). Softening the image edges and similar blending also requires longer drying time than acrylics alone usually allow. When desired, the drying time of oils can be accelerated by the addition of mediums like "Liquin", which can allow the oil paint to dry within a day, depending on the paint layers' thickness. Oil paints require odiferous, flammable solvents such as turpentine for thinning and cleaning of brushes, etc.. Low-odor solvents now exist for oils, but they're quite expensive, and all oil solvents share the hazards of spontaneous combustion; a nasty tendency for solvent-dampened rags to create their own heat if improperly disposed of, often resulting in an unexpected fire. (A volunteer firefighter for nearly my entire adult life, I've seen too many tragic instances of this.) My solvent-dampened rags either go right in the wood stove or are hung out to dry where I won't have to breathe the vapors, then discarded.
Acrylic paints use an acrylic polymer emulsion as a base; this vehicle is essentially a water-based plastic which, when dry, is water-resistant, flexible and tough. Acrylics dry quickly; in minutes for thin layers, hours for thick (and the use of a hair dryer will speed up the process when necessary). Fast drying means more fine work without smudging nearby paint that was recently added, a reduced amount of overall time spent on a painting, and an ability to add more glazes or washes of color to enhance shadows and subtle variations in hue in a given time period. With an oil painting, the artist has to wait longer until the previous paint has dried before adding a glaze or wash layer(s). The small to mid-size, high detail paintings I enjoy creating work better for me in acrylic, primarily for the above reasons. Additionally, brushes used with acrylics clean up with soap and water... and water is the usual thinning solvent. This last point is a vast improvement over oils.
The drying time of acrylics can be made LONGER by adding retarding mediums to the paint, thus allowing some additional time for blending and reworking... but the drying times will never match that of oils. Another downside is that acrylics darken ever so slightly as they dry. When touching up a previously painted area and striving for an exact color match, or working with very minor differences in hue, the artist must anticipate the color variation of the acrylic paint when dry. It's taken me years to get to know the exact amount lighter to mix a color so when it dries, I'll have a perfect match; it's a shortcoming with the medium I've grown to live with and master. With the right amount of time, technique and experience, acrylic paintings can be created to mimic the look and feel of completed oil paintings in virtually every way.
Folks are often surprised to discover that most of my paintings ARE acrylics. They usually assume that they're oils; evidence again that the two are nearly impossible to distinguish between. If I can't smell the varnish or oil odor on another's painting, I can't tell if it's oil or acrylic. For the client, there essentially is no difference.
Mike Mayone - Middlebury, Vermont
