I am focusing on paint viscosity while working on some big pictures that required a large quantity of oil paint. This got me thinking about a series of related issues on the thickness of oil paint. While it may seem rather pedestrian to ponder the thickness of paint, it has multiple ramifications in conjunction with application methods.
Without introducing additives, paint engineered to be thick has specific characteristics. I define thick paint as when the tube is squeezed, the paint coming out on a palette like a piece of cooked spaghetti or more technically put, an extrusion that matches the round shape of the inner diameter of the hole in neck of the paint tube. Medium thick paint comes out somewhat round but flattens a bit and does not hold a crisp circular shaped diameter. Of course, thin paint nearly pours out of the tube with little force needed to press out a puddle of very fluid paint.
It stands to reason that you can make thick paint thinner by adding a medium. For argument’s sake, you can’t easily make a thin paint thicker. Doing so requires material that changes the medium to solids ratio, which, in turn, can interfere with proper film formation. (Within the confines of this essay, we won’t discuss additives that can chemically increase viscosity.)
With medium or thin oil paint, the need to add a diluent like a solvent is lessened. Adding more oil medium increases the chance that the paint will darken more over time and also be more glossy than surrounding paint that has not be modified with more oil. Uneven gloss leads to potentially uneven finished areas of an oil painting. It gets tricky to balance gloss when one part of a painting is super glossy while another part is matte. It put an artist’s varnishing skills to the test.
Back to thick paint for the moment: A large amount of time can be spent “taming” very thick paint. Smashing it with a stiff palette knife and trying to incorporate it with either solvent or a medium to achieve a homogeneous consistency is time consuming. Forgoing an additive and painting with thick paint sans medium, is like trying to spread taffy on the surface of your substrate. Unless a palette knife is used to apply paint to a surface, thick paint relies on some sort of diluent to break down the gooey, stickiness of a very full body oil paint.
Fortunately, you don’t find a lot of paint brands that feature very high viscosity oil colors. Most are medium to medium-thin by design. They provide good spread characteristics along with a sufficient pigment load.
It becomes easy to sniff out overly thick paint that has no covering power. For example, dig out a tube of natural green earth that you might likely have somewhere in the bottom of your paint box. If it is old enough, it spreads poorly. Is very granular and is so transparent it should be labeled as a greenish glaze. Good quality natural green earths have disappeared for the most part. The source of the pigment has been mostly exhausted.
Finally, the point of all this is to avoid the trap of having to employ large amounts of oil or solvents to modulate the viscosity of the paint you are using. As described, it becomes a balancing act between having paint that can be applied as intended and not adding too much to it that it amplifies the gloss or potential for yellowing over time or is thinned to the point of starving it of its binding medium.
This conundrum hits home when I think about my own collection of tube of lead white and some earth colors that have been aging like a fine wine. The common thread between both pigment groups is the presence of metals that promote drying. My lead white tubes are becoming harder and harder over time. At some point they will be unusable unless I decide to curtail painting with titanium white very soon and switch to using up my lead white paints. Some of my older earth colors are very hard or completely solid.
In conclusion, it does not pay to hold on to a number of metal-based, inorganic colors and some earth colors too long. They don’t get better over time. It becomes a juggling act to determine which painting get “special” treatment by getting out some coveted tubes of beautiful real manganese blue, lead white, genuine lapis lazuli, vermilion, aureolin, genuine rose madder and several other colors I have that are no longer being produced by anyone or are outrageously expensive if they are being manufactured.
It is also not a healthy practice to have to resort to solvents to modulate the thickness of some oil colors and the old vintage color just described. Paint holds up remarkably well over time in a properly constructed collapsible tube. But, oil paints in tubes do not last forever. The paint is reactive to heat and cold. Oils try to migrate out of suspension or ooze out of the crimp or cap and turn rubbery and brown.
You know the frustration of squeezing out a tube and only linseed oil emerges. However, it is all part of the charm and allure of the practice of painting. Pictures don’t paint themselves. We strategize to meet whatever challenges are placed before us.
The Syntax of Color