 |
| Photo thanks to Anita Thomhave Simonsen |
Size, primer and gesso. If you're anything like me, you're probably as confused as *** when it comes to these and preparing a surface for painting. Not any more. This post aims to clean away the mud and is a companion post to the one last month on painting substrates. So if you want to know what substrate is usable and whether it requires sizing or priming or both, check it out. This one is about the actual size and primer options. And some of it is straight from the wonderful AMIEN forums. Go check them out for all your technical artist materials questions.
Why Size?
Okay, here we go. Raw canvas, or any oil painting substrate, requires a barrier between itself and the oil paint. This is to prevent the paint sinking in to the surface (makes for terrible colour) and also to prevent the oils in the paint attacking and rotting the substrate (a nasty habit of oil paints).
In the case of acrylic paints, a barrier between the paint and substrate is required to prevent Support Induced Discoloration (SID) which is something that occurs in acrylic paints and mediums only. Many common artist supports have impurities that can discolor a translucent acrylic gel layer or color glaze, and a barrier must be applied to ensure the products stay clear as the films dry.
As an acrylic paint film cures, the water exits two ways: through the surface of the paint and through the back of the support, if it is porous enough. Canvas, linen, wood and masonite are all porous enough to allow water to absorb into them. During this drying process, the water is actually in equilibrium moving back and forth between the acrylic paint and the support. The water extracts water-soluble impurities such as dirt, sap, starches, etc., from the support and deposits them into the acrylic film. The result is a discolored (typically amber) film, with the degree of discoloration dependent on the amount of contaminants deposited and the inherent level of inpurities in the support.
SID contamination often goes undetected. In most cases, the paints applied contain a sufficient level of pigment, thus a strong enough color, to conceal the yellowing. However, in a transparent glaze and especially in thick translucent gel layers, SID becomes quite noticeable. SID can transform the appearance of an Ultramarine Blue glaze into a lower chroma, greenish color. Gesso alone will not stop SID, and different gels and mediums have varying degrees of blocking capabilities.
The acrylic dispersion mediums, paints and primers do absorb and expell moisture for about a year after they are applied -- or less time if the environment is warm and dry. After that's finished the resins coalesce into a more continuous film. They are still susceptible to moisture penetration, but not to the degree that puts them at the same risk as RSG. And it's preventable by coating the finished paintings.
In summary, for both oil and acrylic paints, it is wise to put a barrier between the paint and it's substrate. This barrier is known as size. It is meant to be a penetrating sealer, not a coating. It is therefore very thin, using it is good practice, and it's very easy to do.
Ancient and Modern Size
Traditional size for oil paints is a (disgusting I say) concoction of rabbit skin collagin heated with water. Although this has been used for hundreds of years it is know known by conservators to cause more problems than it solves. How? RSG is hygroscopic. It continuously absorbs moisture from the atmosphere, causing it to continuously swell and shrink. Over time, this constant flexing causes the oil paint on top, which is quite brittle, to crack. In fact, RSG is now believed to be the main cause of cracking in old oil paintings.
Instead of RSG underneath oil paintings, it is now recommended to use a pH neutral PVA or any acrylic medium whose manufacturer recommends it as a size. Please remember that PVA size or acrylic medium does not tighten fabric like rabbit skin glue.
Underneath acrylic paints, the best choice is acrylic medium whose manufacturer recommends it as a size that can block SID - a blocking size. A blocking size will be formulated to inhibit the migration of soluble organic materials from the support through to the ground. Two that are often mentioned are Gamblin’s PVA size and Golden Acrylics’ GAC100. Liquitex makes one as well, as I am sure others do too.
Two coats of size on the front of a substrate are sufficient. They should be touch dry and not cold to touch (which indicates moisture) before the next layer is added.
Why Prime?
Priming is the adding of an absorbent coating to a substrate. The aim is to provide the paint with a porous surface to adhere to. Primer is also known as ground or gesso. It is not size and will not seal or create a barrier to the paint. It does exactly the opposite. It is absorbent and provides tooth, or texture for the paint. It also tends to stiffen the substrate.
Usually two coats of primer is used.
Just for the record, you don't need to prime unless you want the surface that priming gives. You can paint directly onto modern size (but not RSG).
All About Gesso
Traditional gesso
Real or traditional gesso is for hard inflexible surfaces only. It is too brittle for canvas and will crack. Real gesso is like a plaster and is ideal on wood panels. It is made from a thin base of RSG and inert white pigments. The pigment is powdered chalk, calcium or gypsum. Sometimes titanium white pigment is also added to the mix for brightness.
This zinc oxide in the ground is a problem in terms of archival quality. Paint over zinc oxide and within a few years time the painting is likely to be delaminating or cracking.
Modern gesso
Gesso changed in the 20th century. In 1955, Liquitex, an acrylic paint company developed the first water-based acrylic gesso. It provided a consistent and inexpensive primer layer for both acrylic and oil paintings. Technically it is an acrylic dispersion primer, not a gesso, but it is mostly known as acrylic gesso or acrlyic primer. This is the only choice of primer available for use under acrylic paints.
Oil gesso
It's really useful to remember that an oil-primed canvas can only accept oil paints. Although oil paint can be applied to acrylic gesso, acrylic paint will not permanently adhere to an oil-primed canvas. The acrylic gesso will eventually peel off the oil-primed canvas.
Today there are two alternatives to traditional gesso with it's RSG content. There are oil/alkyd grounds which are the quickest drying option. And there is lead white oil primer. The latter is somewhat hazerdous to use. Don't smoke or eat in your studio. Use a barrier cream on your hands or wear thin rubber gloves so the lead can't penetrate your skin. Use odorless mineral spirits to thin the primer, not gum turpentine; odorless mineral spirits is hazardous but not as hazardous as regular mineral spirits and definitely not as hazardous as gum turpentine. In any event, you will want good ventilation when using these solvents: at least work near an exhaust fan. Thoroughly wash your hands when you are finished.
For the lead white oil primer you will have to apply at least 2 thinned coats. Allow perhaps a week for the drying of the initial coating, depending on the temperature and humidity of your studio. If it is dry to the touch after a few days, then you can go ahead and apply the second coat. After you apply the second coat, check to see that the coverage is good. You might want a third coat.
So there are three choices for priming for oil paints. Acrylic gesso and two kinds of oil based gesso.
Acrylic grounds under oils
In recent years, some artists have begun to question whether or not acrylic gesso is the right product to use under oil paint. Acrylic dispersion grounds retain their flexibility as they age, and the oil/alkyd grounds get stiffer and more brittle as they age. There is concern that this difference in flexibility may cause oil paintings on acrylic grounds to delaminte as they age.
The best current knowledge is that they are considered very good grounds for oil paints, with two caveats. 1.) You must purchase the highest quality ground you can find. That means: do not by acrylic dispersion grounds by price, and be sure the label tells you what's in the stuff. If you're unsure about the latter, call the manufacturer. 2.) Oil paints ought to be painted on rigid substrates. If the acrylic dispersion ground is applied to a panel, or to a fabric mounted on a panel, there should be no problems.
For a better bond with oil paint, after the acrylic primer is dry (when it's no longer cool to the touch is a good indicator) you should wipe this surface down with warm water and a clean rag to remove surfactants and allow to dry again (24-48 hrs) before applying oil based paints.
Linky Goodness
More art technique articles
This article is one in an ongoing series of technical articles for artists, all archived together and accessible from here. The topics range from details on materials, to the business of art, to specific art techniques. Please make use of this resource.
And remember to check out my artworks on Flickr, and have an insider peek at my life as an artist on Facebook.
Posts
