Discussion about Paint Mediums (3.2)
Here we discuss the basics of paint mediums
A paint medium is a medium for our paint 🙂
Mediums mix with the paint from our tubes to thin and extend the paint, and also confer to it different working properties and appearences.
Oil paint is versatile because there are a great amount of natural materials that have been used as mediums with the pigment.
The easiest way to understand medium is to break it down into three categories:
BINDER
Binder ‘binds’ the pigments to the paint surface. It is the glue that makes paint stick so that it does not just fall off the surface we apply it to. In the case of oil paint, oil is the binder. In the case of acrylic paint, acrylic is the binder. In the case of watercolor, gum arabic is the binder. In the case of egg tempera, egg proteins are the binder.
Drying oils are a class of oils that actually set like a glue. They become a gummy film that eventually harden with time. This happens by the oil ‘breathing in’ air or more precisely oxygen molecules, from which monomers (singular, disassociated molecules) become polymers (bonded molecule matrix).
Therefore the most important and most principle medium in oil painting is drying oils, principally from the flax plant (linseed oil). Other drying oils can also be used, such as walnut, safflower and poppy oil.
Cold pressed linseed oil is the most common oil, and is the oil that most pigments in oil paint tubes is already mixed with. So many artists just use small amounts of cold pressed linseed oil to extend and give flow to their paint.
The upside of using only cold pressed linseed oil is that it is completely natural and physically benign, and when it does dry it creates a well bonded and flexible paint film. The downside of cold pressed linseed oil is that its slow drying and can run and pull with gravity. If overused, it is too fine and slippery.
So the ancients devised stand oil. Stand oil is cold pressed linseed oil that has been treated with a long summer season, where it is left to stand, to absorb oxygen and u.v. It becomes thicker like honey and faster drying.
But trying to paint with only stand oil presents its own problems – it is thick, gummy and tacky by itself. It does not flow. So many people add a solvent to the stand oil in order to make it finer.
This brings us to our second important ingredient in an oil painting medium
SOLVENTS
Solvents are not binders. They confer no sticking power, they do not adhere the paint to the canvas. They will stretch paint, its true, but it will make a weak paint film and result in dull and unbinded colors. Solvents are used with the binder. They are essentially used to cut the fat so the medium is leaner, so the fat content is lowered, so the paint dries faster because the solvent does not ‘dry’ – it evaporates straight off the canvas.
A solvent can be used to make stand oil thinner. This way, added to stand oil, one may calibrate a medium so it is thicker than cold pressed linseed oil, but not as thick as stand oil alone. So it enables stand oil to be workable.
The downside is that one will be dealing with a volatile solvent that will evaporate into ones working environment. Most people can tolerate the exposure to a solvent in the short term, but may develop intolerance or even reactions and health issues with repeated, reckless exposure. Therefore solvents are to be used with care.
The main two solvents used to cut the stand oil are mineral spirits and gum turpentine. Mineral spirits are a factional distillation from oil. Gum turpentine is the volatile from pine tree sap. Mineral spirits are actually better tolerated than gum turpentine because mineral spirits do not pass through the skin as much. Gum turpentine can pass through the skin, so one should be careful with ones occupational exposure either way – keep the space well ventilated. Paintings should be allowed to dry out of ones direct living environment. One should try minimise exposure with skin.
So a basic recipe is thus:
Stand oil + Mineral Spirit (or gum turpentine)
The ratio is not something that is set in stone. One should aim to thin the stand oil to a workable consistency – this will take trial and error. A good ratio to start may be 1 part gum turpentine (or mineral spirit) to 4 parts stand oil. If the stand oil is still too thick and gloopy, one may try adding slowly more gum turps (or mineral spirit) until a good working viscosity is obtained.
When painting, one should only dispense as much medium as one needs to work. Not leave any jars open. Because the solvent will continue to evaporate and change the ratio of your medium, plus exposing you unneccesarily to excessive fumes.
What about pre-made Paint Mediums?
There are a lot of pre-made paint mediums available out there. It is impossible to summarise how each of these may behave. One approach is to check out the ingredients and see if the ingredients correspond to traditional principles.
Some oil paint mediums are made with newer recipes such as alkyd resins. They have different working properties. Eg Liquin, Liquol, Galkyd. These mediums may suit some people but there are some aspects where they diverge from drying oils.
Check your mediums, if they have stand oil and gum turpentine or mineral spirit. They may also have resins, these resins, such as dammar or canada basalm or mastic. You will usually find that dammar is the resin that is included in these mixes. Resins are that third category of paint mediums, they are added to impart varnish shine and lustre. Their downside is they can be sticky and do not help when it comes to paint film strength. Dammar may also yellow over time. However, a small amount of resin can give deeper darks and brighter colors, as well as a pleasing lustre.