Paint Basics

Knowing Paint Better

Watercolour Paint - Daniel Smith, QoR

I have wondered for a long time about the true nature of artists paints.  I have been searching for answers to some questions about how paint functions and how watercolour, oil and acrylic paint compare.  Here is what I found out.

What makes paint dry?  What are artists paints like on a microscopic level? 

Watercolour

Watercolour paint is a solution of gum arabic (the binder) and water and microscopic-sized pigment particles.  Gum arabic is an exudate from a gum tree.  More precisely it is the sap of two species of acacia trees.  Microscopically it is a long chain that is soluble in water.  The pigment particles are in suspension in the liquid.

When watercolour paint dries the water evaporates leaving the gum arabic to become a solid which holds the pigment particles in place. You can lift dried watercolour paint because the gum arabic will redissolve in water. 

Watercolour Paint in Palette

Acrylic Paint 

Acrylic paint is a suspension of spherical polymer particles (about 1 micrometer in size) in water.  The pigment particles are also suspended in the water.  When acrylic paint dries the water evaporates, the polymer particles coalesce to form solid sheets which trap the pigment particles.  You cannot lift this paint because the polymer is now a water-insoluble solid.

Gesso is made up of water, acrylic polymer and gypsum (calcium sulfate) particles. 

Oil Paint

Oil paint is a mixture of an organic solvent like turpentine or mineral spirits and drying oil (linseed oil, tung oil) and pigment particles.  The oil is soluble in the solvent so they do not separate.  The pigment particles are so small they remain in suspension.  When oil paint dries the solvent evaporates leaving the oil and pigment on the surface.  When on the surface the oil oxidizes and becomes a solid which is no longer soluble in either solvent or water.  

Why can you put oil paint on top of acrylic paint but not the reverse?  Oil paint can be put on top of acrylic paint (or gesso) because the dried acrylic paint is essentially a plastic sheet and the oil  will bond to it.  However, you cannot put acrylic paint on top of oil paint because the dried oil paint is hydrophobic and it repels the water in the acrylic paint thus causing it to flake off.  

How do each of these paints stick to a paper/canvas substrate?  They use a process called wetting in which they form a strong physical bond with the surface.  

Why do we need gesso on a canvas before painting it?  It helps to keep the paint from absorbing into the substrate and it also smooths out the surface. 

What does water do when added to watercolour paint (oil to oil paint or water to acrylic paint)? It reduces the viscosity, which means it dilutes the paint.  If too much is added to acrylic paint, it will not harden.  When considerable dilution is desired you should use acrylic medium which contains acrylic polymer particles and so it acts the same as the paint. 

Paints usually have a fourth component termed an ‘additive’ which is used to make the paint flow well, maintain its quality in the tube, keep it homogeneous, etc.

Water Miscible Oils

Water Miscible Oil Paint is a modern invention that may be going to revolutionize the oil painting industry.  Time will tell how well it is accepted.  Right now it is just in the trial stage by most artists.  How is it different?  It can be thinned by just adding water and brushes can be cleaned in water only.  No solvent is needed.  How is it formulated so this can happen?  

The oils in oil paint are made of long-chain fatty acids.  Most of these fatty acids are hydrophobic (water repelling) so the paint is incompatible with water.  However, in normal oil paint there are normally some hydrophilic fatty acids (water loving).  If this portion of hydrophilic fatty acids is increased, the paint becomes water soluble.  Water can then become the 'solvent' for the oil paint!  Just like magic, the paint can then be thinned with water and clean-up done with water.  Organic solvents (turpentine, mineral spirits) are no longer needed.  

How does water miscible paint dry?  The water evaporates leaving the oil and pigment on the substrate.  The oil then oxidizes and hardens, trapping the pigment. 

Pigment particles 

Pigment particles are made from natural and artificial compounds which are pulverized into ultra-small particles (0.1 to 100 microns).  Some pigments are derived from naturally occurring compounds like iron oxides (ochre, umber, sienna), carbon black, lapis lazuli.  Others are from manmade pigments which are usually based on metal compounds like cadmium, chromium, cobalt, copper, lead, titanium and zinc.  When a tube of paint is labelled ‘hue’ it means that the colour will be almost identical to the genuine paint for which it is named.  However, the pigment is not the same as in the original. It is often a cheaper or a blended form. 

Golden has recently marketed an innovative watercolour paint called QoR, an acronym for ‘Quality of Results’.  They are advertising it as possessing some nice features like:  incredibly smooth transitions, good flowing while maintaining liveliness on the paper, excellent resolubility in water, excellent glazing qualities, vivid depth of colour in one stroke, greater resistance to cracking and flaking, more density of colour than traditional watercolours, and easy clean-up.  Besides these stated advantages they have marketed new  grounds and mediums to go with the paint.  These apparently increase gloss, improve flow or wetting properties, and allow you to achieve different textured surfaces.  It is exciting to be part of modern advances in paints and materials.

QoR paint has a newly formulated binder called Aquazol.  It appears that the company is keeping a tight lip on its formulation.  I have not been able to determine its chemistry but expect it is an acrylic or an acrylic/gum arabic mixture.  I have used QoR paints only minimally so far and find them bright and easy to use.  The only negative is that I have found the Aquazol has separated somewhat from the pigment in one or two of my tubes.  Time will tell how successful they are.  They are probably the forerunner in a lot of new products in the watercolour industry.

There is some discussion about QoR paints not ‘lifting’ as well as traditional watercolours.  I have done a short, uncontrolled test on my own and find that to be true.  See the photo below which shows lifting in traditional paint compared to QoR paint. The two on the left show lifting from traditional paint and the one on the right shows lifting from QoR paint.   I used the same amount of water, the same bristle brush and the same number of strokes on each followed by dabbing with dry paper towel.  It appears the traditional paints lift better than the QoR paints.  However, this is too simple a test to draw any conclusions.  One would need to do many more samples and under more controlled conditions.  I like the QoR paints and will continue to use them and time will tell how they work out.  

Lifting Watercolour Paint; Traditional Paint Left and Centre, QoR Right