How Paints Work
Knowing what is supposed to happen can help if you are making your own.
Colours are part of nature and are all around us, affecting our thoughts and our mood, inspiring us and comforting us. Clearly, as a species, we have felt that we need to incorporate colours into our lives, whether that is in the form of clothes,
gems, metals or a garden full of flowers. Paintings are a logical extension of this and virtually anything can be painted so how does this work?
Light can enter our eyes having been reflected and modified by a surface or directly from a coloured light source. For a painting to be seen properly, it should be illuminated by a light source that our eyes can adjust to such as daylight or artificial lights - tungsten, fluorescent, LED lights and so on (unless the art is designed to be viewed under different conditions, such as if it is made using fluorescent pigments and is meant to be viewed under ultraviolet light, for example).
Using reflected light, paintings attempt to reproduce the images that we see in our everyday lives or, in the case of surrealism and related art forms, those that the artist thinks that we should be able to see. There are limits to what you can reproduce in a piece of art but people's eye's adjust to many limits and there are effects that can be used, such as local contrast, to extend the dynamic range of the artist's work.
Black and White
The brightest we can see is white and the darkest we can see is black. Whilst black and white are not achievable as pigments (black reflects some light and white certainly doesn't reflect it all), they do not exist in nature either. The very dark brown-grey pigment that is made from soot, that we use as black in paintings, reflects light in the same way that real soot does. Titanium white pigment is seen as a bright white but it still absorbs some deep blue light and reflects greeny-blue light (500nm) around 5% more than deep red (700nm) so it is far from perfect. However, if a painting has been created using those two colours, our eyes adjust to it and see the colours in between in the same way that if lead white was used as the white (interestingly, lead still slopes down by the same amount towards the red but it doesn't have the blue absorption that titanium oxide does, it carries on reflecting blue, right into the UV so, contrary to popular opinions, lead white actually reflects light better than titanium white throughout the spectrum).
You can see in the picture of the spray from the garden hose catching the light from the low sun and breaking it down into its colours to form a rainbow (hose-bow?) - showing that sunlight has what is called a continuous spectrum - it pretty much contains all of the colours that we can see. The process that water uses to split light in this situation is refraction insofar as different colours are refracted to different extents thus separated - this is called dispersion. However, you can see from the diagram on the left that light extends in both directions beyond the red into the infrared and in the other direction beyond blue into the ultraviolet - if you have a camera that is sensitive to these wavelengths and use that in the rainbow experiment, you will see that they are dispersed in the same way as the visible colours of the spectrum. It is all one big continuum and our eyes can see just a little bit of it.
Incidentally, whilst our eyes cut off pretty much exactly at 400nm, at the red end of the spectrum, you can see down to around 760nm or so and if you look through an infrared filter, you will see that the grass and leaves are very light and the sky is almost black although I must caution you to not do this for too long as there is a lot of light going into your eye if you do this and just because your vision is not particularly sensitive to it does not mean that it cannot do your eye any harm. Another thing that you might notice is that I say up to 400nm and down to 700nm. Where the values for these numbers might indicate that 400 is smaller than 700, if I'm talking about something like wavelengths, up and down refer to the energies and light at 400nm has a lot more energy than light at 700nm. There is more about light sources and the way that our eyes see light in the page on Colour Reproduction.
Representing Colours using Absorption
Move the mouse over the colour strips in the diagram on the right to see how the colours that are absorbed and reflected are related to each other.
The main way that pigments and dyes appear coloured is from the absorbing different wavelengths of light to different extents. As light is electromagnetic with the wavelengths having different amounts of energy, these are absorbed to different extents according to the electronic configuration of the molecules that form the dyes and pigments. For example, copper salts are capable of existing in two forms (known as copper I and copper II - these are different oxidation states) and the carbonate salt of each of these absorbs light differently so copper I carbonate is azurite which is a blue with a slight greenish tinge (the energy of light in the red to yellow part of the spectrum is just right for azurite to absorb). On the other hand, copper II carbonate is malachite which is green (the energy of light in the red and blue parts of the spectrum is just right for malachite to absorb). Basically, your eyes see the light that is left over after all of these electronic shenanigans.
There are many chemicals around us that can modify light to make colours - rocks, minerals, plant dyes and even dyes made from insects and then there are metals such as gold, silver, tin, copper and more recently, palladium and aluminium which, if they are not used strictly as a pigment in art, their reflective properties are used to reflect the light in a way that is very different to normal pigments. To qualify as a useful pigment or a dye, it has to: change the light that is shone onto it; not react with its environment; and last long enough to be useful. However, if you just paint a suspension of pigment in water or other volatile liquid onto a smooth surface, after a while, a lot of it would probably fall off. Clearly, we need something to hold it in place, some sort of transparent medium - a glue of some sort.
Then, we need to be able to put the paint on the surface so it needs to be in a form that we can apply. Normally, paints are quite viscous so they need to have some sort of solvent added to them in order to thin them down so that they can be applied effectively. The solvent depends upon the medium but for a given solvent type, there are ways of making sure that you get the best solvent for the job.
Finally, you need to have a surface that you can apply the paint to - known as a ground - and that needs to be on something that will allow it to be displayed appropriately - the support.
R e f l e c t e d A b s o r b e d This document is on the long side so I have divided it into two parts - any part of which you can get to by clicking below. However, if you wish to see the document as a whole, click here.
- Part One
- Pigments - making colours work
- Dyes - organic dyes also make good colours but you can't just mix the dye with the medium
- Impurities - unless you have a synthetic pigment, it will have impurities
- Particle Size - Different particle size in some pigments can change hue or depth of colour
- Masstone - Thickness and imperfect colours
- Part Two
- Media - Keeping it all in place
- Pigment Volume Concentration (PVC) - how much pigment is in the medium can effect the way it performs
- Solvents / Thinners - why use solvents and which are better than others?
- Grounds and Supports - The paint has to go on something
- Mixing colours - What happens when you mix colours
All images and original artwork Copyright ©2020 Paul Alan Grosse.
